pH-independent inhibition of plasma angiotensin I degradation: implications for renin assay

Valid enzymatic assay of plasma renin (EC 3.4.99.19) requires complete inhibition of enzymes that destroy angiotensin I. Historically, the degree of such inhibition has varied with assay pH and with the combination of protease inactivators employed. To circumvent this problem, we studied the effects of several protease inactivators on both the formation and degradation of angiotensin I by human plasma at pH 5.5 and at pH 7.4. While several reagent combinations effectively suppressed the rate of angiotensin I disappearance at one of the two pH values, three combinations of phenylmethylsulfonyl fluoride with chelating agents accomplished this at both pH values. Of these, however, only the combination of 5 mmol/l ethylenediamine tetraacetate, 1.5 mmol/l 8-hydroxyquinoline, and 7.5 mmol/l phenylmethylsulfonyl fluoride also maximized the rate of angiotensin I formation at both pH 5.5 and 7.4. Thus, the use of the latter reagent combination would facilitate comparison of renin assays performed at different pH values and would permit standardization of the protease inactivators employed in such assays.