SUCCESS! In Clinical Laboratory Science - Chemistry: Enzymes and Cardiac Assessment

A 42-year-old male presents with anorexia, nausea, fever, and icterus of the skin and mucous membranes. He noticed that his urine had appeared dark for the past several days. The physician orders a series of biochemical tests. Based on the following test results, what is the most likely diagnosis? Serum alkaline phosphatase—slightly elevated Serum alanine aminotransferase—markedly elevated Serum aspartate aminotransferase— markedly elevated Serum gamma-glutamyltransferase— slightly elevated Serum total bilirubin—moderately elevated Urine bilirubin—positive Fecal urobilinogen—decreased A. Acute hepatitis B. Alcoholic cirrhosis C. Metastatic carcinoma of the pancreas D. Obstructive jaundice

A. Acute hepatitis A. Acute hepatitis is characterized by markedly elevated levels of serum alanine aminotransferase and aspartate aminotransferase, which may range from 10- to 100-fold greater than the reference values. Although alkaline phosphatase and gamma-glutamyltransferase are increased, their elevations are less notable than the aminotransferases. Alkaline phosphatase may range up to two times the reference range whereas gamma-glutamyltransferase may go as high as five times the reference range in acute hepatitis. Due to leakage of conjugated bilirubin from the hepatocytes, the urine bilirubin will be positive. With less conjugated bilirubin reaching the intestines, fecal urobilinogen will be less than normal

To aid in the diagnosis of skeletal muscle disease, which of the following serum enzyme measurements would be of most use? A. Creatine kinase B. Alkaline phosphatase C. Aspartate aminotransferase D. Alanine aminotransferase

A. Creatine kinase A. To aid in the diagnosis of skeletal muscle disease, measurement of creatine kinase would be most useful. CK yields the most reliable information when skeletal muscle disease is suspected. Other enzymes that are also useful to measure are aspartate aminotransferase and lactate dehydrogenase. Both of these enzymes will be moderately elevated, whereas CK is significantly increased.

If elevated, which of the following is associated with increased risk for coronary heart disease? A. Homocysteine B. Vitamin B6 C. Myoglobin D. pro-BNP

A. Homocysteine A. Elevated homocysteine levels are associated with increased risk for coronary heart disease. Increased homocysteine contributes to the damage of arterial walls preceding formation of plaques. Individuals at risk need to be evaluated for vitamin B levels, because low levels of folic acid, vitamin 65, and vitamin 612 are associated with increased levels of homocysteine.

When measuring enzyme activity, if the instrument is operating 5°C lower than the temperature prescribed for the method, how will the results be affected? A. Lower than expected B. Higher than expected C. Varied, showing no particular pattern D. All will be clinically abnormal.

A. Lower than expected A. Factors that affect enzyme assays include temperature, pH, substrate concentration, and time of incubation. For each clinically important enzyme, the optimum temperature and pH for its specific reaction are known. When lower than optimum temperature or pH is employed, the measured enzyme activity will be lower than the expected activity value. As temperature increases, the rate of the reaction increases. Generally, a twofold increase in reaction rates will be observed with a 10°C rise in temperature. However, once the optimum temperature is exceeded, the reaction rate falls off as enzyme denaturation occurs at temperatures ranging from 40 to 70°C.

Which of the following does not accurately describe properties associated with lactate dehydrogenase? A. Optimum pH for the catalysis of lactate to pyruvate is 7.4-7.8. B. LD is increased in a hemolyzed serum specimen. C. LD catalyzes the oxidation of lactate to pyruvate with mediation of nicotinamide-adenine dinucleotide. D. LD-4 and LD-5 are labile in the cold.

A. Optimum pH for the catalysis of lactate to pyruvate is 7.4-7.8. A. Lactate dehydrogenase (LD, also abbreviated LDH) is found in all body tissues and is especially abundant in red and white blood cells. Hence hemolyzed serum will give falsely elevated results for LD. The enzyme catalyzes the conversion of lactate to pyruvate at pH 8.8-9.8 and pyruvate to lactate at pH 7.4-7.8, mediated by nicotinamide adenine dinucleotide (NAD+). Each of these reactions is associated with its own unique reference range. LD exists in five isomeric forms called isoenzymes. The isoenzymes can be separated by electrophoresis. Serum specimens for LD isoenzyme determinations can be stored at room temperature for 2 or 3 days without appreciable loss of activity. Room temperature storage is necessary because LD-4 and LD-5 are labile in the cold. This is in contrast to most enzymes, which are more stable when refrigerated or frozen.

For assessing carcinoma of the prostate, quantification of PSA has virtually replaced the measurement of which of the following enzymes? A. Alkaline phosphatase B. Acid phosphatase C. Alanine aminotransferase D. Trypsin

B. Acid phosphatase B. The quantification of serum prostate-specific antigen has replaced measurement of serum acid phosphatase for assessing carcinoma of the prostate. PSA measurement in conjunction with the digital rectal examination is recommended for prostate cancer screening. In addition, PSA can be used to stage and monitor therapy of prostatic cancer.

