Amino acid transport disorders

AA Transport - The Cellular Doormen

• Mechanism: Primarily secondary active transport, co-transporting AAs with Na⁺. This process is driven by the Na⁺/K⁺-ATPase pump. These transporters are part of the Solute Carrier (SLC) gene family.
• Specificity: Transporters are often shared among groups of chemically similar amino acids.
  • Basic: Cysteine, Ornithine, Lysine, Arginine 📌 COLA
  • Neutral: Tryptophan & other neutral AAs
  • Acidic: Aspartate & Glutamate

⭐ Defects in these shared transporters cause aminoacidurias, where excess amino acids are excreted in urine (e.g., Cystinuria, Hartnup disease).

Cystinuria - COLA Stone Party

• Defect: Autosomal recessive mutation in SLC3A1 or SLC7A9 genes, leading to a defective dibasic amino acid transporter in the Proximal Convoluted Tubule (PCT) & intestine.
• Affected AAs (📌 COLA): Impaired reabsorption of Cystine, Ornithine, Lysine, and Arginine.

• Clinical Presentation: Recurrent nephrolithiasis in a young patient.
• Diagnosis:
  • Urinalysis: Pathognomonic hexagonal cystine crystals.
  • Test: Positive urinary cyanide-nitroprusside test.

⭐ Exam Favorite: While all four COLA amino acids are excreted in excess, only cystine is poorly soluble in urine (especially at low pH), leading to stone formation.

• Treatment:
  • Vigorous hydration (> 3L/day).
  • Urinary alkalinization (e.g., potassium citrate) to maintain urine pH > 7.5.
  • Chelating agents (e.g., penicillamine, tiopronin) for severe cases.

Hartnup Disease - Tryptophan Trouble

• Genetic Defect: Autosomal recessive mutation in the SLC6A19 gene, causing a defective neutral amino acid transporter (especially Tryptophan) in the proximal convoluted tubules and intestinal enterocytes.

• Pathophysiology: Impaired intestinal absorption and renal reabsorption of Tryptophan leads to ↓ Niacin (Vitamin B3) synthesis, resulting in Pellagra-like symptoms.

• Clinical Presentation: Symptoms are often intermittent and triggered by stress, illness, or poor diet.

  • 📌 The '3 D's': Dermatitis (photosensitive rash), Diarrhea, and Dementia (or ataxia/neuropsychiatric issues).

• Diagnosis: Generalized neutral aminoaciduria detected on urine chromatography. Plasma amino acid levels are typically normal.

• Treatment:

  • High-protein diet.
  • Niacin (nicotinamide) supplementation.

⭐ High-Yield: Despite significant aminoaciduria, most patients remain asymptomatic throughout life. Clinical manifestations are episodic and often related to nutritional status.

High‑Yield Points - ⚡ Biggest Takeaways

• Cystinuria is an AR disorder from a defective dibasic amino acid transporter (COLA) in the renal PCT.
• It classically presents with recurrent, radiopaque hexagonal cystine kidney stones from a young age.
• The sodium cyanide-nitroprusside test turns purple, confirming the diagnosis.
• Hartnup disease is an AR defect in neutral amino acid transport, primarily affecting tryptophan.
• ↓ Tryptophan leads to ↓ niacin (B3), causing pellagra-like symptoms (dementia, dermatitis, diarrhea).