Regulation of mitochondrial glycine decarboxylase from pea mitochondria

Reactions of glycine cleavage were assayed in mitochondria isolated from cotyledons of germinating pea seeds. These reactions, which included the exchange of bicarbonate with C-1 of glycine and an NAD-stimulated decarboxylation of glycine, were maximal under aerobic conditions at pH 7·8. The apparent Michaelis-Menten constants for glycine and bicarbonate in the exchange reaction were 1·8 and 12·5 mM respectively. The K_m for NAD in the decarboxylation reaction was 47 μM. Maximal enzyme activity was observed when mitochon-drial integrity was maintained. Up to 40% inhibition of the decarboxylation reaction was observed when NADH, NADPH or l-methionine were added to the reaction system. When glycine-2-¹⁴C] was incubated with the isolated mitochondria, labelled CO₂ was evolved in nanomolar quantities. It is concluded that glycine decarboxylase may be of importance in supplying C-1 units for the de novo synthesis of methionine in pea mitochondria.