Which of the following enzymes does not belong to the class of enzymes known as the hydrolases? A. Alkaline phosphatase B. Aldolase C. Amylase D. Lipase

B. Aldolase B. Hydrolases are enzymes that split molecules with the addition of water—for example, amylase, lipase, alkaline phosphatase, acid phosphatase, 5'-nucleotidase, and trypsin. They do not usually require coenzymes but often need activators. Aldolase and carbonic anhydrase are examples of the class of enzymes known as the lyases. Lyases are enzymes that split molecules between carbon-to-carbon bonds without the addition of water. The resulting products usually contain carbon double bonds.

In acute pancreatitis, a significant increase in which serum enzyme would be expecteddiagnostically? A. Creatine kinase B. Amylase C. Alkaline phosphatase D. Aspartate aminotransferase

B. Amylase B. Amylase and lipase are the two most important enzymes in evaluating pancreatic function. The values of amylase and lipase activity are significantly elevated in acute pancreatitis and obstruction of the pancreatic duct. In most cases of acute pancreatitis, the lipase activity stays elevated longer than amylase activity.

Which enzyme is measured by methodologies that use small oligosaccharides and 4-nitrophenyl-glycoside for substrates? A. Lipase B. Amylase C. Creatine kinase D. Cholinesterase

B. Amylase B. The function of amylase to catalyze the hydrolysis of starch to dextrins, maltose, and glucose has been used as the basis for several methods over the years. The more commonly used methods today employ small oligosaccharides and 4-nitrophenyl-glycoside as substrates. In general, these methods can be automated, using an oxygen electrode system and UV or visible wavelength spectrophotometry to determine amylase activity

Which of the following enzymes catalyzes the transfer of amino groups causing the interconversion of amino acids and alpha-oxoacids? A. Amylase B. Aspartate transaminase C. Alkaline phosphatase D. Lactate dehydrogenase

B. Aspartate transaminase B. Aspartate and alanine aminotransferases catalyze the transfer of amino groups between amino acids and a-oxoacids. A prosthetic group, pyridoxal-5'-phosphate (P-5'-P), is required for the transfer of the amino group. In the aspartate aminotransferase (AST) reaction, AST catalyzes the transfer of an amino group from L-aspartate to a-oxoglutarate, with the amino group transfer mediated by P-5'-P, which is bound to the apoenzyme. The products formed are oxaloacetate and L-glutamate. By coupling this reaction with a malate dehydrogenase reaction, the decrease in absorbance of NADH as it is oxidized to NAD+ can be followed at 340 nm. The change in absorbance will be proportional to the AST activity present in the serum specimen.

Which of the following is false about myoglobin as it relates to acute myocardial infarction (AMI)? A. Measure serially B. Cardiac specific C. Initial increase occurs in 1-3 hours D. Doubling of initial value within 1-2 hours suggestive of AMI

B. Cardiac specific B. In acute myocardial infarction (AMI), the initial increase in serum myoglobin levels occurs in 1 to 3 hours following onset of symptoms. Serial measurements need to be made because a single value is not diagnostic. When doubling of the initial value occurs within 1 to 2 hours, this is suggestive of AMI. In AMI, the myoglobin level will peak within 5 to 12 hours, with serum levels returning to normal within 18 to 30 hours. Because myoglobin is found in other tissues and is not cardiac specific, it is usually used in conjunction with cardiac troponin and CK-MB to assess the occurrence of AMI.

Which of the following enzymes would not be useful to quantify in the assessment of liver function? A. Alanine aminotransferase B. Creatine kinase C. Alkaline phosphatase D. Gamma-glutamyltransferase

B. Creatine kinase B. Alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, gammaglutamyltransferase, and lactate dehydrogenase are enzymes for which the serum activities may be assayed to assess liver function. At the cellular level, alkaline phosphatase functions in the membrane border, gamma-glutamyltransferase functions in the cell membrane, and alanine aminotransferase functions both in the cytoplasm and mitochondria. With tissue damage and necrosis, the cells disintegrate and leak their contents into the blood. Because these enzymes are cellular enzymes, any increase in their activity levels in serum is indicative of tissue destruction. It is important to remember that these enzyme levels must be used in conjunction with other clinical data because enzymes generally are not organ specific; they are found in several tissues.

The properties of enzymes are correctly described by which of the following statements? A. Enzymes are stable proteins. B. Enzymes are protein catalysts of biological origin. C. Enzymes affect the rate of a chemical reaction by raising the activation energy needed for the reaction to take place. D. Enzyme activity is not altered by heat denaturation

B. Enzymes are protein catalysts of biological origin. B. Enzymes are protein in nature. Like all proteins, they may be denatured with a loss of activity as a result of several factors (e.g., heat, extreme pH, mechanical agitation, strong acids, and organic solvents). Enzymes act as catalysts for the many chemical reactions of the body. Enzymes increase the rate of a specific chemical reaction by lowering the activation energy needed for the reaction to proceed. They do not change the equilibrium constant of the reaction; but rather, enzymes affect the rate at which equilibrium occurs between reactants and products.

