Binding properties of acid-activated prorenin to (pro)renin receptor [(P)RR] was investigated in vitro to discuss possible roles of such reversibly acid-activated prorenin in the renin angiotensin (RA) system. Prorenin was acidified at pH 3.3, 4.5, 5.5, 6.5, and its activation level was measured at 1, 2, 4, 8, 12, and 25 h. Prorenin, activated non-proteolytically in time- and pH-dependent manners, was verified by Western blot analyses. Acidification of prorenin for 25 h at pH 3.3, 4.5, 5.5, and 6.5 showed 78%, 54%, 34%, and 20% activities, respectively when compared with the renin activity of trypsinized prorenin as 100%. Additionally, the binding properties of acidified prorenin to (P)RR were elucidated both at the equilibrium state and in the kinetic state using BIAcore. BIAcore assay showed that acidified prorenin at pH 3.3, 4.5, 5.5, and 6.5 had apparent K(D) of 1.57 × 10(4), 14.1, 8.29, and 8.04 nM, respectively while native prorenin at pH 7.4 had a K(D) of 7.8 nM. At equilibrium state, K(D) of native prorenin was 1.42 nM whereas apparent K(D) varied from 1.25 to 5.0 nM for the prorenin acidified at pH 4.5, 5.5, and 6.5. The K(m) values of free forms of acidified prorenin at different pH (0.33-0.5 μM) was almost similar to those of (P)RR-bound forms of acidified prorenin (0.5-0.77 μM). These in vitro data indicate that prorenin acidified in vivo possibly modulate RA system in receptor-dependent and/or -independent manners which could ultimately lead to the pathogenesis of diseases.