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Nonketotic Hyperglycinemia: An in vitro Study of the Glycine-serine Conversion in Liver of Three Patients and the Effect of Dietary Methionine

Extract: Liver biopsies were performed on three patients suffering from the nonketotic form of hyperglycinemia. Direct assay of the enzymes involved in glycine-serine conversion revealed that the enzymic defect underlying this disease was due to nonfunction or absence of a glycine cleavage system. This enzyme system catalyzes the conversion of glycine to hydroxymethyltetrahydrofolic acid, carbon dioxide, and ammonia. In patients with hyperglycinemia, serine to glycine conversion is normal and patients depend on this metabolic conversion for their supply of C1 units. As glycine formed in this reaction cannot participate as a C1-unit donor, it cannot be metabolized further and consequently will accumulate.A therapeutic approach was devised to supply the organism with sufficient C1 units and was achieved by addition of methionine to a diet devoid of serine and glycine.As a consequence, the patient with hyperglycinemia does not rely so heavily on the glycine-serine conversion to fulfill his requirements for C1 units and glycine will not accumulate. Levels of glycine in plasma fell to normal values after feeding a diet with 0.3 g methionine/kg/24 h for a period of 10 days. It was demonstrated that the decrease in plasma glycine by administration of methionine was not due to increased excretion of glycine in the urine.Speculation: These studies have yielded evidence that the nonketotic form of hyperglycinemia is due to absence or nonfunction of the enzymic system responsible for the conversion of glycine to CO2, NH3, and hydroxymethyltetrahydrofolic acid. To meet the requirements of the organism for C1 units the conversion of serine to glycine proceeds at an accelerated rate. This can be compensated by addition of methionine to the diet, which serves as an alternate C1-unit donor. Patients with hyperglycinemia that do not respond to serine- and glycine-free diets may respond to diets rich in methionine.