A 4-year-old male child is brought to the pediatrician because the parents are concerned about the child's frequent falling, which results in bruising. The parents indicate that the child has difficulty running, walking, standing up, climbing stairs, and even sitting up straight. The child also appears somewhat weak. Which of the following results is not consistent with the most likely diagnosis? A. Moderately elevated AST B. Moderately elevated ALP C. Moderately elevated LD D. Markedly elevated CK

B. Moderately elevated ALP B. The child's symptoms are consistent with Duchenne dystrophy, which is an X-linked recessive disorder. It is characterized by muscle weakness, which is caused by destruction of muscle fibers. Symptoms are seen in male children starting at 3 to 7 years of age. The most notable enzyme increase is in creatine kinase, which may increase 50-100 times the reference range. Aspartate transaminase and lactate dehydrogenase would also be increased, because both enzymes are present in skeletal muscle tissue. Alkaline phosphatase is not present in skeletal muscle tissue and is measured to assess hepatobiliary and bone disorders.

Which of the following statements is not associated with serum cholinesterase? A. Inhibited by organic insecticides B. Referred to as "true" cholinesterase C. Decreased level causes prolonged apnea after administration of succinyldicholine D. Acts on the substrate propionylthiocholine

B. Referred to as "true" cholinesterase B. Cholinesterase is a serum enzyme synthesized by the liver. It is also known as pseudocholinesterase to distinguish it from "true" cholinesterase (acetylcholinesterase) of erythrocytes. Although a number of disease states are associated with abnormal levels of this enzyme, cholinesterase levels are especially important in detecting organic insecticide poisoning of workers in the chemical industry and agriculture. Decreased cholinesterase levels and atypical enzyme forms are associated with prolonged apnea after succinylcholine administration during surgery. Propionylthiocholine is a commonly used substrate for measuring serum cholinesterase activity.

Based on the following graph of velocity of an enzyme reaction versus substrate concentration, you are designing a new method to measure the activity of an enzyme of clinical interest. To formulate the new methodology so that enzyme activity is assessed using zero-order kinetics, which concentration of substrate should you initially determine experimentally? A. Substrate concentration a B. Substrate concentration b C. Substrate concentration c D. Substrate concentration d

B. Substrate concentration b B. Michaelis and Menten proposed a basis for the theory of enzyme-substrate complexes and rate reactions. By measuring the velocity of the reaction at varying substrate concentrations, it is possible to determine the Michaelis constant (^m) for any specific enzymatic reaction. Km represents the specific concentration of substrate that is required for a particular reaction to proceed at a velocity that is equal to half of its maximum velocity. The A^m value tells something about the affinity of an enzyme for its substrate. When [S] = Km, the velocity of the reaction is expressed as V = 1/2 Vmax. In the graph shown with this question, the Km of the reaction is represented by b. Because substrate must be present in excess to obtain zero-order kinetics, the substrate concentration necessary would have to be at least 10 times the Km, which is represented by d. Usually substrate concentrations 20-100 times the Km are used to be sure that substrate is present in excess. Thus it is critical that the ^m value be determined experimentally

In the assay of an enzyme, zero-order kinetics are best described by which of the following statements? A. Enzyme is present in excess; rate of reaction is variable with time and dependent only on the concentration of the enzyme in the system. B. Substrate is present in excess; rate of reaction is constant with time and dependent only on the concentration of enzyme in the system. C. Substrate is present in excess; rate of reaction is constant with enzyme concentration and dependent only on the time in which the reaction is run. D. Enzyme is present in excess; rate of reaction is independent of both time and concentration of the enzyme in the system.

B. Substrate is present in excess; rate of reaction is constant with time and dependent only on the concentration of enzyme in the system. B. Enzymes are proteins that act as catalysts. It is not practical to measure enzyme concentrations in a body fluid specimen, but rather to assay enzymes according to their activity in catalyzing an appropriate reaction; that is, the conversion of substrate to product. An enzyme acts by combining with a specific substrate to form an enzymesubstrate complex, which then breaks down into product plus free enzyme, which is reused. A general form of the reaction is [E] + [S]±<->[ES]->[P] + [E] where [E] = concentration of enzyme, [S] = concentration of substrate, [ES] = concentration of enzyme-substrate complex, and [P] = concentration of product of the reaction. Because the rate of such a reaction is used as a measure of enzyme activity, it is important to consider the effect of substrate concentration on the rate of the reaction. The kinetics of the reaction are initially of the first order (i.e., the rate varies with the concentration of substrate as well as the concentration of enzyme) until there is sufficient substrate present to combine with all enzyme. The reaction rate then becomes zero order (i.e., the rate is independent of concentration of substrate and directly proportional to concentration of enzyme as measured by reaction rate) when substrate is present in excess. Hence it is desirable to use conditions that provide zeroorder kinetics when assaying enzyme activity

