BIOCHEM BLOCK 4

When he was finally able to tolerate a full diet, Al Martini's stools became bulky, glistening, yellow-brown, and foul smelling. They floated on the surface of the toilet water. What caused this problem?

"-Al Martini's stool changes are characteristic of steatorrhea (fat-laden stools caused by malabsorption of dietary fats), in this case caused by a lack of pancreatic secretions, particularly pancreatic lipase, which normally digests dietary fat. -Steatorrhea also may be caused by insufficient production or secretion of bile salts. Therefore, Michael Sichel might also develop this condition."

What does lipoprotein lipase do with chylomicrons and VLDL?

LPL converts chylomicrons to chylomicron remnants and VLDL to intermediate density lipoprotein (IDL). These products, which have a relatively low triacylglycerol content, are taken up by the liver by the process of endocytosis and degraded by lysosomal action. IDL may also be converted to low density lipoprotein (LDL) by further digestion of triacylglycerol. Endocytosis of LDL occurs in peripheral tissues as well as the liver (Table VI.1), and is the major means of cholesterol transport and delivery to peripheral tissues.

What is the action of lipoproteins in the blood?

Lipoproteins carry TG and cholesterol esters in blood circulation to other tissues.

PLs are hydrolyzed to?

Lisophospholipids

LPL (lipoprotein lipase)

Metabolism of chylomicron into chylomicron remnants and VLDL into VLDL remnants

Short chain FFA require?

NO emulsification (4-12C)

HDL

Retrieves cholesterol from tissues; "good cholesterol"

Pancreatic lipase + colipase?

TG --> FFA + MG

They are synthesized from the same gene

The apoproteins B-48 and B-100 are similar with respect to what?

High-density lipoproteins

The conversion of nascent chylomicrons to mature chylomicrons requires what?

What is CKK?

The hormone cholecystokinin secreted by the intestine signals the gallbladder to contract and release bile acids, and the pancreas to release digestive enzymes.

What do eneterocytes do with absorbed fatty acids?

The intestinal epithelial cells will resynthesize triacylglycerol from free fatty acids and 2-monacylglycerol and will package them with a protein, apolipoprotein B-48, phospholipids, and cholesterol esters into a soluble lipoprotein particle known as a chylomicron. The chylomicrons are secreted into the lymph and even- tually end up in the circulation, where they can distribute dietary lipids to all tis- suesofthebody.

Where is ApoB48 located?

The intestines

Where is ApoB100 located?

The liver

What is the principle function of HDL?

The principal function of high density lipoprotein (HDL) is to transport excess cholesterol obtained from peripheral tissues to the liver and to exchange proteins and lipids with chylomicrons and VLDL. The protein exchange converts "nascent" particles to "mature" particles.

Elevated triglyceride levels

Type III hyperlipidemia is caused by a deficiency of apoprotein E. Analysis of the serum of patients with this disorder would exhibit what?

Triglyceride

What is the most abundant compound of chylomicrons?

HDL/LDL/VLDL/chylomicrons

What sequence places the lipoproteins in the order of most dense to least dense?

ApoC-II

Which apoprotein acts as a cofactor activator of the enzyme lipoprotein lipase (LPL)?

Lymphatic Transport(Blockage) release of chylomicrons is blocked

Whipples disease

The oversupply of cholesterol activates?

ACAT, which converts cholesterol to CE, suing Fatty Acyl CoA (monosaturated) and forms droplets.

What does the LDL receptor bind to? (recognizes specifically)

Apo B100

Lecithin-Cholesterol Acyl Transferase (LCAT) is activated by?

ApoA (from HDL)

What does the Chylomicron remnant receptor recognize?

ApoB48 and ApoE

What activates LPL?

ApoC-II

What inhibits the interaction between LDL receptors and ApoB100 on the LDL?

ApoE in chylomicrons

Statins (e.g Lipitor)

Decrease isoprenoid intermediates, which inhibit eNOS.

