Regulation of the glycine cleavage system in the isolated perfused rat liver.

The catabolism of glycine in the isolated perfused rat liver was investigated by measuring the production of 14CO2 from [1-14C]- and [2-14C]glycine. Production of 14CO2 from [1-14C]glycine was maximal as the perfusate glycine concentration approached 10 mM and exhibited a maximal activity of 125 nmol of 14CO2 X g-1 X min-1 and an apparent Km of approximately 2 mM. Production of 14CO2 from [2-14C]glycine was much lower, approaching a maximal activity of approximately 40 nmol of 14CO2 X g-1 X min-1 at a perfusate glycine concentration of 10 mM, with an apparent Km of approximately 2.5 mM. Washout kinetic experiments with [1-14C]glycine exhibited a single half-time of 14CO2 disappearance, indicating one metabolic pool from which the observed 14CO2 production is derived. These results indicate that the glycine cleavage system is the predominant catabolic fate of glycine in the perfused rat liver and that production of 14CO2 from [1-14C]glycine is an effective monitor of metabolic flux through this system. Metabolic flux through the glycine cleavage system in the perfused rat liver was inhibited by processes which lead to reduction of the mitochondrial NAD(H) redox couple. Infusion of beta-hydroxybutyrate or octanoate inhibited 14CO2 production from [1-14C]glycine by 33 and 50%, respectively. Alternatively, infusion of acetoacetate stimulated glycine decarboxylation slightly and completely reversed the inhibition of 14CO2 production by octanoate. Metabolic conditions which are known to cause a large consumption of mitochondrial NADPH (e.g. ureogenesis from ammonia) stimulated glycine decarboxylation by the perfused rat liver. Infusion of pyruvate and ammonium chloride stimulated production of 14CO2 from [1-14C]glycine more than 2-fold. Lactate plus ammonium chloride was equally as effective in stimulating glycine decarboxylation by the perfused rat liver, while alanine plus ammonium chloride was ineffective in stimulating 14CO2 production.