A rare disease due to abnormal absorption and excretion of tryptophan and other amino acids, characterized clinically by rash and CNS abnormalities. Hartnup disease is inherited as an autosomal recessive trait. Consanguinity is common.
Heterozygotes are normal.
Small intestine malabsorption of tryptophan, phenylalanine, methionine, and other monoaminomonocarboxylic amino acids occurs. Accumulation of unabsorbed amino acids in the GI tract increases their metabolism by bacterial flora. Some tryptophan degradation products including indoles, kynurenine, and serotonin are absorbed by the bowel and appear in the urine. Renal amino acid resorption is also defective, causing a generalized aminoaciduria involving all neutral amino acids except proline and hydroxyproline. Conversion of tryptophan to niacinamide is also defective.