Chylomicrons are released to lymphatic system and distributed to tissues via?

Bloodstream

What is the core of LDL and HDL?

Cholesterol ester

Cholesterol Ester Transfer Protein (CETP) transfers?

Cholesterol esters from HDL to VLDL; also TG from VLDL to HDL

LCAT's Lecithin component comes from?

Cholesterol primarily from extrahepatic cell membranes

LCAT converts?

Cholesterol to Cholesterol Ester (Lecithin: Phosphatidylcholine --> Lysolecithin)

Exchange of apoproteins using HDL occurs with?

Chylo, VLDL, and LDL

Dietary lipids: TG packed with phospholipids, cholesterol, cholesterol esters and apoprotein B48 to form?

Chylomicrons (lipoprotein)

Liprotein Lipase (LPL)

Converts TG of lipoproteins to free fatty acids and glycerol. Bound to capillary walls of adipose cells by heparan sulfate.

What is the structure of serum lipoproteins?

Core of hydrophobic lipids (TG, cholesterol, esters), surface of phospholipid monolayer, free cholesterol and apoproteins, and stable non-covalent hydrophobic interactions.

Patient showing absence of chylomicrons, LDL, and VLDL would be expected to have?

Defect in the ability to express ApoB100 gene.

LDL

Degradative product of VLDL

Synthesis of HDL is stimulated by?

Dietary fat, estrogens, exercise, and alcohol

Chylomicrons are what kind of lipoprotein?

Digested lipoprotein

ApoB48 is involved in ___ (block I haunts you.

From same gene as ApoB100. Durning transcription process, you have RNA editing to convert C to to U, creating a stop codon. (Shorter version of same gene)

ACAT is stimulated by?

Elevated intracellular cholesterol

After LPL hydrolyzes TG, what happens to the products?

Fatty acids - adipose cells (reactivated to Fatty AcylCoA --> TG storage), and glycerol returned to liver (make more TG or fed to glycolytic pathway)

Which lipoprotein is primarily (40%) protein, 25% cholesterol, 5% TG, and 30& PL?

HDL

Describe HDL formation and metabolism

HDL is released from the liver (contains ApoC, ApoA, and ApoE). It encounters nascent chylomicrons and donates ApoC and ApoE. It encounters VLDL, donates ApoC and ApoE again. Comes in contact with cell membranes, and these contain cholesterol. You can recycle the cholesterol from cell membranes is transferred to HDL facilitated by ABC-1 transporter. LCAT activated by ApoA takes surface cholesterol from HDL and converts it to CE (hydrophobic core). When it encounters VLDL, and exchange of TG for CE, assisted by CETP, and PL moves from VLDL to HDL. HDL is recognized and bound to SR-B1 in steroidogenic cells or liver.

Decreasing isoprenoid intermediates by inhibiting HMGCoA reductase leads to?

Increase in NO, and vasodilation

What are some side effects of an increase in cholesterol concentration?

Inhibition of HMGCoA synthase, so you stop synthesis of cholesterol. You also inhibit synthesis of LDL receptors.

AcylCoA Cholesterol Acyl Transferase (ACAT) is found?

Inside the cells (not serum)

What stimulates LDL synthesis?

Insulin, thyroxin, and steroids

Where do you produce chylomicrons?

Intestinal mucosal cell

What is the function of serum lipoproteins?

Intravascular transport of TG and cholesterol. Contain cell-targeting signals

HDL is the site of the ____ reaction, which is activated by _____.

LCAT, ApoA

Which lipoprotein is 50% cholesterol/cholesterol ester, 22% PL, and 20% protein?

LDL

LDLs transport cholesterol esters of Hepatic and Intravascular origin. How are these synthesized/broken down?

