Inhibition mechanisms of lead, cadmium and nickel for the purified bovine liver NADPH-cytochrome P450 reductase

Very little information is available on the effects of metals in veterinary animals, despite the fact that they are exposed to these chemicals via contaminated plants and water sources. NADPH-Cytochrome P450 reductase is an essential component of the microsomal cytochrome P450 monooxygenase system. In this study, our aim was to elucidate in vitro effect of Pb2+, Cd2+ and Ni2+ on the purified bovine NADPH-cytochrome P450 reductase. NADPH-cytochrome P450 reductase was purified from bovine liver microsomes. The effects of metals were studied by adding various concentrations of them on the reaction mixture. All of the metal ions caused inhibition of the enzyme activity. The IC50 values of Pb2+, Cd2+ and Ni2+ were estimated to be 0.027 mM, 0.059 mM and 2.9 mM, respectively. Of the metal ions tested, Pb2+ exhibited much higher inhibitory effect at lower concentrations. Kinetic analysis was done to determine type of inhibition of purified bovine cytochrome P450 reductase. All of the metal ions displayed noncompetitive type of inhibition mechanism for the purified NADPH-cytochrome P450 reductase as analyzed by Lineweaver-Burk Plot and Dixon plot. Ki values of Pb2+, Cd2+ and Ni2+ were calculated from Dixon plots as 0.03 mM, 0.04 mM and 2.0 mM, respectively. The results of this study clearly indicate that mechanism of inhibition of lead, cadmium and nickel for cytochrome P450 reductase in bovine is noncompetitive type.