Influence of angiotensinase inhibitors on the enzymatic activity of renin

Abstract

Asp 1-Ileu 5-angiotensin I, added to pooled normal human plasma at pH 5.5 or at 7.4, was shown to be fully protected from enzymatic degradation and conversion to angiotensin II by addition to plasma of a combination of angiotensinase inhibitors including ethylenediaminetetraacetic acid, diisopropylfluorphosphate, dimercaptopropanol, and 8-hydroxyquinoline hemisulfate. Addition of Dowex resin to the incubation media (pH 5.5) did not increase the protection of angiotensin I afforded by this combination of inhibitors. For these studies, both bioassay and radioimmunoassay were employed, which allowed for evaluation of N-terminal and C-terminal alteration of the peptide. In the presence of Dowex resin (pH 5.5), plasma renin activity was shown to be depressed. By the use of an angiotensinase-free isolated system of human angiotensinogen and homologous renin, this decrease in activity was found to be due to binding of renin, and not of the renin substrate, to the resin. At pH 5.5, although 3.4 m m 8-hydroxyquinoline was shown to exert maximal inhibition of angiotensin degradation, yielding an increase in measured plasma renin activity, at higher concentrations of 8-hydroxyquinoline an overall inhibition of angiotensin generation was observed. At pH 7.4, on the other hand, 8-hydroxyquinoline inhibited renin activity at all concentrations studied. The inhibition of renin activity at pH 7.4 was minimized by reduction of the 8-hydroxyquinoline concentration to 1.7 m m. This latter concentration was of equal effectiveness in preventing angiotensin degradation. Although protection of angiotensin I was complete at pH 5.5 and 7.4, it was found that plasma renin activity was increased at pH 7.4. It was concluded that maximization of angiotensin I generation in plasma, by endogenous renin, was dependent not only on the pH of incubation, but also upon the type and concentration of angiotensinase inhibitors employed which affect the renin reaction itself.