ADP-REGENERATING ENZYME-SYSTEMS IN MITOCHONDRIA OF GUINEA-PIG MYOMETRIUM AND HEART

Any enzyme or enzyme system that produces ADP in proximity to the mito chondria may be capable of stimulating respiration. Hexokinase (HK), a denylate kinase (AK), and mitochondrial creatine kinase (Mi-CK) all ca talyze reactions that produce ADP and thus may play a role in cellular nucleotide metabolism or control of mitochondrial oxidative phosphory lation. Respiratory characteristics and enzyme activities of mitochond ria simultaneously isolated from heart and uterus of the gravid guinea pig were compared. The abilities of AMP, glucose, and creatine to sti mulate mitochondrial respiration via AK, HK, and Mi-CK systems, respec tively, were examined. Although the uterine Mi-CK activity is low comp ared with the values found in heart, the activities of HK and AK were significantly greater. Furthermore, the abilities of HK and AK to stim ulate respiration (functional activity) were greater in the uterine mi tochondria. Indeed, the activity of AK was sufficient to generate maxi mal(state 3) respiration. The apparent Michaelis constant (K-m) for AD P to stimulate respiration in the isolated uterine mitochondria was si gnificantly different from that of the heart mitochondria (9.6 +/- 0.9 and 5.1 +/- 1 mu M ADP, respectively). It is concluded that uterine m itochondria can use HK and AK systems in addition to the CX system in enhancing local ADP concentration, which may aid in the mitochondrial responses to energetic demands.