A 68-year-old male in an unconscious state is transported to the emergency department after being involved in a one-car crash, where he drove off the road and hit a tree. Because he was alone at the time and there was no apparent cause for the accident, it is assumed that he blacked out, which caused him to lose control of the car. He was not wearing a seat belt and has a broken leg, multiple contusions, and cuts. Blood samples were drawn upon arrival to the ED and in 3-hour intervals for 12 hours; all control values were within acceptable range. Selected test results follow: Test - Initial Values - 3 hrs - 9hrs -- Reference Range Myoglobin - 57 ng/mL - 140 ng/mL - 281 ng/mL -- 30-90 ng/mL Total CK - 112U/L - 1 70 U/L - 390 U/L -- 15-1 60 U/L CK-MB - 3 ng/mL - 6 ng/mL - 8 ng/mL -- 0-5 ng/mL Troponin I - 0.10 ng/mL - 0.12 ng/mL - 0.11 ng/mL -- <0.40 ng/mL What do these test results suggest? A. The man had a myocardial infarction, which caused the accident. B. The elevated results are from the skeletal muscle injuries sustained in the car crash. C. The elevated results are a combination of the car crash injuries and a myocardial infarction. D. The elevated total CK and CK-MB results indicate that the man had a stroke.

B. The elevated results are from the skeletal muscle injuries sustained in the car crash. B. The controls were within acceptable limits, so it is assumed that all test results are accurate. The elevated myoglobin, total CK, and CK-MB with a troponin I that showed no change and remained in the reference range suggest that the elevated results were due to the skeletal muscle injuries sustained in the car crash. Myoglobin is not tissue specific and may be increased in skeletal muscle injuries, muscular dystrophy, and AMI. The same is true for creatine kinase, which is not tissue specific and may be increased in skeletal muscle disorders as well as cardiac muscle disorders. CK-MB, although it is associated with cardiac muscle and the occurrence of AMI, is not tissue specific and will increase with skeletal muscle injury (but to a lesser degree than CK-MM). Cardiac troponin I is tissue specific and not expressed by skeletal muscle; thus it would remain within the reference range in the absence of an AMI. Total CK and CK-MB do not provide information to assess if a stroke has occurred.

Which of the following is not associated with assessment of an AMI? A. Elevated serum cTnl level B. Elevated serum CK-MB level C. Abnormal serum alkaline phosphatase isoenzyme pattern D. Blood collected upon presentation and serially in 3- to 6-hour intervals

C. Abnormal serum alkaline phosphatase isoenzyme pattern C. Quantification of serum total creatine kinase, CK-MB (or CK-2) isoenzyme, and cardiac troponin I (cTnl) or cardiac troponin T (cTnT) is very useful in determining an AMI. Determining the presence and activity level of CK-MB is valuable, because CK-MB levels can increase following an infarct, ranging from 6 to 30% of the total CK. Serial assessment of serum specimens is recommended, with the initial specimen obtained at presentation, followed by blood collection at 3-6 hours, 6-9 hours, and 12-24 hours from the initial time. Because alkaline phosphatase isoenzymes are associated with liver, bone, intestinal, and placental tissues, their analysis would not contribute any significant information to determining the occurrence of an AMI.

Which of the following disorders is not associated with an elevation of serum creatine kinase? A. Cerebrovascular accidents B. Hypothyroidism C. Bone disease D. Intramuscular injection

C. Bone disease C. Increased serum creatine kinase (CK), formerly called creatine phosphokinase (CPK), values are caused primarily by lesions of cardiac muscle, skeletal muscle, or brain tissue. CK increases in the early stages of Duchenne-type progressive muscular dystrophy. Assays of total CK and CK isoenzymes are commonly used in the diagnosis of myocardial infarction. Hypothyroidism causes a moderate increase in CK values. Elevation of this enzyme also occurs after vigorous muscular activity, in cases of cerebrovascular accidents (stroke), and after repeated intramuscular injections. In addition to quantifying total CK activity, isoenzymes may be determined by using electrophoretic, immunologic, or ion-exchange chromatography methods. Three isoenzymes have been identified: CK-1 or BB, primarily found in brain and nerve tissues with some in thyroid, kidney, and intestine; CK-2 or MB, primarily found in heart muscle; and CK-3 or MM, primarily found in skeletal muscle but present in all body tissues. CK is not elevated in bone disease.

When an AMI occurs, in what order (list first to last) will the enzymes aspartate aminotransferase (AST), creatine kinase (CK), and lactate dehydrogenase (LD) become elevated in the serum? A. AST, LD, CK B. CK,LD,AST C. CK,AST, LD D. LD, CK,AST

C. CK,AST, LD C. When an AMI occurs, CK is the first enzyme to become elevated in the blood, rising within 4 to 6 hours following chest pain. AST exhibits a rise in the serum level within 6 to 8 hours. LD shows an increase in 8 to 12 hours following infarction. Measurement of these three enzymes to assess acute myocardial infarction has been replaced by cardiac troponin, myoglobin, and CK-MB. However, awareness of the CK, AST, and LD patterns as well as other biochemical tests is useful in assessing organ complications that may arise during the period of AMI.

