Angiotensin II-Producing Enzyme III From Acidified Serum of Nephrectomized Dogs: Activation of Proenzyme III to Enzyme III by Cathepsin G

Abstract

Enzyme III and its inactive precursor proenzyme III have been obtained from the acidified serum of bilaterally nephrectomized dogs. Enzyme III occurs in a concentration producing angiotensin at a rate 50 times higher than the residual renin. Much higher concentrations of enzyme III have been obtained by three activation procedures: a) by storage for several months in the frozen state; b) by treatment at 0 degrees C and pH 3.0; or c) by incubation at 38 degrees C and pH 7.7. These procedures yielded levels of enzyme III that produced up to 400 ng angiotensin II/mL serum/h, i.e., 4000 times higher than the endogenous renin. Of further significance, the angiotensin II produced by enzyme III represents the octapeptide with the highest known vasoconstrictor activity. This is in contrast to renin, enzyme I, and enzyme II, all of which produced angiotensin I, the decapeptide without appreciable vasoconstrictor activity. The endogenous activating enzyme has been identified as cathepsin G by the following six experimental observations: both the endogenous activating enzyme and exogenous cathepsin G produced enzyme III from proenzyme III according to the same unusual sigmoid kinetics; they were blocked identically by antibody to human cathepsin G. Also, both were inhibited similarly by alpha-1-antitrypsin, lima bean trypsin inhibitor, diisopropyl fluorophosphate, or by an inhibitor present in dog serum. Thus, removal of this inhibitor and activation of proenzyme III by endogenous or by added cathepsin G are prerequisites to obtaining enzyme III; this provides a novel mechanism for the rapid formation of angiotensin II from serum protein substrate or from angiotensin I.