Folate-responsive homocystinuria and "schizophrenia". A defect in methylation due to deficient 5,10-methylenetetrahydrofolate reductase activity.

Homocystinuria and homocystinemia without hypermthioninemia, but with reccurent episodes of folate responseive schizophrenic-like behavior, was documented in a mildly retarded adolescent girl who lacked the habitus associated with cystathionine synthase deficiency. Enzymes involved in homocysteine-methionine metabolism were demonstrated to be normal. A defect in the ability to reducte N-5-10--methylenetetrahydrofolate to 5-methyltetrahydrofolate was demonstrated. Methylenetetrahydrofolate reductase was 18 per cent of control values. Methyltetrahydrofolate is used for the methylation of homocysteine to methionine, and a deficiency of this compound could explain the homocystinemia and homocystinuria.