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 P3 fraction of the heart and skeletal muscles, angiotensin I-converting enzyme (ACE) and neutral endopeptidase 24.11 (NEP) are the major kininases, as determined first with specific substrates and second with bradykinin. ACE activity was highest in guinea pig heart (2.7 ± 0.07 μmol·h−1 · mg protein−1) but decreased in other species in this order: dog atrium, rat heart, dog ventricle, and human atrium. The specific activity of NEP was lower: 0.45 μmol · h−1 · mg protein−1 in cultured neonatal cardiac myocytes and varying between 0.12 and 0.05 μmol · h−1 · mg protein−1 in human, dog, rat, and guinea pig heart. In the skeletal muscle P3, ACE was most active in guinea pig and rat (1.2 and 1.1 μmol · h−1 · mg protein−1, respectively) but less so in dog (0.09 μmol · h−1 · mg protein−1). NEP activity was higher in dog P3 (0.28 μmol · h−1 · mg protein−1) but lower in rat and guinea pig (0.19 and 0.1 μmol · h−1 · mg protein−1, respectively). Continuous density gradient centrifugation enriched NEP activity in dog and rat (from 0.3 to 1.0 and 0.49 μmol · h−1 · mg protein−1, respectively). Immunoprecipitation with antiserum to purified NEP proved the specificity of the rat enzyme. Bradykinin (0.1 mmol/1) was inactivated in the presence and absence of inhibitors by rat skeletal muscle NEP, as measured by high-performance liquid chromatography. Here, 36% of the activity was caused by NEP and 19% by ACE. In radioimmunoassay (bradykinin 10 nmol/1), 46 and 55% of kininase in rat and dog skeletal muscle P3, respectively, was due to ACE; 36 and 28%, respectively, was due to NEP. Aside from these enzymes, an aminopeptidase in rat P3 also inactivates bradykinin. Thus, in conclusion, heart and skeletal muscle membranes contain kininase II-type enzymes, but their activity depends on the species.

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