In this paper, the relationship between adenylate kinase and apyrase activities in controlling the adenylate pool of pea stem mitochondria was examined. Non-purified mitochondria possess an adenylate kinase, localised on the outer surface of the inner membrane, and an apyrase on the surface of the outer membrane. These organelles, energised by succinate, exhibit a cyanide-insensitive ATP synthesis, which is inhibited by P1,P5-di(adenosine-5′)pentaphosphate (Ap5A) and carboxyatractylate (CAtr). This synthesis can also be followed either as AMP-induced dissipation of the electrical potential, that is reversed by CAtr and prevented by Ap5A, or as state 4 respiration which is partially inhibited by Ap5A and CAtr. However, when Percoll-purified mitochondria are used, state 4 respiration is insensitive to both inhibitors. Commercial apyrase exogenously supplied to purified mitochondria (which do not have apyrase activity) restores Ap5A- and CAtr-sensitive state 4 oxygen consumption. These results can be interpreted as follows. Apyrase, probably of cytoskeleton or endoplasmic reticulum origin and still adhering to non-purified mitochondria, produces AMP by hydrolysing ADP. AMP, acting as co-substrate with endogenous ATP, is utilised by adenylate kinase (reverse reaction) to generate ADP which, once entered the mitochondrial matrix, is available for ATP synthase.
