Studies of the Glycine Metabolism in a Patient with D-Glyceric Acidemia and Hyperglycinemia

Summary: A mentally retarded boy exhibiting both hyper-D-glyceric acidemia and hyperglycinemia and in whom a deficiency of D-glycerate dehydrogenase had previously been demonstrated was investigated to elucidate the ethiology of the glycine accumulation and its relationship to the D-glyceric acid accumulation. It was found that a positive correlation existed between excretion of D-glyceric acid and glycine (coefficient of correlation: r = 0.62, P < 0.001), that part of the IV injected [14C]glycine was metabolized to D-glyceric acid whereas no [14C]glyceric acid was metabolized to glycine, and that the in vivo degradation of IV injected [14C]glycine to 14CO2was diminished. Measurement of glycine cleavage activity in autoptic liver tissue from the patient showed only 10% of normal activity. It is argued that this diminished activity could be caused by an endogenous inhibitor, D-glyceric acid is demonstrated not to possess such an inhibitory effect. Based on the finding of increased urinary excretion of both free and conjugated isobutyric acid, 2-methylbutyric acid, and isovaleric acid, it is hypothesized that the diminished glycine cleavage activity might be due, at least partially, to inhibition by 2-methylbutyryl-CoA and isobutyryl-CoA, two compounds that are known to inhibit the glycine cleavage system.Speculation: The deranged branched-chain amino acid metabolism demonstrated in this patient is. like the hyperglycinemia, considered to be a secondary phenomenon, possibly caused by the D-glyceric acidemia.