Measurement of Renin Activity in Human Plasma

A method is described for estimating plasma renin activity by using renin substrate present in plasma. This method differs from other indirect renin assay methods by (1) incubation in the absence of ions thus establishing conditions for zero order kinetics for the reaction between endogeneous renin and substrate and (2) the use of angiotensinase inhibitors di-sodium ethylenediamine tetraacetic acid ( EDTA ) and d-isopropylfluorophosphate (DFP). Recoveries of renin added to plasma in levels similar to those occurring in plasma are 85% SD ±7%. The incubation was done at pH 5.5 which was shown to be the optimum for human renin reacting with human substrate. By incubating human plasma samples with known quantities of human renin, evidence was obtained suggesting that factors other than enzyme or total substrate concentrations affect the velocity of angiotensin formation. This variability of reaction rate may be explained by the existence of an inhibitor or activator in this system or by a variation in the type of substrate.