4 - Purines

Depletion of ATP --->

--> significantly increased AMP AMP --> Hypoxanthine --> Xanthine -->Uric Acid --------> HYPERURICEMIA

PRPP is Important for...

... is a key intermediate both in the de novo and salvage pathways of purine and pyrimidine nucleotide biosynthesis .

Dietary Purines

...are not precursors for purine nucleotide synthesis in the DeNovo or Salvage pathways.

Regulation of DeNovo Purine Nucleotide Synthesis

1) PRPP stimulates the committed step (pace-setting also) - feed forward stimulation. 2.) AMP, GMP, IMP inhibit the committed step - feed back inhibition. 3.) AMP, GMP, IMP also inhibit PRPP synthetase (otherwise, you will have a mess!)

Origin of Purine Ring Atoms

Amine: Aspartate C2 and C8: Formate N3 and N9: Glutamine Amide N7: Glycine C6: HCO3-

Purine Metabolism

Breakdown of purines results in Xantine ---> goes to uric acid to be urinated out

APRT

Catalyzes Adenine ---> AMP

HGPRT (Hypoxanthine - Guanine Phosphoribosyl Transferase)

Catalyzes Hypoxanthine + PRPP --> IMP (IMP then -->--> AMP) Catalyzes Guanine --> GMP

Gouty Attack

Gout is caused by excess purine synthesis --> too much uric acid --> leads to monosodium urate crystal deposits. Joints - deposits cause inflammation Soft tissues - (deposits called tophi), results in chronic tophaceous gout Kidney - uric acid stones Deposits start as acute gout and then progress to chronic gouty arthritis if untreated Often affecting distal joins such as the toes, hands

Purines

Guanine Adenine

Lesch Nyhan Syndrome

HGPRT (also called HPRT) is absent. X-linked genetic disorder Excess PRPP and excess uric acid - treatable Neurological abnormalities; self-mutilation (cause unknown) - not treatable Poor prognosis - death by 2 years None of these severe neurological problems with APRT deficiency! (APRT = AMP only, HGPRT = AMP and GMP)

PRPP Accumulation Causes...

Hyperuricemia due to increased Purine Synthesis PRPP accumulation --> increased DeNovo Purine Nucleotide synthesis --> increased nucleotide breakdown (into free purine bases + ribose+Pi's) --> increased Purine base concentrations --> increased Uric Acid in urine

IMP --> GMP or AMP

IMP can be made into AMP or GMP IMP --> XMP --> GMP or IMP --> Adenylosuccinate --> AMP

IMP -> GMP

IMP is converted to XMP via the addition of Oxygen XMP uses one ATP and converts a Glutamine --> Glutamate (substrates) to produce GMP

IMP

IMP is the first intermediate purine nucleotide formed in the DeNovo pathway Inosine monophosphate, IMP, is Hypoxanthine ribose monophosphate

IMP --> AMP

IMP uses a GTP and adds Aspartate to produce Adenylosuccinate which is then converted to AMP.

AMP, GMP, IMP

Inhibit PRPP Synthetase which, by reducing PRPP, reduces PRPP activation of de novo Purine nucleotide synthesis Inhibit first step of de novo Purine nucleotide synthesis

DeNovo Pathway of Purine Nucleotide Synthesis

Most active in the Liver Cytosolic Pathway

Glutamine PRPP Amidotransferase Activators

PRPP

Purine Nucelotide Salvage Pathway

PRPP + Purine -> Purine Nucleotide

Metabolic Uses of Dietary Nucleic Acids

Phosphate and Ribose Sugar --> used in other metabolic pathways Free Nitrogenous base (ring) --> Xanthine --> Uric Acid for excretion

Nucleotides consist of...

Phosphate, Ribose Sugar, and Nitrogenous base (ring)

Gout Cause

Poor regulation of PRPP Synthetase --> increased [PRPP] or Deficiency of HGPRT/APRT Both lead to: Excessive Purine Synthesis--> Excessive URIC ACID production

Hypoxanthine

Purine nucleotide base that becomes the Nucleotide IMP in salvage pathway

Gout - Etiology (things that can cause/contribute to)

Purine rich foods Ethanol Drugs - Loop diuretics

Diuretics

Raise levels of uric acid in blood

Purine Degradation

Relevant in reperfusion Injury. During ischemia, purine degredation results in the production of hypoxanthine and xanthine, which are used by xanthine oxidase to create H2O2 in the tissues, which can cause tissue damage. [In ischemia, xanthine dehydrogenase is converted to xanthine oxidase]

Allopurinol (Zyloprim)

Structural analog of hypoxanthine used to treat gout through the inhibition of XO, reducing Uric Acid production.

