Nonproteolytic "activation" of prorenin by active site-directed renin inhibitors as demonstrated by renin-specific monoclonal antibody

Incubation of human plasma prorenin (PR), the enzymatically inactive precursor of renin (EC 3.4.23.15), with a number of nonpeptide high-affinity active sitedirected renin inhibitors induces a conformational change in PR, which was detected by a monoclonal antibody that reacts with active renin but not with native inactive PR. This conformational change also occurred when inactive PR was activated during exposure to low pH. Nonproteolytically acid-activated PR, and inhibitor-"activated" PR, as well as native PR, were retained on a blue Sepharose column, in contrast to proteolytically activated PR. Kinetic analysis of the activation of plasma prorenin by renin inhibitor (INH) indicated that native plasma contains an open intermediary form of prorenin, PR_oi, in which the active site is exposed and which is in rapid equilibrium with the inactive closed form, PR_C. PR_oi reacts with inhibitor to form a reversible complex, PR_oi·INH, which undergoes a conformational change resulting in a tight complex of a modified open form of prorenin, PR_o, and the inhibitor, PR_oi·INH → PR_o·INH. The PR_oi-to-PR_o conversion leads to the expression of an epitope on the renin part of the molecule that is recognized by a reninspecific monoclonal antibody. Presumably, PR_o corresponds to the enzymatically active form of PR that is formed during exposure to low pH. Thus, it seems that the propeptide of PR interacts with the renin part of the molecule not only at or near the enzyme's active site but also at some distance from the active site. Interference with the first interaction by renin inhibitor leads to destabilization of the propeptide, by which the second interaction is disrupted and the enzyme assumes its active conformation. The results of this study may provide a model for substrate-mediated prorenin activation and increase the likelihood that enzymatically active prorenin is formed in vivo.