Effect of mercury, cadmium, nickel, chromium and zinc on kinetic properties of NADPH-cytochrome P450 reductase purified from leaping mullet (Liza saliens)

Information on the mechanism of metal ion inhibition of NADPH-cytochrome P450 reductase is limited. The purpose of the present paper was to elucidate in vitro effect of Hg+2, Cd+2, Ni+2, Cr+3 and Zn+2 ions on the purified mullet NADPH-cytochrome P450 reductase. NADPH-cytochrome P450 reductase was purified from detergent-solubilized liver microsomes from leaping mullet (Liza saliens). All of the metal ions caused inhibition of the enzyme activity except Zn+2. At 50 μM metal concentration, Hg+2 inhibited the cytochrome P450 reductase activity completely (100%), while, at the same concentrations, Cd+2, Cr+3 and Ni+2 caused 66%, 65% and 37% inhibition, respectively. At 50 μM metal concentration, Zn+2 had no apparent effect on cytochrome P450 reductase activity. The IC50 values of HgCl2, CrCl3, CdCl2 and NiCl2 were estimated to be 0.07 μM, 24 μM, 33 μM and 143 μM, respectively. Of the metal ions tested, Hg+2 exhibited much higher inhibitory effect at lower concentrations, so it was evidently a more potent inhibitor than the others. All four metal ions displayed noncompetitive type of inhibition mechanism for the purified reductase as analyzed by Dixon plot. Ki values of Hg+2, Cr+3, Cd+2, and Ni+2 were calculated from Dixon plots as 0.048 μM, 18 μM, 73 μM and 329 μM, respectively.