LDL is produced from VLDL metabolism. LDL binds to the receptor, forms a coated vesicle and enters into the endosome. Ensodome recycles most of B100 receptors back to surface. The rest of ApoB100 is broken down in lysosome. CE (core) is broken down to free cholesterol, release FA and Amino acids. Increase in cholesterol concentration.

Serum lipoproteins are macromolecular complexes of?

Lipids and apoproteins

Phospholipids, TG, and cholesterol are transported as?

Lipoproteins

VLDL

Lipoproteins that endogenously synthesize TG from liver, peripheral tissue

Scavenger Receptor Type B Class 1 (SR-B1) is primarily in?

Liver and steriodogenic tissue

Inside the intestinal mucosal cell, what happens during chylomicron metabolism?

MAG + FFA reform TG, TG+c + ApoB48 + PL are packaged into large macromolecular complex called chylomicron. Sent out to lymph, into the bloodstream. In the blood, it picks up more Apoproteins (ApoE & ApoC from HDL). ApoC enables it to bind and activate the lipoprotein lipase in the capillary bed near the adipose tissue. Activated lipoprotein lipase: TG hydrolyzed to FFA-albumin + Glycerol + FA to adipose. After most of the TG have been depleated, the remnant chylomicron is taken up by ApoB48:E receptors in liver cells.

Bile salts, lysophosphatidylcholine, FFA all form?

Mixed micelles

Dietary TGs are broken down to FFA and monoglycerides by?

Pancreatic lipase/colipase

Why is HDL "good" cholesterol?

Take cholesterol from cell membranes and move it back to liver.

What does SR-B1 do?

Recognizes HDL particle's apoproteins

What does the Chylomicron remnant receptor do in macrophages?

Recognizes damaged LDL

HDL's main fucntion is?

Return of peripheral tissue cholesterol to liver, source of exchangeable apoproteins

Which lipoprotein has a 60% TG, 20% cholesterol, and 15% PL, and only 5% protein?

VLDL

Free fatty acids are transported by?

Serum Albumin

Cell receptors for lipoproteins bind?

Specific apoproteins to begin endocytosis

Chylomicron remnant receptor is?

Specifically in the liver

Chylomicrons are predominantly made of?

TG

What is the core of chylomicrons?

TG

What is the core of VLDL?

TG (some cholesterol esters)

Endogenously synthesized TG (VLDL) production

TG+CE+ApoB100 --> VLDL complex. VLDL sent out to blood, pick up ApoE and ApoC. ApoC activates LPL, hydrolyze TG --> FFA (taken up by adipose). Remove TG, VLDL remnants released and continue in blood. Some continue to liver to be taken up by ApoE receptors. Other remnants acted on by hepatic lipase, removes more TG and the intermediate density lipoproteins (IDL), are converted to LDL. Take back ApoE and ApoC-II. ApoB100 is left, and CE in the core of the LDL, binds to LDL receptors on tissue or liver. Taken to lysosome and broken down.

LDL is formed from?

VLDL and IDL by the action of lipoprotein lipase

Cholesterol

These are digested by cholesterol esterase

What is the function of chylomicrons?

Transport dietary lipids

HDL participates in the exchange of CE for TG & cholesterol in?

VLDL and LDL

HDL

Transport protein by which we take cholesterol from peripheral tissues to liver

What is the Difference between ApoB48 and ApoB100?

"-ApoB100 (liver) and ApoB48 (intestine) are encoded by the same ApoB gene. -In intestine, an RNA editing enzyme converted a CAA to UAA (stop codon) in mRNA. -This shortened RNA transcript encodes ApoB48, which is 48% of the size of ApoB100. ApoB48 does not have LDL receptor binding site, which is located in the C-terminal region of ApoB100. Therefore, chylomicrons can not be taken up by LDL receptor."

What is ApoCII?

