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 IC 50 values of HgCl 2, CrCl 3, CdCl 2 and NiCl 2 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. K i 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.