Homocystinuria associated with decreased methylenetetrahydrofolate reductase activity

Summary A new type of homocystinuria is described. A variety of evidence indicates that patients with this type of homocystinuria are not deficient in cystathionine synthase activity. Fibroblasts from these patients were unable to grow as rapidly as control fibroblasts if homocystine replaced methionine in the culture medium. N 5 -Methyltetrahydrofolate-homocysteine methyltransferase activity in these cells was not markedly decreased, whereas methylenetetrahydrofolate reductase activity was significantly below normal. A deficiency of this reductase activity can explain the biochemical abnormalities in these patients.