"-Apolipoprotein C2 or Apolipoprotein C-II is a protein that in humans is encoded by the APOC2 gene. -The protein encoded by this gene is secreted in plasma where it is a component of very low density lipoproteins and chylomicrons. -This protein activates the enzyme lipoprotein lipase in capillaries,[1] which hydrolyzes triglycerides and thus provides free fatty acids for cells. -Mutations in this gene cause hyperlipoproteinemia characterized by xanthomas, pancreatitis -Lab tests will show elevated blood levels of triglycerides, cholesterol, and chylomicrons[2]"

What fat pathway can acetyl-CoA go down?

"-Becomes cholesterol Cholesterol can go to: 1. Membranes;2. Bile Salts;3. Steroid Hormones"

Describe the action of pancreatic lipase.

"-Bile acids activate pancreatic lipases. -Colipases increase lipase activity to digest triglycerides to Monoacylglycerol and free fatty acids (Fig 32.4). Cholesterol esterases hydrolyze cholesterol esters to free cholesterol and fatty acids (Fig 32.5.A). -Phospholipids are hydrolyzed by Phospholipase A2"

Describe the transport of lipids in the blood

"-Chylomicrons (CM) are TG-rich. Nascent CMs are secreted into the lymphatic system and enter the blood through the thoracic duct. Nascent CMs receive ApoCII and ApoE from HDL in blood to form mature CM. -ApoE is recognized by membrane receptors, LDL receptor-related proteins (LRPs). -ApoCII activates lipoprotein lipases (LPL) located on the surface of capillary endothelial cells in muscle and adipocytes, and digest TG to release free fatty acids from CM and VLDL."

What proteins make up chylomicrons, and what proteins to chylomicrons interact with?

"-Chylomicrons contain ApoB48, which is synthesized in the intestine. -CM's accept ApoCII and ApoE from HDL. -ApoE is recognized by LDLR related protein (LRPs). -ApoCII activates lipoprotein lipase in muscle and adipocytes to release fatty acids from TG."

Describe the action of bile salts.

"-Dietary fat leaves the stomach and enters the small intestine, where it is emulsified by bile acids. -Bile acids are synthesized in the liver, stored in the gallbladder, and secreted into the intestinal lumen. Bile salts are derived from cholesterol -Bile salts act as detergents, binding to the globules of dietary fat. The emulsified fat is attached by digestive enzymes from the pancreas."

How is cholesterol transported in the body?

"-Endocytosis of LDL occurs in peripheral tissues as well as the liver , and is the major means of cholesterol transport and delivery to peripheral tissues. -LDL comes from degraded intermediate density lipoproteins"

What is the major route for triglyceride degradation?

"-The main route for degradation of triglycerides involves hydrolysis to fatty acids and 2- monoacylglycerol in the lumen of the intestine. -Limited digestion of these lipids occurs in the mouth (lingual lipase) and stomach (gastric lipase). These lipases preferentially hydrolyze short- and medium-chain fatty acids."

Summarize the digestion of triacyglycerols

"-Triacylglycerols are the major fat in the human diet, consisting of three fatty acids esterified to a glycerol backbone. -Limited digestion of these lipids occurs in the mouth (lingual lipase) and stomach (gastric lipase) because of the low solubility of the substrate. In the intestine, however, the fats are emulsified by bile salts that are released from the gallbladder. This increases the available surface area of the lipids for pancreatic lipase and colipase to bind and to digest the triglycerides. -Degradation products are free fatty acids and 2-monoacylglycerol. -When partially digested food enters the intestine, the hormone cholecystokinin is secreted by the intestine, which signals the gallbladder to contract and release bile acids, and the pancreas to release digestive enzymes."

How are triglycerides transported?

"-Triglycerides are transported in lipoprotein particles. -Intestinal cells package TG together with proteins and phospholipids in chylomicrons. Chylomicrons contain 85% TG, Apo B-48, cholesterol esters (CE) and phospholipids (PL), and also fat-soluble vitamins."

Where are triglycerides and their transporting proteins synthesized?

