PRIMARY or congenital cystathioninuria was first described as an inborn error of methionine metabolism in 1959 by Harris et al.1 These workers demonstrated that the conversion of cystathionine to cysteine did not proceed in the normal manner. A plausible mechanism for this defect suggested by Frimpter2,3 was a diminished affinity of the enzyme cystathionase for its coenzyme pyridoxal phosphate. The few patients found to have primary cystathioninuria to date are listed in Table 1.
Secondary cystathioninuria resulting from vitamin B6 deficiency has been observed by Scriver et al.8 Cystathioninuria secondary to other disorders also has been encountered by Gjessing in patients with neuroblastoma,9-14 argentaffinoma,9 hepatoblastoma,15 and following administration of thyroxine.16
Studies of a girl with primary cystathioninuria, who incidentally also has phenylketonuria, are described in the present communication.* Preliminary observations on excessive excretion of cystathionine in patients with other specific disorders also are