Which of the following sets of tests would be the most useful in diagnosing an AMI? A. AST, LD, CK-MB B. LD, CK-MB, troponin C. CK-MB, troponin, myoglobin D. LD, troponin, myoglobin

C. CK-MB, troponin, myoglobin C. For many years, the diagnosis of an AMI was facilitated by assaying serum levels of aspartate aminotransferase (AST), lactate dehydrogenase (LD), creatine kinase (CK), and LD and CK isoenzymes. Today the clinical usefulness of AST and LD has been replaced primarily by cardiac troponin and to a lesser degree by myoglobin, whereas CK isoenzymes continue to play a role. Although myoglobin will increase above the upper reference interval in 1-3 hours following AMI, it is not tissue specific for cardiac muscle and its application has found limited usefulness. Myoglobin will also be increased following skeletal muscle trauma. Troponin I and troponin T have proven to be useful markers, because each has a cardiac-specific isoform, cTnl and cTnT. cTnl appears to be more specific for cardiac muscle, because it has not been identified in regenerating or diseased skeletal muscle, whereas cTnT is made in small amounts by skeletal muscle. Total CK is elevated in AMI and takes 4-6 hours to rise above the upper reference interval. It is the increased level of CK-2 (CKMB) that is more helpful in diagnosing AMI, but caution needs to be exercised here, because skeletal muscle injury can cause a similar increase.

Which of the following statements correctly describes alkaline phosphatase? A. Decreased in Paget disease B. Decreased in third trimester of a normal pregnancy C. Increased in obstructive jaundice D. Primarily found in cardiac muscle

C. Increased in obstructive jaundice C. The main sources of alkaline phosphatase are liver, bone, intestine, and placenta. Elevated seaim alkaline phosphatase is associated with liver disease and with both obstructive jaundice and intrahepatic jaundice. In most cases, the serum alkaline phosphatase value in obstructive jaundice is higher than in intrahepatic jaundice. Increased serum values are also found in bone diseases such as Paget disease; in pregnant women (placental origin), especially in the third trimester of a normal pregnancy; and in normal growing children. In the presence of the latter conditions, when liver disease is also suspected, a GOT assay may be performed to aid in a differential diagnosis. Serum GOT levels are normal in these conditions but are elevated in liver disease.

Which of the following is a true statement concerning serum enzymes? A. The presence of hemolyzed red cells is of no significance for an accurate assay of most serum enzymes. B. Serum asparate transaminase (AST), but not serum lactate dehydrogenase (LD), is usually elevated in acute myocardial infarction. C. Increased serum alkaline phosphatase may be found in bone disease. D. Aspartate transaminase was formerly known as glutamate pyruvate transaminase.

C. Increased serum alkaline phosphatase may be found in bone disease C. Serum alkaline phosphatase is elevated in several disorders, including hepatobiliary and bone diseases. For an accurate assay of most serum enzymes, the presence of hemolyzed red blood cells must be avoided because many enzymes are present in red cells. Serum aspartate transaminase (formerly known as glutamate-oxaloacetate transaminase, GOT) and lactate dehydrogenase are both enzymes that are elevated in acute myocardial infarction and liver disease.

A 53-year-old female presents with fatigue, pruritus, and an enlarged, nontender liver. The physician orders a series of blood tests. Based on the following serum test results, what is the most likely diagnosis? Alkaline phosphatase—markedlyelevated Alanine aminotransferase—slightly elevated Lactate dehydrogenase—slightly elevated Gamma-glutamyltransferase— markedly elevated Total bilirubin—slightly elevated A. Alcoholic cirrhosis B. Infectious mononucleosis C. Intrahepatic cholestasis D. Viral hepatitis

C. Intrahepatic cholestasis C. Obstruction of the biliary tree is also referred to as intrahepatic cholestasis. This disorder is characterized by significant elevations in the serum levels of alkaline phosphatase and gamma-glutamyltransferase. The serum levels of alanine and aspartate aminotransferases and lactate dehydrogenase are only slightly elevated. Early in the disease, the serum bilirubin level may be normal or only slightly elevated. In alcoholic cirrhosis, viral hepatitis, and infectious mononucleosis, only a slight to moderate elevation of alkaline phosphatase would be seen.

Which of the following enzyme activities can be determined by using a dilute olive oil emulsion substrate, whose hydrolyzed product is monitored as a decrease in turbidity or light scatter? A. Alkaline phosphatase B. Amylase C. Lipase D. Trypsin

C. Lipase C. Lipase activity can be determined using a dilute olive oil emulsion as the substrate. The fatty micellar complexes absorb light as well as scatter light. Lipase catalyzes the hydrolysis of these triglyceride complexes, forming fatty acid and glycerol products. With the degradation of the micellar complexes, clearing of the reagent mixture occurs, causing changes in turbidity and light scatter. The rate at which the turbidity decreases can be monitored spectrophotometrically at 400 nm, or the decrease in light scatter can be measured using a nephelometer. The rate of these changes can be equated to the lipase activity present in the serum specimen.

