Standard methods for determining prorenin-renin concentrations in plasma (PRC) and other tissues require the addition of exogenous renin substrate (angiotensinogen) to improve the kinetics of the renin reaction. We studied the effects of substrate prepared from normal human plasma fraction Cohn IV-4, or from nephrectomized (2NX) sheep plasma, on PRC of normal and 2NX human plasmas before and after prorenin activation by acid, cold, and trypsin, and compared the results with plasma renin activities (PRA, no added substrate). Plasmas from 2NX men exhibited negligible basal PRA, indicating that very little, if any, renin had been formed from the extrarenal prorenin they contained, and suggesting the lack of an endogenous prorenin activating mechanism, or "convertase," of probable renal origin. Prorenin was demonstrable by tryptic activation, more than by acid or cold, at up to about 30% of normal. Addition of Cohn IV-4 substrate to 2NX plasma unexpectedly produced (i) a basal PRC value higher than in normal plasma, (ii) total renin values after activation by acid, cold, and trypsin that were much closer to normal values than reflected by PRA methodology, without a commensurate increase (if anything a decrease) in prorenin as a percentage of total renin estimated by all activation methods, and (iii) substantial equalization of activation effects such that trypsin was no longer more effective than acid and cold (and this was also noted with normal plasma). The skewing effect of adding Cohn IV-4 substrate on the PRC of 2NX plasma was much greater than in normal plasma, even though 2NX plasma already had an above normal level of endogenous substrate and should have been influenced less. Enhancement of PRC was very pronounced even when Cohn IV-4 was added to make up only 9% of total (endogenous + exogenous) substrate in the incubation system, suggesting that it was not the added substrate but a renin-generating contaminant that inflated the PRC. Such inflation could be blocked by adding protease inhibitors, suggesting that the responsible protease(s) acted as a prorenin "convertase" that generated new renin from renal and (or) extrarenal prorenin contributed by the added substrate, as well as by the plasma being assayed. One component of convertase could be kallikrein, which was identified by chromogenic assay, the importance of which relative to total convertase activity is unknown.(ABSTRACT TRUNCATED AT 400 WORDS)