"-Triglycerides produced in the smooth endoplasmic reticulum (SER) of intestinal epithelial cells. - ApoB48 is synthesized in the rough ER (RER). -Chylomicrons are assembled in both ER and Golgi complex, and secreted into lymph."

What is VLDL?

"-Very low density lipoprotein (VLDL) is produced in the liver, mainly from dietary carbohydrate. Lipogenesis is an insulin-stimulated process through which glucose is converted to fatty acids, which are subsequently esterified to glycerol to form the triacylglycerols that are packaged in VLDL and secreted from the liver. -Thus, chylomicrons primarily transport dietary lipids, and VLDL transports endogenously synthesized lipids."

What are the roles of Apolipoproteins?

"1) Provide structural Stability 2) Serve as enzyme cofactors 3) serve as receptor ligands"

What is the purpose of cholesterol?

"1. Cholesterol stabilizes the phospholipid bilayer of the membranes 2. Precursor of bile acids, physiological detergents 3. Precursor of steroid hormones"

What are some common lipids in the body?

"1. Fatty acids and triglycerides 2. Glycerophospholipids and sphingolipids 3. Eicosanoids 4. Cholesterol, bile acids and steroid hormones 5. Fat-soluble vitamins"

What pathways can Arachidonic acid (or EPA) go down?

"1. Leukotrienes 2. Prostaglandins 3. Thromboxanes"

What pathways can Fatty Acids go down?

"1. Oxidation (CO2+H2O) 2. Synthesis of Triacyglycerol (adipose tissue) 3. Synthesis of Phospholipids and sphingolipids"

What happens to the fatty acids released from the Chylmicron?

"The fatty acids Enter the muscle for energy production or the fat cell for storage. The glycerol released is metabolized only in the liver"

Describe the absorption of dietary lipids.

-Bile acids emulsify fatty acids, monoacylglycerol to form micelles. Bile salts in the intestinal reach 5 to 15 mM (critical micelle concentration, CMC) to form micelles. Other dietary lipids, such as cholesterol, lysophospholipids, and fat-soluble vitamins, are also packaged in micelles. Lipids in micelles are absorbed in the microvilli on the surface of the intestinal epithelial cells, where triglycerides are resynthesized from fatty acids and monoacylglycerol.

Describe the enterohepatic circulation of bile acids.

-Enterohepatic circulation of bile acids: Bile acids are quantitatively reabsorbed (95%) in the ileum, and circulated via portal vein to liver, secreted into the bile for storage in the gallbladder, and ejected into the intestinal lumen. Bile acids are recycled 6 to 7 times a day.

What are Triacylglycerols?

-Triglycerides are the major fat in the human diet. They consist of three fatty acids esterified to a glycerol backbone.

What is ApoE?

APOE [2] is 299 amino acids long and transports lipoproteins, fat-soluble vitamins, and cholesterol into the lymph system and then into the blood. It is synthesized principally in the liver, but has also been found in other tissues such as the brain, kidneys, and spleen. In the nervous system, non-neuronal cell types, most notably astroglia and microglia, are the primary producers of APOE, while neurons preferentially express the receptors for APOE. There are seven currently identified mammalian receptors for APOE which belong to the evolutionarily conserved low density lipoprotein receptor gene family.

ApoCII

Activates lipoprotein lipase (LPL), which cleaves free fatty acids from triglycerides on the chylomicrons, releasing them for uptake by peripheral cells; found on chylomicrons, VLDL, and HDL

Inadequate Chylomicron formation

Alpha Beta - lipoproteinemia/ Hypo-betalipopoteinemia

What is ApoB?

Apolipoprotein B (APOB or ApoB) are the primary apolipoproteins of chylomicrons and low-density lipoproteins (LDL - known commonly by the misnomer "bad cholesterol" when in reference to heart disease), which is responsible for carrying cholesterol to tissues

What are apolipoproteins?

