Synergistic modulation of cystinyl aminopeptidase by divalent cation chelators

Abstract

Membranes of Chinese hamster ovary (CHO-K1) cells were used to study the opposite modulation of enzyme activity and [ 125 I ]Ang IV binding to cystinyl aminopeptidase (EC 3.4.11.3) by divalent cation chelators. Whereas ethylene diamine tetraacetic acid (EDTA) or ethylene glycol-bis(2-aminoethylether)- N, N, N′, N′-tetraacetic acid (EGTA) alone only slightly affected the enzyme activity, 1,10-phenanthrolin (1,10-PHE) produced a complete and concentration-dependent inhibition. Interestingly EDTA (≥0.05 mM) or EGTA (≥0.15 mM) enhanced the inhibitory effect of 1,10-PHE. Two-site analysis of the corresponding inhibition curves revealed that EDTA and EGTA converted enzymes with low sensitivity towards 1,10-PHE into enzymes with high sensitivity. The combined inhibition by EDTA (0.1 mM) and 1,10-PHE (0.1 mM) could be prevented and reversed by addition of Zn 2+ (at about 0.04–0.1 mM). In contrast, specific binding of [ 125 I ]Ang IV was enhanced in the presence of 1,10-PHE. Binding was only slightly affected by EDTA or EGTA alone. Furthermore, the stimulatory effect of 1,10-PHE was potentiated by EDTA (≥0.05 mM) as well as EGTA (≥0.15 mM). In the presence of EDTA (0.1 mM) and 1,10-PHE (0.1 mM), specific [ 125 I ]Ang IV binding was completely inhibited by Zn 2+ (IC 50 = 39.7 ± 6.2 μM). The present data show that divalent cations such as Zn 2+ are essential for the enzyme activity of cystinyl aminopeptidase and inhibitory for [ 125 I ]Ang IV binding. Modulation of the effects of 1,10-PHE by other chelators such as EDTA or EGTA, suggests that, in addition to the binding site for zinc in the catalytic site, cystinyl aminopeptidase also bears a regulatory divalent cation binding site.