Synergistic inhibition of the enzymatic activity of aminopeptidase N by divalent metal ion chelators.

Membranes of HEK293 cells that were transfected with human aminopeptidase N (AP-N, CD13, EC 3.4.11.2) and purified soluble porcine kidney AP-N were used to study inhibition of its enzyme activity by divalent cation chelators. Whereas pre-incubation for 10 min with ethylenediaminetetraacetic acid (EDTA), did not or only weakly affected the enzyme activity, the bidentate chelator 1,10-phenanthroline produced a complete and concentration-dependent inhibition of AP-N. The corresponding curves had Hill slopes of 2.50 +/- 0.23 and 2.73 +/- 0.01 for soluble and recombinant AP-N respectively. EDTA increased the potency of 1,10-phenanthroline till a limit, at which Hill slopes became close to unity. In the absence of EDTA, the inhibition by 1,10-phenanthroline was only weakly affected by the substrate concentration. On the other hand, competition between 1,10-phenanthroline and the substrate took place in the presence of EDTA. Similar findings were reported for the related metallopeptidase cystinyl aminopeptidase and point towards a model in which 1,10-phenanthroline inhibit enzyme activity by decreasing the free Zn2+ concentration. Moreover, EDTA is capable of removing a modulatory ion from an allosteric site at the enzyme, facilitating the direct interaction between 1,10-phenanthroline and the catalytic Zn2+. Compatible with this model, Ca2+ may bind to this allosteric site resulting in the potentiation of Zn2+-mediated re-activation of the enzyme activity in the presence of EDTA and 1,10-phenanthroline.