Glycine Decarboxylation in Mitochondria Isolated from Spinach Leaves

Abstract

Mitochondria isolated from spinach leaves contain at least two glycine decarboxylating systems. One system is stimulated by ADP and evidently couples to the electron transport chain. The other system, three times as active, is stimulated by NAD+ and oxaloacetate and is not coupled directly to electron transport; however, comparative studies with uncouplers and inhibitors indicate it may depend on a membrane potential generated by electron transport. In this system, the role of oxaloacetate appears to be the regeneration of NAD+, via mitochondrial malate dehydrogenase, as an electron acceptor during glycine decarboxylation. Mitochondria isolated from spinach leaves also catalyse a rapid glycine-dependent exchange of bicarbonate into acid-stable products. This reaction is stimulated by the addition of lipoamide dehydrogenase. The activity of the glycine decarboxylation and exchange reactions are irreversibly lost when mitochondria are broken. When corrections are applied to account for mitochondrial breakage, the rates of glycine decarboxylation and the exchange reaction are comparable to the rates of CO*2 evolution from leaves of C*3 plants in air. The role of these processes in vivo and relationship to other sources of CO*2 in the glycollate pathway are discussed.