Homocystinuria due to cystathionine synthase deficiency: the effect of pyridoxine.

We investigated the effect of pyridoxine administration in three patients with homocystinuria due to cystathionine synthase deficiency. The drug decreased the plasma concentration and urinary excretion of methionine and homocystine and the urinary excretion of homolanthionine and the homocysteine-cysteine mixed disulfide. Urinary cystine rose somewhat. Oral methionine tolerance tests before and during the patients' response to pyridoxine indicated that during response they remained deficient in their capacity to convert the sulfur of methionine to inorganic sulfate, although this capacity increased somewhat. During pyridoxine response only, the methionine loads caused increased homocystinuria. There was no indication that pyridoxine stimulated an alternate pathway of metabolism. The values for specific activity of cystathionine synthase in liver biopsy specimens from two patients in pyridoxine response were 3 and 4% of the mean control value. When these patients were not receiving pyridoxine, comparable values were 2 and 1%, respectively. The hepatic enzyme activity of the mutant patients was similar to normal enzyme activity with respect to trypsin activation, heat inactivation, and stabilization by pyridoxal phosphate. Approximate estimates were made of the relation between total body capacity to metabolize methionine and hepatic cystathionine synthase activity. These estimates suggested that because of the large normal reserve capacity of cystathionine synthase, a few per cent residual activity is sufficient to metabolize the normal dietary load of methionine. Thus, small increases in residual capacity may be of major physiological importance. However, many liver biopsies would be required to establish unequivocally that such changes were due to the administration of a particular therapeutic agent rather than to biological variation. All the data in the present study are consistent with the interpretation that pyridoxine does act by causing an increase in residual cystathionine synthase activity.