Angiotensin I converting enzyme and kinin-hydrolyzing enzymes along the rabbit nephron

Angiotensin I converting enzyme (ACE) and kininase activities were measured in various segments of the rabbit nephron. ACE was determined with tritiated hippuryl-glycylglycine as substrate. Lysyl-bradykinin (LBK) hydrolysis (kininase activity) was measured by radioimmunoassay. ACE was only found in the glomerulus and in the two parts of proximal tubule: the convoluted proximal tubule and the pars recta (PR). It was distributed along a concentration gradient which increased from the glomerulus to PR. Kininase activity was found in both proximal and distal parts of the nephron. Besides intense LBK-hydrolyzing activity in the proximal tubule, a kininase activity was also found in the medullary collecting tubule (MCT). Kininase activity in the glomerulus and the proximal tubule was completely inhibited by chelating agents. Captopril inhibited this activity only in the PR and at high concentrations (above 10(-7) M). These results indicate that several types of enzymes other than ACE hydrolyze kinins in the glomerulus and in the proximal tubule. The contribution of ACE to kinin hydrolysis appears only minimal. The kininase activity found in MCT was different from ACE and other proximal tubule kininases because it was not inhibited by chelating agents. This kininase may play a physiological role in inactivating the kinins formed by kallikrein at or beyond the connecting tubule.