KININASE II-TYPE ENZYMES - THEIR PUTATIVE ROLE IN MUSCLE ENERGY-METABOLISM

Because of the importance of bradykinin in improving heart function in some conditions or in enhancing glucose uptake by skeletal muscle, we investigated kininases in these tissues. In P-3 fraction of the heart and skeletal muscles, angiotensin I-converting enzyme (ACE) and neutr al endopeptidase 24.11 (NEP) are the major kininases, as determined fi rst with specific substrates and second with bradykinin. ACE activity was highest in guinea pig heart (2.7 +/- 0.07 mu mol . h(-1). mg prote in(-1)) but decreased in other species in this order: dog atrium, rat heart, dog ventricle, and human atrium. The specific activity of NEP w as lower: 0.45 mu mol . h(-1). mg protein(-1) in cultured neonatal car diac myocytes and varying between 0.12 and 0.05 mu mol . h(-1). mg pro tein(-1) in human, dog, rat, and guinea pig heart. In the skeletal mus cle P-3, ACE was most active in guinea pig and rat (1.2 and 1.1 mu mol . h(-1). mg protein(-1), respectively) but less so in dog (0.09 mu mo l . h(-1). mg protein(-1)). NEP activity was higher in dog P-3 (0.28 m u mol . h(-1). mg protein(-1)) but lower in rat and guinea pig (0.19 a nd 0.1 mu mol . h(-1). mg protein(-1), respectively). Continuous densi ty gradient centrifugation enriched NEP activity in dog and rat (from 0.3 to 1.0 and 0.49 mu mol . h(-1). mg protein(-1), respectively). Imm unoprecipitation with antiserum to purified NEP proved the specificity of the rat enzyme. Bradykinin (0.1 mmol/l) was inactivated in the pre sence and absence of inhibitors by rat skeletal muscle NEP, as measure d by high-performance liquid chromatography. Here, 36% of the activity was caused by NEP and 19% by ACE. In radioimmunoassay (bradykinin 10 nmol/l), 46 and 55% of kininase in rat and dog skeletal muscle P-3, re spectively, was due to ACE; 36 and 28%, respectively, was due to NEP. Aside from these enzymes, an aminopeptidase in rat P-3 also inactivate s bradykinin. Thus, in conclusion, heart and skeletal muscle membranes contain kininase II-type enzymes, but their, activity depends on the species.