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Gout, Lesch-Nyhan, Purine Metabolism, Vitamin D, B12, B9 & Antioxidant Vitamins

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1. GOUT ⭐⭐⭐ (Clinical)

Definition & Pathogenesis

• Overproduction — enzymatic defects in purine synthesis (e.g., partial HGPRT deficiency), tumor lysis syndrome
• Underexcretion (~90% of primary gout) — reduced renal excretion; urate is filtered, almost completely reabsorbed by the proximal tubule, then a fraction is secreted distally

Types

Mechanism of Acute Arthritis

1. MSU crystals precipitate in joints
2. Resident macrophages phagocytose crystals → activate the NLRP3 inflammasome → caspase-1 activation → IL-1β release
3. Massive neutrophil recruitment → cytokines, free radicals, proteases, lysosomal enzyme release
4. Results in acute arthritis → spontaneously remits in days to weeks
5. Repeated attacks → tophi (aggregates of urate + inflammatory tissue) in synovial membrane and periarticular tissue → cartilage damage

Clinical Features

• Podagra — classic first attack: hot, red, swollen 1st metatarsophalangeal joint (MTP joint)
• Attacks often nocturnal, precipitated by: high-purine meals, alcohol, dehydration, surgery, trauma
• Tophi — in pinna of ear, Achilles tendon, periarticular tissue (after 20–30 years of hyperuricemia)
• Uric acid nephrolithiasis — radiolucent stones
• Gouty nephropathy — urate deposition in renal interstitium

Stages of Gout

1. Asymptomatic hyperuricemia
2. Acute gouty arthritis (intermittent attacks)
3. Intercritical gout (asymptomatic between attacks)
4. Chronic tophaceous gout

Risk Factors

• Male sex, postmenopausal women
• Obesity, hypertension, metabolic syndrome
• Alcohol (especially beer — high in guanosine)
• Medications: diuretics (thiazides, furosemide), cyclosporine, low-dose aspirin
• Chronic renal disease

Investigations

• Serum uric acid >6.8 mg/dL
• Joint aspiration (gold standard): needle-shaped, negatively birefringent MSU crystals (yellow when parallel to compensator)
• X-ray: "punched-out" erosions with overhanging edges ("rat-bite erosions"), soft-tissue tophi
• 24h urinary uric acid to distinguish overproducers vs. underexcreters

Management

• Colchicine (first-line; inhibits microtubule polymerization → impairs neutrophil chemotaxis and inflammasome activation)
• NSAIDs (indomethacin preferred) — avoid in renal impairment
• Corticosteroids — if NSAIDs/colchicine contraindicated

• Target serum urate <6 mg/dL
• Allopurinol — xanthine oxidase inhibitor; start low (100 mg/day), titrate; avoid in azathioprine patients (or reduce AZA dose by 75%) because xanthine oxidase metabolizes azathioprine
• Febuxostat — non-purine selective xanthine oxidase inhibitor; 40–80 mg/day; no renal dose adjustment; ⚠️ higher CV mortality vs. allopurinol in high-risk patients
• Probenecid — uricosuric (blocks URAT1 renal transporter); avoid in nephrolithiasis
• Pegloticase — recombinant PEGylated uricase; converts uric acid → allantoin (soluble); for refractory gout; risk of antibody formation and infusion reactions
• Start ULT not during acute flare (can prolong attack); cover with colchicine prophylaxis when initiating

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2. LESCH-NYHAN SYNDROME ⭐⭐⭐ (Clinical)

Genetics & Enzyme Defect

• X-linked recessive (affects males; females are carriers)
• Gene: HPRT1 on chromosome Xq26–q27
• Enzyme deficiency: Hypoxanthine-Guanine Phosphoribosyltransferase (HGPRT/HPRT)
• Complete (or near-complete) absence of HGPRT

Pathophysiology

• HGPRT catalyzes the salvage of hypoxanthine → IMP and guanine → GMP using PRPP
• Without HGPRT:
  • Hypoxanthine is not recycled; instead degraded → xanthine → uric acid (overproduction hyperuricemia)
  • PRPP accumulates (no longer consumed by salvage) → drives de novo purine synthesis further → more uric acid
  • IMP and GMP levels fall → loss of feedback inhibition → amplified de novo synthesis
• CNS mechanism unclear but likely involves dopaminergic pathway dysfunction in basal ganglia

Clinical Features (onset ~3–6 months)

Diagnosis

• Clinical features + elevated uric acid
• HGPRT enzyme assay (in RBCs)
• HPRT1 gene mutation analysis — accurate for carriers and affected males

Management

• Allopurinol — controls hyperuricemia and prevents uric acid nephropathy, but does NOT improve neurological/behavioral symptoms
• Behavioral management (physical restraints to prevent self-harm)
• Baclofen, benzodiazepines for spasticity
• Fluphenazine — reported to suppress self-mutilation when haloperidol failed
• No cure for neurological manifestations