Lactate dehydrogenase (LD) catalyzes the following reaction: Lactate+ NADH+ <-LD-> pyruvate + NADH As the reaction is written, which of the following techniques can be used to assess LD activity? A. Measure the colorimetric product pyruvate. B. Measure the colorimetric product NADH. C. Measure the increase in absorbance at 340 nm as NADH is produced. D. Measure the decrease in absorbance at 340 nm as NADH is produced.

C. Measure the increase in absorbance at 340 nm as NADH is produced. C. Enzymes catalyze specific reactions or closely related groups of reactions. Lactate dehydrogenase (LD), with nicotinamide adenine dinucleotide (NAD+) as a hydrogen acceptor, catalyzes the oxidation of L-lactate to pyruvate and the reduction of NAD+ to NADH. Because NAD+ does not absorb light at 340 nm but NADH does, the production of NADH can be monitored as an increase in absorbance at 340 nm and related to the LD activity present in the specimen. Because this reaction is reversible, either the forward or reverse reaction can be used in the laboratory to quantify LD activity. Although the reaction equilibrium favors the formation of lactate from pyruvate, this reaction is less commonly used. It should be noted that the reference ranges for the two reactions are considerably different. Elevation of serum LD is associated with acute myocardial infarction, liver disease, pernicious anemia, malignant disease, and pulmonary embolism. It is also seen in some cases of renal disease, especially where tubular necrosis or pyelonephritis exists

To what class of enzymes does lactate dehydrogenase belong? A. Isomerases B. Ligases C. Oxidoreductases D. Transferases

C. Oxidoreductases C. The oxidoreductases are enzymes that catalyze the addition or removal of hydrogen from compounds. These enzymes need a coenzyme, such as nicotinamide adenine dinucleotide (NAD+) or its phosphorylated derivative NADP+, as a hydrogen acceptor or donor in order to function. Lactate dehydrogenase and glucose-6-phosphate dehydrogenase are examples of oxidoreductases. Isomerases are those enzymes that catalyze intramolecular conversions such as the oxidation of a functional group by an adjacent group within the same molecule. Glucose phosphate isomerase is an example of this class of enzymes. Ligases are those enzymes that catalyze the union of two molecules accompanied by the breakdown of a phosphate bond in adenosine triphosphate (ATP) or a similar triphosphate. An example is glutamine synthetase.

Which statement best describes the clinical use of measuring NT-proBNP? A. Used to assess risk of coronary heart disease B. Used to assess risk of angina C. Used to assess individuals treated with nesiritide D. Used to assess individuals treated with vitamin B

C. Used to assess individuals treated with nesiritide C. N-terminal proBNP is released when BNP (Btype or brain natriuretic peptide) is cleaved from precursor proBNP. NT-proBNP is released in a 1:1 ratio to BNP; however, NT-proBNP has a half-life of hours as compared to BNP's short half-life of approximately 22 minutes. The longer half-life of NT-proBNP contributes to its clinical utility. In addition, when measuring NT-proBNP, there is no interference when an individual is being treated with nesiritide, which is human recombinant BNP used in treating congestive heart failure

Which of the following statements concerning creatine kinase is false? A. Rises within 4-6 hours after acute myocardial infarction B. Catalyzes the phosphorylation of creatine by ATP C. Requires Ca2+ for activity D. Found mainly in skeletal and cardiac muscles and in brain tissue

C. Requires Ca2+ for activity C. Creatine kinase (CK) is found mainly in skeletal muscle, cardiac muscle, and brain tissue. CK catalyzes the following reversible reaction: pH9.0 creatine + adenosine triphosphate (ATP) < > pH 6.7 phosphocreatine+adenosine diphosphate (ADP) Mg2+ is required as an activator. The direction in which the reaction takes place, and hence the equilibrium point, depends on the pH. Measurement of CK activity is valuable in the early diagnosis of acute myocardial infarction. Its level rises 4 to 6 hours after infarction, reaches its peak at 12 to 24 hours, and returns to normal by the third day. In addition to quantifying total CK activity, electrophoresis may be performed to ascertain the presence of an MB band, which represents the heart tissue isoenzyme. Electrophoretically, the MB band moves to an intermediary position between the BB and the MM bands. The BB band travels fastest toward the anode and the MM band travels slowest, remaining in the gamma-globulin region. Electrophoretic separation of CK-MB has been widely replaced by immunologic methods that can be performed on automated instruments

What does an increase in the serum enzyme levels indicate? A. Decreased enzyme catabolism B. Accelerated enzyme production C. Tissue damage and necrosis D. Increased glomerular filtration rate

