Cystathioninuria from pyridoxine deficiency complicating treatment of hypercalcaemia in a cretin.

Rats with vitamin B6 deficiency excrete cystathionine in the urine (Hope, 1957). Cystathionine is an amino acid in the pathway of synthesis of cysteine from methionine (Fig. 1). A methyl group is removed from methionine to produce homocysteine, which condenses with serine to yield cystathionine. This condensation is catalysed by a transulphurase enzyme requiring pyridoxal phosphate as a co-enzyme. Finally, cystathionine splits to yield cysteine and homoserine. This reaction is catalysed by cystathionase and also requires pyridoxal phosphate (White, Handler, and Smith, 1964). Pyridoxal and its phosphate are forms of vitamin B6, the others being pyridoxamine and its phosphate, and pyridoxine. Rats with vitamin B6