Apolipoproteins are proteins that bind lipids (oil-soluble substances such as fat and cholesterol) to form lipoproteins. They transport the lipids through the lymphatic and circulatory systems.

ApoCII Activates lipoprotein lipase

As the chylomicron loses triaglyceride, its density INCREASES and it becomes a chylomicron remnant.

Describe the degradation of a chylomicron

As the chylomicron loses triglyceride, its density increases and it becomes a chylomicron remnant, which is taken up by the liver by receptors that recognize apolipoprotein E. In the liver, the chylomicron remnant is degraded into its component parts for further disposition by the liver.

Familial hypercholesterolemia

Autosomal dominant mutations in the LDL-R; can be caused by a defect in ApoB100; most frequent cause of hyperlipidemia; characterized by xanthomas, marked elevation of LDL cholesterol, and predisposition to heart attacks at an early age

Deconjugation of Bile Acids

Bacterial Overgrowth syndrome

Pancreas secretes?

Bicarbonate and several lipases

What is the fate of bile acids?

Bile acids are reabsorbed in the ileum and recirculated back to liver via portal blood.

Cholecystokinin gives the signal to release?

Bile from gallbladder

Bile contains?

Bile salts and phosphatidylcholine (emulsifiers)

ApoB100

Binds LDL receptors; found on VLDL and LDL; indicated in familial hypercholesterolemia

ApoE

Binds to the LRP receptor on hepatocytes to return the remnant chylomicron to the liver

What happens to a chlyomicron after it is used?

CMs are rapidly cleared in 2-3 hours after a meal.

Chylomicron remnants

Chylomicrons become this after transferring triglycerides

What is the difference between VLDL and chylomicrons?

Chylomicrons primarily transport dietary lipids, and VLDL transports endogenously synthesized lipids.

What is the end usage of lipids that are transported to muscle, adipocytes?

Chylomicrons transport dietary lipids to muscle for energy metabolism and adipocytes for storage of TG.

Defects in Pancreatic enzyme Synthesis and Secretion?

Cystic Fibrosis or Chronic Pancreatis

LDL

Delivers cholesterol to tissues; "bad cholesterol"

VLDL

Delivers endogenously synthesized triglycerides from the liver to peripheral tissues

Bile acts a?

Detergent to emulsify lipids for digestion and absorption (speeds up the process)

Where are dietary fats absorbed?

Dietary fats and nutrients are absorbed in the intestine. Bile acids facilitate fat absorption.

How are dietary fats transported?

Dietary fats are assembled to form chylomicrons, which transport TG and CE to liver, muscle and adipocytes for storage and energy metabolism.

What fatty acid changes happen during fasting?

During fasting, fatty acids and glycerol are released from adipose triacylglycerol stores (Fig. VI.5). The glycerol travels to the liver and is used for gluconeogenesis. Only the liver contains glycerol kinase, which is required for glycerol metabolism. The fatty acids form complexes with albumin in the blood and are taken up by muscle, kidney, and other tissues, where ATP is generated by their oxidation to CO2 and water. Liver also converts some of the carbon to ketone bodies, which are released into the blood. Ketone bodies are oxidized for energy in muscle, kidney, and other tissues during fasting, and in the brain during prolonged starvation (see Chapter 23).

ApoB48

Found on chylomicrons and chylomicron remnants

ApoAI

Found on the HDL; activates LCAT, which causes HDL to unload cholesterol on an IDL, forming an LDL

Triglycerides are hydrolyzed to?

Free FA and monoglycerides

HL (hepatic lipase)

Helps convert IDL to LDL

Bile Acid Deficiency due to interruped enterohepatic circulation?

Ileal resection or dysfunction, biliary obstructction

Describe the digestion of phospholipids and cholesterol

In addition to triacylglycerols, phospholipids, cholesterol,, and cholesterol esters (cholesterol esterified to fatty acids) are present in the foods we eat. Phos- pholipids are hydrolyzed in the intestinal lumen by phospholipase A2, and cholesterol esters are hydrolyzed by cholesterol esterase. Both of these enzymes are secreted from the pancreas.

