Species differences in the kinetics of the renin-substrate reaction in plasma

In view of the substrate-dependence of renin, it was of interest to examine the kinetics of the renin-angiotensinogen reaction in plasma of various species to establish differences in stoichiometry. The results also provide an indication of assay conditions appropriate for accurate measurement of the reaction velocity. Plasma from hogs, dogs, and rats served as the source of renin for incubation with homologous angiotensinogen. The rate of production of radioimmunoassayable angiotensin I increased with increasing concentrations of angiotensinogen. This substrate-dependence of renin conformed to conventional Michaelis-Menten kinetics. The concentration of angiotensinogen was less than that required for half maximum velocity in dog and rat plasma. In contrast, endogenous substrate in hog plasma was sufficient to sustain near maximal rates of generation of angiotensin I. Purification of angiotensinogen altered the catalytic properties of angiotensinogen making it a poor representative substrate for renin. Hog and rat renin were saturable with high concentrations of unextracted plasma angiotensinogen. In contrast, it was not possible to saturate the dog enzyme with unextracted substrate. The interspecies differences in stoichiometry of the reaction indicate that standardization of assay conditions for various species of renin is not justified.