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Hyperglycinemia is an error of amino acid metabolism in which there are increased amounts of glycine and normal amounts of other amino acids in blood, urine, and cerebrospinal fluid. There are a t least two different types of hyperglycinernia. We recently reported on the metabolism of glycine in nonketotic hyperglycinernia [I]. A defect was found in the formation of14C02 from glycine-1-14C and in the conversion of the carbon 2 of glycine to the carbon 3 of serine. These findings were consistent with a defect in a n enzyme system catalyzing the formation of CO, and hydroxymethyltetrahydrofolate from glycine, but did not exclude a defect in glycine oxidase. It was decided, therefore, to assess the metabolism of glyoxylate in nonketotic hyperglycinernia.T h e conversion of glyoxylate-1-14C to 14C02 ( fig. 1) was similar to that of glycine-1-14C. Oxidation to CO, was rapid in control subjects, but in the patient, a flat curve was obtained. The transfer of isotope to serine was slower in the patient than in the control subjects. No isotope was found in carbon 3 of serine in the patient, while a significant amount of the isotope of carbon 3 of glyoxylate was incorporated into carbon 3 of serine in the control subjects.The data obtained indicate that the pathway of preference for the metabolism of glyoxylate is transamination to glycine and rule out a defect in glycine oxidase in nonketotic hyperglycinernia. SpeculationThe data obtained in this investigation contribute to the hypothesis that the enzymic defect in nonketotic hyperglycinernia is situated in the reaction that forms CO,, NH, and F H 4 C H 2 0 H from glycine.This reaction is probably catalyzed by a n enzyme system with a number of component enzymes. Further investigations will be necessary in order to determine exactly the enzyme that is deficient.