Sulfur-containing amino acid metabolism

Methionine Metabolism - The Sulfur Start

• Essential amino acid Methionine is converted to S-adenosylmethionine (SAM), the universal methyl donor, in an ATP-dependent reaction.
• SAM donates its methyl group, becoming S-adenosylhomocysteine (SAH), then Homocysteine.
• Homocysteine (Hcy) is a critical branch point.
  • Remethylation: Hcy → Methionine. Requires vitamins B12 and Folate.
  • Transsulfuration: Hcy → Cysteine. Requires vitamin B6 (PLP).

⭐ Elevated homocysteine is an independent risk factor for thrombotic events (atherosclerosis, DVT) and ectopia lentis.

📌 Mnemonic: To get UP to Methionine, you need B12/Folate. To go OUT to Cysteine, you need B6.

Homocysteine's Crossroads - Recycle or Remove

Homocysteine (Hcy) is metabolized via two main pathways: remethylation (recycling) or transsulfuration (removal). The pathway choice depends on methionine levels.

• Remethylation (Recycle): Active when methionine is low.

  • Hcy is converted back to methionine by methionine synthase.
  • Requires Vitamin B₁₂ and Folate (B₉).

• Transsulfuration (Remove): Active when methionine/cysteine are high.

  • Hcy is irreversibly converted to cysteine.
  • Key enzyme: Cystathionine β-synthase (CBS).
  • Requires Vitamin B₆ (pyridoxine).

⭐ Classic homocystinuria is most commonly caused by a deficiency in Cystathionine β-synthase (CBS), leading to elevated homocysteine and methionine. Patients present with thromboembolic events, lens dislocation, and skeletal abnormalities.

Homocystinuria - A Sticky Situation

Autosomal recessive deficiency of cystathionine β-synthase (CBS), leading to ↑ homocysteine & methionine. Requires co-factor pyridoxal phosphate (B6).

• Clinical Features:
  • Ocular: Ectopia lentis (lens dislocation) → downward & inward.
  • Skeletal: Marfanoid habitus, osteoporosis, kyphosis.
  • Vascular: Thromboembolism (atherosclerosis, DVT, stroke) is a major cause of morbidity/mortality.
  • Neurologic: Intellectual disability, seizures.

⭐ Homocysteine is prothrombotic, causing endothelial damage and increasing the risk of premature atherosclerosis, venous thrombosis, and stroke.

• Diagnosis & Treatment:
  • Screening: ↑ Homocysteine in urine/plasma.
  • Treatment: High doses of Vitamin B6 (pyridoxine) for responsive forms. Supplement with folate and Vitamin B12. Betaine promotes remethylation of homocysteine to methionine.

Cystinuria - The COLA Problem

• Pathophysiology: Autosomal recessive defect in the dibasic amino acid transporter (SLC3A1/SLC7A9 genes) in the proximal convoluted tubule & intestine.
  • Impairs reabsorption of Cystine, Ornithine, Lysine, Arginine (📌 COLA).
• Clinical Manifestations: Recurrent hexagonal cystine kidney stones, often presenting in young adults. Cystine is poorly soluble in acidic urine.
• Diagnosis:
  • Urinalysis: Pathognomonic hexagonal crystals.
  • Positive sodium cyanide-nitroprusside test.
• Management:
  • ↑ Fluid intake (> 3L/day).
  • Urine alkalinization (e.g., potassium citrate).
  • Chelating agents (penicillamine, tiopronin) for severe cases.

⭐ Unlike radiolucent uric acid stones, cystine stones are radiopaque due to their sulfur content.

High‑Yield Points - ⚡ Biggest Takeaways

• Methionine generates S-adenosylmethionine (SAM), the universal methyl donor, and homocysteine.
• Homocystinuria (most often cystathionine β-synthase deficiency) causes marfanoid habitus, thromboembolism, and downward lens dislocation.
• Key cofactors: Vitamin B6 (pyridoxine) for the transsulfuration pathway and Vitamin B12/Folate for remethylation.
• Cystinuria, a transporter defect, leads to recurrent hexagonal cystine kidney stones.
• High homocysteine is a major risk factor for atherosclerosis.