Reversible acid-activation of inactive renin: Evidence favouring a unimolecular reaction

Purified inactive renin from human kidney or plasma was activated to a similar extent by incubation with trypsin or by dialysis to pH 3.3. Trypsin- or acid-activated inactive renin was like active renal renin in terms of pH optimum (pH 5.5-6.0) and inhibition by monospecific antibody. The apparent K(m) of homologous angiotensinogen for acid-activated inactive renin (1.11 μmol/l) was the same as for trypsin-activated inactive renin (1.12 μmol/l) or active renal renin (1.07 μmol/l). In contrast with trypsin-activated renin, acid-activated renin was reversibly inactivated in time-dependent manner during incubation at 37°C above pH 5. The maximal rate of reversal occurred in the pH 7-8 range. Reversal was prevented by prior incubation with trypsin, plasmin, glandular kallikrein or plasma kallikrein. By kinetic analysis, the reversal of activation was found to be a first-order decay process with a reaction half-time (7-8 min at 37°C, pH 7.4) that was independent of the initial concentration of activated enzyme. These results indicate that reversible acid-activation of inactive renin is a unimolecular reaction, consistent with the hypothesis that acid pH induces a conformational change in a single polypeptide chain.