Key Distinction: Partial vs. Complete HGPRT Deficiency

• Partial deficiency (Kelley-Seegmiller syndrome): hyperuricemia + gout, but no neurological features, no self-mutilation
• Complete deficiency: full Lesch-Nyhan syndrome

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3. PURINE DE NOVO SYNTHESIS ⭐⭐⭐

Overview

• Synthesizes purine ring from scratch using small molecule precursors
• End product: IMP (inosine monophosphate)
• 11 steps, energetically expensive (multiple ATPs, glutamine, glycine, aspartate, folate derivatives consumed)
• Occurs primarily in the liver

Precursors of the Purine Ring

Key Steps

1. PRPP synthesis (ribose-5-phosphate + 2 ATP → PRPP) by PRPP synthetase
2. Glutamine donates amide N → 5-phosphoribosylamine (committed step, catalyzed by PRPP glutamyl amidotransferase)
3. Sequential additions of glycine, formyl-THF (C8), glutamine (N3), CO₂ (C6), aspartate (N1), formyl-THF (C2) → closes the ring → IMP
4. From IMP:
   • IMP → AMP: adenylosuccinate synthetase (GTP-dependent) → adenylosuccinate lyase
   • IMP → GMP: IMP dehydrogenase → GMP synthetase (ATP-dependent)
   • Cross-regulation: AMP synthesis requires GTP; GMP synthesis requires ATP (mutual balance)

Regulation

• Rate-limiting factor: concentration of PRPP
• PRPP synthetase inhibited by AMP, ADP, GMP, GDP (feedback)
• PRPP glutamyl amidotransferase inhibited by AMP and GMP (feedback)
• Multifunctional enzymes in eukaryotes facilitate channeling of intermediates

Pharmacological Inhibition (anticancer/antifolate drugs)

• Methotrexate, aminopterin — inhibit dihydrofolate reductase → deplete THF → block steps 4 (C8 addition) and 10 (C2 addition)
• Azaserine, DON (6-diazo-5-oxo-L-norleucine) — glutamine analogs → block steps 2, 5, 8
• Mycophenolate mofetil (MMF) — inhibits IMP dehydrogenase → blocks de novo GMP synthesis → selectively affects T and B lymphocytes (which lack efficient salvage pathway)

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4. PURINE SALVAGE PATHWAY ⭐⭐

Overview

• Recycles free purine bases (from diet or nucleic acid degradation) back into nucleotides
• Far less energy-intensive than de novo synthesis
• Particularly important in tissues with low de novo capacity: brain, RBCs, PMNs

Key Enzymes

Mechanism

Purine base + PRPP → Purine-5'-monophosphate + PPi
(phosphoribosylation)

Regulation Feedback

• Salvage-generated AMP, GMP, IMP feedback inhibit PRPP synthetase and PRPP glutamyl amidotransferase, suppressing de novo synthesis
• This is why HGPRT deficiency → ↑ free PRPP → ↑ de novo synthesis → ↑ uric acid

Clinical Relevance

• Lesch-Nyhan syndrome — HGPRT deficiency (see above)
• ADA deficiency (adenosine deaminase) — accumulation of dATP inhibits ribonucleotide reductase → severe combined immunodeficiency (SCID)
• PNP deficiency (purine nucleoside phosphorylase) — dGTP accumulates → T-cell deficiency, normal B cells
• Azathioprine — prodrug metabolized to 6-mercaptopurine → incorporated via salvage → inhibits de novo purine synthesis → immunosuppression

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5. VITAMIN D

Synthesis & Activation (Two-Step Hydroxylation)

7-Dehydrocholesterol (skin)
        ↓ UV light (290–320 nm)
Previtamin D3 → Vitamin D3 (cholecalciferol)  [enters dermal capillaries]
        ↓ Liver — 25-hydroxylase
25-hydroxyvitamin D [25(OH)D3] — storage form, measured in serum
        ↓ Kidney — 1α-hydroxylase (regulated by PTH, hypophosphatemia)
1,25-dihydroxyvitamin D3 = Calcitriol (active form)

• Dietary sources: fortified milk, fatty fish (salmon, sardines, tuna, cod), fish oil
• Ergocalciferol (D2) from plant sources joins the same pathway at the liver step

Actions of Calcitriol (1,25(OH)₂D₃)

• Intestine: ↑ calcium and phosphate absorption (nuclear receptor → transcription of calbindin)
• Bone: stimulates osteoclast-mediated bone resorption → releases Ca²⁺ and PO₄
• Kidney: ↑ tubular reabsorption of Ca²⁺ and phosphate
• Immune: activates macrophages via Toll-like receptor-triggered VDR expression; protective against TB

Deficiency — Rickets/Osteomalacia

• Children: Rickets — failure of bone mineralization, bowing of long bones, rachitic rosary, craniotabes
• Adults: Osteomalacia — bone pain, proximal myopathy, insufficiency fractures

Genetic Disorders of Vitamin D Metabolism

Emerging Associations

• ↓ vitamin D linked to: hypertension, ↑ fasting glucose/insulin, cardiovascular disease, hip fractures, colon cancer mortality, ↑ all-cause mortality