PABA Analogs

Sulfa drugs, prevent bacteria from making nucleic acids Bacteria make their own Folate for THF. (Humans need dietary folate for THF production) Sulfa abx prevent bacteria from making Folate. No Folate = No THF = No Nucleic Acid Synthesis

PRPP Synthetase

Synthesizes PRPP (by adding PPi (from ATP) to a phosphoribose )--> activates de novo purine nucleotide synthesis via increased [PRPP]

Gout Treatment

Treat with allopurinol (Zyloprim is the trade name), a structural analog of hypoxanthine; Xanthine Oxidase (XO) converts allopurinol to alloxanthine, a suicide inhibitor (permanently binds to and inhibits) of XO. Modify food habits.

Phosphoribosylamine

formed in the first (committed) step of DeNovo purine nucleotide synthesis. First "intermediate" of IMP synthesis.

Purine rich foods

meat, kidney, liver, seafood, anchovies, oatmeal, certain vegetables (peas, beans, lentils, mushrooms, cauliflower, spinach), sweetbreads

THF (in DeNovo Purine Synthesis)

2 N10-Formyltetrahydrofolates are required for the formation of IMP in DeNovo Purine Nucleotide synthesis (N10-Formyltetrahydrofolates ---> THF) via Formyltransferase

PRPP --> IMP requires how many ATPs? (DeNovo Synthesis)

5 ATP's are spent

Glutamine PRPP Amino Transferase Inhibitors

AMP, GMP, IMP

PRPP

Activator and Substrate for Glutamine PRPP Amidotransferase (First step of DeNovo Purine Nucleotide synthesis)

Salvage Pathway of Purine Nucleotide Synthesis

Active in extrahepatic tissue - uses preformed bases (made by liver & transported by RBC) from surrounding medium or from nucleotide catabolism

Pancreatic Nucleases

Cleave dietary Nucleic Acids into Nucleotides in the small intestine.

Xanthine Oxidase (XO) Normal Function

Converts hypoxanthine --> xantine --> Uric Acid

HGPRT or APRT Deficiency

Deficiency leads to increased levels of PRPP (accumulation) --> increased Purine synthesis --> Hyperuricemia

Diagnosis of Gout

Definitive diagnosis only possible by aspirating and inspecting synovial fluid or tophaceous material and demonstrating MSU crystals. Polarized microscopy - the crystals appear as bright birefringent crystals that are yellow (negatively birefringent).

Hyperuricemia Causes

Depletion of ATP (increased AMP) Hypoxic conditions Ethanol Ingestion Glucose-6-phosphatase Deficiency Ingestion of Fructose Acidosis "Hyper Elephants Gluttonously Ingest Acid"

Fate of Dietary Nucleic Acids

Dietary Nucleic Acids are digested in the small intestine by Pancreatic Nucleases into Nucleotides --> Free Base. Then Xantine Oxidase in the Small Intestine mucosa converts them into Uric Acid, most of which is excreted in the urine and the rest of which is catabolized by bacteria in the large intestine.

Glutamine PRPP Amidotransferase

Enzyme for the first step of DeNovo Purine Nucleotide synthesis (committed step_ Converts 5'-PRPP --> 5'-phosphoribosylamine ACTIVATED by: PRPP INHIBITED by: AMP, GMP, IMP

Formyltransferase

Enzyme in DeNovo Purine Nucleotide synthesis that is used twice during the synthesis of IMP (from PRPP) that requires the reaction: N10-Formyl THF ---> THF

Alcoholic Acidosis

Ethanol --> Acetaldehyde --> Acetate --> Acetyl CoA + AMP Acetate buildup = Alcoholic Acidosis Can cause Hyperuricemia

Hyperuricemia - Alcohol

Ethanol --> Acetaldehyde --> Acetate --> Acetyl CoA + AMP Uses ATP --> AMP to create Acetyl CoA Results in Net increase of intracellular AMP ---> Hyperuricemia

Committed Step of DeNovo Purine Synthesis

First Step of de novo purine nucleotide synthesis is the Committed step. Gln-PRPP Amidotransferase converts 5'-PRPP to Phosphoribosylamine ACTIVATED by: PRPP INHIBITED by: AMP, GMP, IMP