C. Tissue damage and necrosis C. The majority of serum enzymes that are of interest clinically are of intracellular origin. These enzymes function intracellularly, with only small amounts found in serum as a result of normal cellular turnover. Increased serum levels are due to tissue damage and necrosis, where the cells disintegrate and leak their contents into the blood. Thus, elevated serum levels of intracellular enzymes are used diagnostically to assess tissue damage

Which test, if elevated, would provide information about risk for developing coronary artery disease? A. Troponin B. CK-MB C. hs-CRP D. Myoglobin

C. hs-CRP C. C-reactive protein is an acute-phase reactant that is increased in the presence of inflammation. High-sensitivity C-reactive protein (hsCRP) refers to a sensitive method that is able to measure low levels of CRP in serum. One theory is that elevated levels of CRP contribute to the damage of arterial walls that precedes plaque formation. hs-CRP is considered a good predictor test for assessing cardiovascular risk. However, it is also elevated in other conditions, including infection, stress, and trauma. CK-MB, troponin, and myoglobin are tests used to assess if a myocardial infarction has occurred.

Given the following information for a ratereaction, calculate the activity of a serum specimen for alanine aminotransferase in international units per liter (IU/L). Time/ Absorbance 1 min / 1.104 2 min/ 1.025 3 min/ 0.950 4 min/ 0.873 Specimen volume = 20 uL Reagent volume = 3.0 mL Molar absorptivity for NADH at 340 nm = 6.22 x 10^3 L/mol-cm Light path = 1 cm A. 186 B. 198 C. 1857 D. 1869

D. 1869 D. An international unit (IU) is defined as the enzyme activity that catalyzes the conversion of 1 umol of substrate in 1 minute under standard conditions. For determination of enzyme activity when a rate method is employed, the following equation is used: [AA/min X total assay volume (mL) X I0^6 umol/mol] / [Absorptivity coefficient X light path (cm) X specimen volume (mL)] [0.077 X 3.02 mL X 10^6 umol/mol] / [6.22 X 10^3 L/mol • cm X 1 cm X 0.02 mL] = 1869 IU/L It is important to remember that the total assay volume includes the volume of reagent, diluent, and sample used in the particular assay and that the total assay volume and specimen volume should be expressed in the same units.

If elevated, which laboratory test would support a diagnosis of congestive heart failure? A. Homocysteine B. Troponin C. Albumin cobalt binding D. B-type natriuretic peptide

D. B-type natriuretic peptide D. Symptoms are sometimes nonspecific, making it difficult to diagnose congestive heart failure. B-type (brain) natriuretic peptide (BNP) is used to determine if physical symptoms are related to congestive heart failure. BNP is synthesized in and secreted by myocardial ventricles in response to ventricular volume expansion and pressure overload. An increase in BNP causes dilation of blood vessels and promotes sodium and water loss by the kidneys. This reduces fluid load on the heart in an attempt to improve cardiac function. Albumin cobalt binding is a test that measures ischemia-modified albumin, which is a marker for ischemic heart disease.

Which statement concerning gammaglutamyltransferase is false? A. Present in almost all cells of the body B. Elevated in liver and some pancreatic diseases C. Elevated in chronic alcoholism D. Elevated in bone disease

D. Elevated in bone disease D. Gamma-glutamyltransferase (GGT) catalyzes the transfer of gamma-glutamyl groups from peptides to an appropriate acceptor. GGT is found in almost all cells. The highest amount of GGT is found in the kidney, and slightly less is found in the liver and pancreas. Diagnostically, the assay of GGT is widely used to investigate hepatic disease. Increased values are seen in a variety of liver disorders and in conditions that are characterized by secondary liver involvement, including acute pancreatitis, pancreatic carcinoma, infectious mononucleosis, alcoholism, and cardiac insufficiency. Normal GGT levels are seen in bone disorders, in growing children, and during pregnancy.

A 10-year-old female presents with varicella. The child has been experiencing fever, nausea, vomiting, lethargy, and disorientation. A diagnosis of Reye syndrome is determined. Which of the following laboratory results is not consistent with the diagnosis? A. Elevated serum AST B. Elevated serum ALT C. Elevated plasma ammonia D. Elevated serum bilirubin

D. Elevated serum bilirubin D. Reye syndrome is associated with viral infections, exogenous toxins, and salicylate use. The disorder generally manifests itself in children from 2 to 13 years of age. The laboratory findings that support a diagnosis of Reye syndrome include increased levels of serum aspartate and alanine transaminases (greater than 3 times the reference range), increased plasma ammonia level (can exceed 100 (jig/dL), and prolonged prothrombin time (3 sec or more than the control). In Reye syndrome the serum bilirubin level is generally within the reference range.