What is the action of bile salts?

In the intestine, bile salts emulsify fats and facilitate absorption of dietary fats, vitamins and cholesterol into intestinal cells.

How do enterocytes aid with the transport and packaging of dietary lipids?

Intestinal epithelial cells resynthesizes TG and package them to nascent chylomicrons for release into the circulation.

Emulsification is required for absorption by?

Intestinal mucosal cells

Gastric lipase is important in infants due to?

Neutral pH of stomach

Cholesterol, free FA, and vitamins are?

Not modified

Describe the conversion of a nascent chylomicron to a mature chylomicron

Once in circulation, the newly released ("nascent") chylomicrons interact with another lipoprotein particle, HDL (high-density lipoprotein) and acquire two apoproteins from HDL, apoprotein CII and E. This converts the nascent chylomi- cron to a "mature" chylomicron. The apoCII on the mature chylomicron activates the enzyme lipoprotein lipase (LPL), which is located on the inner surface of the capillary endothelial cells of muscle and adipose tissue. The LPL digests the triglyceride in the chylomicron, producing free fatty acids and glycerol. The fatty acids enter the adjacent organs either for energy production (muscle) or fat stor- age (adipocyte). The glycerol that is released is metabolized in the liver.

Secretin gives the signal to release?

Pancreatic contents

LDL

Produced in blood (remnant of IDL after triacylglycerol digestion; end product of VLDL); contains high concentration of cholesterol and cholesterol esters; endocytosed by liver and peripheral tissues

IDL

Produced in blood (remnant of VLDL after triacylglycerol digestion); endocytosed by liver or converted to LDL

Chylomicrons

Produced in intestinal epithelial cells from dietary fat; carry triacylglycerol in blood

HDL

Produced in liver and intestine; exchanges proteins and lipids with other lipoproteins; functions in the return of cholesterol from peripheral tissues to the liver

VLDL

Produced in the liver mainly from dietary carbohydrate; carry triacylglycerol in blood

ApoAI

Promotes cholesterol efflux from cells; found on HDL

Discuss the formation of micelles

The products of enzymatic digestion (free fatty acids, glycerol, lysophospholipids, cholesterol) form micelles with bile acids in the intestinal lumen. The micelles interact with the enterocyte membrane and allow diffusion of the lipid soluble components across the enterocyte membrane into the cell. The bile acids, however, do not enter the enterocyte at this time. They remain in the intestine, travel further down, and are then reabsorbed and sent back to the liver by the enterohepatic circulation. This allows the bile salts to be used multiple times in fat digestion.

What removes TAG's from chylomicrons and VLDL?

The triacylglycerols of chylomicrons and VLDL are digested by LIPOPROTEIN LIPASE (LPL), an enzyme found attached to capillary endothelial cells (see Fig. VI.4). The fatty acids that are released are taken up by muscle and many other tis- sues and oxidized to CO2 and water to produce energy (see Chapter 23). After a meal, these fatty acids are taken up by adipose tissue and stored as triacylglycerols.

Triglycerides

These are digested by lingual lipase, gastric lipase, pancreatic lipase and colpiase, bile salts; degradation products are mostly free fatty acids and 2-monoacylglycerol

Phospholipids

These are digested by phospholipase A2

Lymphatics

This is the pathway that chylomicrons take to enter into the bloodstream

Cholesterol from dead cells

Transported by HDL

Cholesterol synthesized in liver

Transported by LDL

Triacylglycerols synthesized in liver

Transported by VLDL

Fatty acids from storage in adipose tissue

Transported by albumin

Lipids in diet

Transported by chylomicrons

What are chylomicrons?

Triglycerides (TG) are re-synthesized in intestinal epithelial cells and packaged in chylomicrons (CM) and secreted into the lymph.