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6. IMPORTANT ANTIOXIDANT VITAMINS

Vitamin C (Ascorbic Acid)

• Water-soluble antioxidant
• Directly scavenges ROS (superoxide, hydroxyl radicals, singlet oxygen)
• Regenerates vitamin E (reduces tocopheroxyl radical back to tocopherol)
• Co-factor for prolyl and lysyl hydroxylase (collagen synthesis) → deficiency = scurvy
• Scurvy: perifollicular hemorrhages, corkscrew hairs, gingival bleeding/swelling, impaired wound healing

Vitamin E (Tocopherols/Tocotrienols)

• Fat-soluble antioxidant — major lipid-phase antioxidant
• α-tocopherol = most biologically active form
• Protects polyunsaturated fatty acids (PUFAs) in cell membranes from lipid peroxidation
• Mechanism: donates H· to lipid peroxy radical → chain-breaking antioxidant
• Deficiency: hemolytic anemia, spinocerebellar ataxia (in children with fat malabsorption), peripheral neuropathy
• Found in vegetable oils, nuts, seeds, green leafy vegetables

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7. VITAMIN B12 (COBALAMIN) ⭐⭐

Structure & Sources

• Cobalt-containing vitamin; animal products only (meat, liver, dairy, eggs)
• Plant-based diets → risk of deficiency without supplementation

Absorption

• Requires Intrinsic Factor (IF) — secreted by gastric parietal cells (Castle's intrinsic factor)
• IF-B12 complex → absorbed at terminal ileum
• Transported by transcobalamin II in plasma

Causes of Deficiency

• Pernicious anemia — autoimmune destruction of parietal cells → ↓ IF → B12 malabsorption
• Gastrectomy, ileal resection, Crohn's disease (terminal ileum)
• Strict veganism
• Drugs: metformin (↓ IF-B12 absorption), prolonged PPI use, nitrous oxide (oxidizes cobalamin)

Functions

1. Methylmalonyl-CoA mutase — converts methylmalonyl-CoA → succinyl-CoA (requires adenosylcobalamin)
   • Deficiency → ↑ methylmalonic acid (MMA) — marker of B12 deficiency
2. Methionine synthase — converts homocysteine → methionine (requires methylcobalamin); regenerates THF from methyl-THF ("methyl-folate trap")
   • Deficiency → ↑ homocysteine; impaired myelin synthesis

Clinical Features of Deficiency

• Hematologic: macrocytic megaloblastic anemia (impaired DNA synthesis in RBCs), hypersegmented neutrophils
• Neurological (subacute combined degeneration of spinal cord): demyelination of posterior columns (vibration/position sense loss) + lateral corticospinal tracts (UMN signs); peripheral neuropathy
• NB: Folate deficiency also causes megaloblastic anemia but NO neurological features

Diagnosis

• ↑ MCV; megaloblastic changes on blood smear
• ↓ Serum B12
• ↑ Serum methylmalonic acid (most sensitive) + ↑ homocysteine

Treatment

• IM cyanocobalamin or hydroxocobalamin (if absorption defective)
• Oral high-dose B12 (1000 µg/day) if absorption intact

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8. VITAMIN B9 (FOLATE) ⭐⭐

Sources & Absorption

• Liver, leafy green vegetables, legumes, wheat bran, fortified cereals
• Absorbed in jejunum (proximal small bowel)
• Body stores last only 3–4 months (unlike B12 which lasts years)

Active Form

• Tetrahydrofolate (THF) — carrier of single-carbon units
• Used in: purine synthesis (C2, C8 of purine ring), thymidylate synthesis (dUMP → dTMP), amino acid metabolism (serine ↔ glycine interconversion, homocysteine → methionine)

Causes of Deficiency

• Poor diet (alcoholism, poverty)
• Malabsorption: celiac disease, chronic diarrhea, total gastrectomy
• Antifolate drugs:
  • Methotrexate, trimethoprim, pyrimethamine (inhibit dihydrofolate reductase)
  • Oral contraceptives
  • Antiepileptics (phenobarbital, phenytoin — induce hepatic enzymes, deplete folate)
• Increased demand: pregnancy, hemolytic anemia, malignancy

Clinical Features

• Megaloblastic anemia + hypersegmented neutrophils (same as B12)
• Glossitis (smooth tongue), angular cheilitis, mucosal ulcers
• Brown hyperpigmentation in palmar creases and flexures
• No neurological symptoms (key distinction from B12 deficiency)
• Neural tube defects (spina bifida, anencephaly) — with periconceptional deficiency

Treatment

• Folic acid 1–5 mg/day orally
• ⚠️ Must rule out B12 deficiency first — folate supplementation corrects anemia but allows neurological damage from B12 deficiency to progress undetected ("masking")

Supplementation

• All women of childbearing age: 400 µg/day folic acid to prevent neural tube defects
• High-risk women (prior NTD, antiepileptic use): 4–5 mg/day

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Quick Summary Table

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