Which of the following is false about cardiac troponin I (cTnl) as it relates to AMI? A. Increase above reference interval seen in 3 to 6 hours B. Measure initially and serially in 3- to 6-hour intervals C. Remains elevated 5 to 10 days D. Expressed in regenerating and diseased skeletal muscle and cardiac muscle disorders

D. Expressed in regenerating and diseased skeletal muscle and cardiac muscle disorders D. Troponin is a group of three proteins that function in muscle contraction by binding to the thin filaments of cardiac and skeletal striated muscle. The three proteins are known as troponin T (TnT), troponin I (Tnl), and troponin C (TnC). With AMI, the cardiac-specific isoforms of troponin are released into the blood; the two of clinical interest are cTnl and cTnT. Cardiac troponin I (cTnl) will show an increase that exceeds the reference interval in approximately 3-6 hours following an AMI. Quantification should be done serially starting with an initial measurement at presentation followed by testing at 3-6 hours, 6-9 hours, and 12-24 hours. cTnl will remain elevated for 5-10 days. Unlike cTnT, which is expressed in small quantities in regenerating and diseased skeletal muscle, cTnl is not, which makes it specific for cardiac muscle.

What abbreviation has been used in the past to designate alanine aminotransf erase? A. AST B. A AT C. GOT D. GPT

D. GPT D. Alanine aminotransferase (ALT), formerly known as glutamate pyruvate transaminase (GPT), and aspartate aminotransferase (AST), formerly known as glutamate oxaloacetate transaminase (GOT), are categorized as transferase enzymes. These older designations are still seen in conjunction with the current terminology on reagent packaging, on physician test request forms, and on laboratory test result forms. Through the transfer of amino groups, ALT and AST catalyze the interconversion of amino acids and keto acids. ALT catalyzes the interconversion of alanine and oxoglutarate to pyruvate and glutamate. The reaction is reversible. In viral hepatitis, both ALT and AST are elevated. In acute myocardial infarction, AST is elevated and ALT is normal or slightly increased. L-Alanine + a-oxoglutarate ALT P-5'-P pyruvate + L-glutamate

Which of the following disorders is not characterized by an elevated serum myoglobin? A. Renal failure B. Vigorous exercise C. Acute myocardial infarction D. Hepatitis

D. Hepatitis D. The heme protein myoglobin can bind oxygen reversibly and is found in cardiac and striated muscles. In cases of acute myocardial infarction, myoglobin increases within 1-3 hours of the infarct. Myoglobin is not cardiac specific, and increased serum levels also occur in vigorous exercise, intramuscular injections, rhabdomyolysis, and muscular dystrophy. Because myoglobin is a relatively small protein and able to be excreted by the kidneys, elevated serum levels occur in renal failure.

Which of the following is not characteristic of cystic fibrosis? A. Decreased bicarbonate concentration in duodenal fluid B. Decreased lipase activity in duodenal fluid C. Decreased amylase activity in duodenal fluid D. Increased trypsin in feces

D. Increased trypsin in feces D. Cystic fibrosis is inherited as an autosomal recessive trait. It is a systemic disease that affects the exocrine glands, causing gastrointestinal malabsorption, pancreatic insufficiency, and pulmonary disease. Cystic fibrosis is characterized by increased concentrations of chloride and sodium in sweat. With pancreatic insufficiency, the amount of lipase, amylase, trypsin, and bicarbonate secreted into the duodenum is decreased. Because the three enzymes contribute to digestion of fats, starches, and proteins, respectively, children with this disorder suffer from malabsorption.

When measuring CK-MB, which of the following would provide the most sensitive method? A. Electrophoretic B. Colorimetric C. Kinetic D. Mass immunoassay

D. Mass immunoassay D. When measuring CK-MB, the mass immunoassay is more sensitive because it is quantifying the amount of enzyme present. This is in contrast to a kinetic method, which measures enzyme activity by means of the enzyme catalyzing a reaction and the product of that reaction being measured. Electrophoretic methods also measure enzyme activity based on colored product or fluorescent product formation.

A physician orders several laboratory tests on a 55-year-old male patient who is complaining of pain, stiffness, fatigue, and headaches. Based on the following serum test results, what is the most likely diagnosis? Alkaline phosphatase—significantly increased Gamma-glutamyltransferase—normal A. Biliary obstruction B. Cirrhosis C. Hepatitis D. Osteitis deformans

D. Osteitis deformans D. Osteitis deformans, also known as Paget disease, is a chronic disorder of bone. This disorder is characterized by a significant increase in the serum alkaline phosphatase level. Gammaglutamyltransferase will be normal in bone disease, because this enzyme is not found in bone tissue. However, in hepatobiliary disease both enzymes would characteristically be elevated.

Enzymes that catalyze the transfer of groups between compounds are classified as belonging to which enzyme class? A. Hydrolases B. Eyases C. Oxidoreductases D. Transferases

D. Transferases D. There are six major classes of enzymes. The International Commission of Enzymes of the International Union of Biochemistry has categorized all enzymes into one of these classes: oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases. Transferases are enzymes that catalyze the transfer of groups, such as amino and phosphate groups, between compounds. Transferases frequently need coenzymes, such as pyridoxal-5'-phosphate (P-5-P), for the amino transfer reactions. Aspartate and alanine aminotransferases, creatine kinase, and gammaglutamyltransferase are typical examples.