Folic acid coenzyme distribution patterns were examined in the liver and kidney of two patients with homocystinuria due to different inborn errors of metabolism affecting the remethylation of homocysteine to methionine. One patient, with severe mental retardation (and death at 3 1/2 yr), had greatly reduced levels of methylenetetrahydrofolic acid (THF) reductase in fibroblasts as well as in liver and kidney. Chromatographic separation of folate coenzymes in liver showed an abnormal pattern with THF as the main component and almost no methyl-THF but total folate was normal. The other patient, who was dystrophic, microcephalic, and had megaloblastic anemia died at age 4 months. He had reduced levels of methionine synthetase in liver and kidney due to a defect of intracellular cobalamin metabolism. Chromatographic analysis of his tissues showed methyl-THF to be the principal folate form and a markedly reduced total folate. These results support the "methyl-THF trap" hypothesis and offer information with respect to the possible therapy of these two disorders.

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