Distribution of sulfur in urine of patients with cystathioninuria before and during administration of pyridoxine.

During administration of 400 mg daily of vitamin B6 as pyridoxine hydrocholoride to two patients with homozygous cystathioninuria, cystathionine excretion fell in a characteristic manner. Inorganic sulfate excretion did not change significantly when considered by itself. Total sulfur excretion, determined as sulfate after oxidation, decreased slightly in both patients and this decrease was statistically significant in one.

Renal clearance of cystathionine in homozygous and heterozygous cystathioninuria, cystinuria, and the normal state.

Cystathionine is more readily cleared from the plasma than other amino acids. This is because the amino acid has a very low tubular maximum (Tm), in the order of 1 mumole per minute per 1.73 square meters body surface area (BSA).

Cystathioninuria: nature of the defect.

A homogenate of liver obtained by biopsy from two patients with cystathioninuria, an inborn error of metabolism, cleaved radioactive cystathionine only slightly until an excess of pyridoxal phosphate was added. The apoenzyme failed to bind the coenzyme normally. Pyridoxine therapy of familial cystathioninuria thus has a sound basis.