Metabolism of Purines, Pyrimidines, and Gout: Exhaustive Study Guide

Nucleoprotein Composition: Nucleoproteins are essential constituents found in every living cell.
Nucleoside Definition: A nucleoside consists of a Nitrogenous base joined to a Ribose sugar.
Nucleotide Definition: A nucleotide consists of a Nitrogenous base, a Ribose sugar, and a phosphate group.

Building Blocks: Nucleotides serve as the primary assembly units for nucleic acids, specifically DNA and RNA.
Energy and Transport: They are integral to energy storage, muscle contraction, active transport mechanisms, and the maintenance of ion gradients.
Activated Intermediates: They function as activated intermediates in various biosynthetic processes; examples include: - UDP-glucose - S-adenosylmethionine
Coenzyme Components: Nucleotides are structural components of critical coenzymes, including: - $NAD^+$ - $NADP^+$ - $FAD$ - $FMN$ - $CoA$
Metabolic Regulators: They acting in several regulatory capacities: - Second Messengers: Such as $cAMP$ and $cGMP$ . - Signal Transduction: ATP serves as a phosphate donor in signal transduction pathways. - Enzyme Regulation: Regulation of specific enzymes occurs via processes of adenylation and uridylylation.

Synthesis Sites: While purines are synthesized by most tissues, the liver is the major site of production. The subcellular site for this synthesis is the cytoplasm.
Pathways of Synthesis: - De novo Synthesis: This is the major pathway, involving the synthesis of purine nucleotides from various small molecules derived as intermediates from multiple metabolic pathways. - Salvage Pathway: This is a minor pathway used for recycling.
De novo Synthesis Process: - Purines are synthesized step-by-step using 5-PhosphoRibose (R-5-P) as the starting material. - PRPP (5-Phosphoribosyl-1 Pyrophosphate): Serves as the active donor of R-5-P. - IMP (Inosine-5'-Monophosphate): This is the first purine nucleotide biosynthesized in the De novo pathway. It contains Hypoxanthine as its nitrogenous base. - Precursor Role: IMP is the precursor for both AMP and GMP; it is converted into these two nucleotides.
Regulation: Purine nucleotide biosynthesis is regulated through Feedback inhibition.

Formation of Free Purines: Free purines (adenine, guanine, and hypoxanthine) are produced during the normal turnover of nucleic acids and are also obtained from dietary sources.
Pathway Mechanism: The salvage pathway converts these free purines back into their corresponding nucleotides.
Key Enzymes: - Adenine phosphoribosyl transferase: Catalyzes the formation of AMP from adenine. - Hypoxanthine-guanine phosphoribosyl transferase (HGPRT): Converts guanine to GMP and hypoxanthine to IMP.
Ribose Donor: Phosphoribosyl pyrophosphate (PRPP) acts as the donor of ribose 5-phosphate in this pathway.
Tissue Importance: The salvage pathway is particularly vital in tissues where De novo synthesis is not operative, such as the brain and erythrocytes.

Uric Acid Overview: Uric acid is the final end product of purine metabolism in the human body.
Normal Serum Concentrations: - Adults generally range from $3-7\,mg/dl$ . - Women typically have levels approximately $1\,mg$ lower than men.
Daily Excretion: The body excretes approximately $500-700\,mg$ of uric acid daily.
Hyperuricemia: Defined as an elevation in serum uric acid concentration. It is sometimes associated with Uricosuria (increased uric acid excretion).
Pathophysiology of Gout: - Gout is a metabolic disease linked to the overproduction of uric acid. - At physiological pH, uric acid exists in a more soluble form as sodium urate. - Crystal Deposition: In severe hyperuricemia, sodium urate crystals precipitate and deposit in soft tissues and joints. - Tophi: These deposits are known as tophi and cause joint inflammation, leading to painful acute gouty arthritis. - Progression: Untreated, it can progress to chronic gouty arthritis, urolithiasis, and renal damage.

Primary Gout: - Metabolic Causes: - Abnormal PRPP glutamylamidotransferase that is active but insensitive to feedback control. - Variant forms of PRPP synthetase that are not subject to allosteric control. - Deficiency of salvage pathway enzymes, specifically HGPRT deficiency (leading to Lesch-Nyhan syndrome). - Renal Causes: Failure in the excretion of uric acid.
Secondary Gout: - Overproduction of Uric Acid: Linked to enhanced nucleic acid turnover rates. - Increased tissue turnover (e.g., psoriasis). - Rapidly growing malignant tissues (e.g., leukemias). - Increased tissue breakdown following treatment for large tumor masses via radiation or chemotherapy. - Reduced Excretion of Uric Acid: - Increased alcohol consumption leading to lactic acidosis; lactic acid inhibits the excretion of uric acid. - Thiazide diuretics, which inhibit the tubular secretion of uric acid. - Renal failure.
Other Related Conditions: Von Gierke's disease, which involves elevated glutathione reductase.

General Management: - Low purine diet. - Restriction of alcohol.
Pharmacological Treatment: - Uricosuric Drugs: Probenecid, salicylates, halofenate. - Enzyme Inhibitors: Allopurinol.
Palliative Treatment: - Anti-inflammatory drugs: Colchicine, indomethacin, ibuprofen. - Steroids.
Lesch-Nyhan Syndrome: - Inheritance: X-linked recessive (only males affected). - Enzyme Defect: Deficiency of HGPRT in the salvage pathway. - Mechanism: Decreased salvage rate leads to accumulation of intracellular PRPP and a decrease in inhibitory nucleotides (GMP and IMP), causing increased production and degradation of purines. - Clinical Features: Excess uric acid, nephrolithiasis, self-mutilation, and neurological abnormalities (mental retardation, aggressive behavior, learning disabilities). The brain's dependence on the salvage pathway likely causes neurological symptoms.
Immunodeficiency Disorders: - Adenosine deaminase (ADA) deficiency: Inherited as autosomal recessive; affects both T-cells and B-cells. - Purine nucleoside phosphorylase deficiency: Inherited as autosomal recessive; affects T-cells while B-cells remain normal.

Biosynthesis of Pyrimidine Ribonucleotides: - Simpler process than purine synthesis. - Precursors: Aspartate, glutamine, and $CO_2$ contribute atoms to the pyrimidine ring. - Ring Assembly: The pyrimidine ring (a heterocyclic ring) is synthesized first and then attached to ribose 5-phosphate. This differs from purine synthesis where the ring is built upon the ribose. - Step 1: Synthesis of Carbamoyl Phosphate.
Degradation of Pyrimidine Nucleotides: - Involves dephosphorylation, deamination, and cleavage of glycosidic bonds to liberate nitrogenous bases (cytosine, uracil, thymine). - Bases degrade into highly soluble products: $\beta$ -alanine and $\beta$ -aminoisobutyrate. - These amino acids undergo transamination to produce acetyl CoA and succinyl CoA.
Orotic Aciduria: - Type I: Deficiency of Orotate phosphoribosyl transferase and OMP-decarboxylase. - Type II: Rare; deficiency of only OMP decarboxylase. - Characteristics: Autosomal recessive; results in excessive production (UMP typically inhibits OMP decarboxylase). - Symptoms: Anemia (affecting rapidly growing cells), retarded growth, and urinary obstruction due to crystal excretion. - Treatment: Both types respond to uridine, which is converted to UTP and acts as a feedback inhibitor.