Superoxide-independent reduction of vanadate by rat liver microsomes/NAD(P)H: Vanadate reductase activity

It has been reported that vanadate-stimulated oxidation of NAD(P)H by microsomal systems can proceed anaerobically, in contrast to the general notion that the oxidation proceeds exclusively by an O₂^-1-dependent free radical chain mechanism. The current study indicates that microsomal systems are endowed with a vanadate-reductase property, involving a NAD(P)H-dependent electron transport cytochrome P450 system. Our ESR measurements demonstrated the formation of a vanadium (IV) species in a mixture containing vanadate, rat liver microsomes, and NAD(P)H. This vanadium(IV) species was identified as the vanadyl ion (VO²⁺) by comparison with the ESR spectrum of VOSO₄. The initial rate of vanadium(IV) formation depends linearly on the concentration of microsomes. The Michaelis-Menten constants were found to be: k_m = 1.25 mM and V_max = 0.066 μmol (min)^-1 (mg microsomes)^-1, respectively. Pretreatment of the microsomes with carbon monoxide or K₃Fe(CN)₆ reduced vanadium(IV) generation, suggesting that the NAD(P)H-dependent electron transport cytochrome P450 system plays a significant role in the microsomal reduction of vanadate. Measurements under argon or in the presence of superoxide dismutase caused only minor (less than 10%) reductions in vanadium(IV) generation. The VO²⁺ species was also detected in NAD(P)H oxidation by fructose plus vanadate, a reaction known to proceed via an O₂^--mediated chain mechanism. However, the amount of vanadium(IV) generated by this reaction was an order of magnitude smaller than that by the microsomal system and was inhibitable by superoxide dismutase, affirming the conclusion that the microsomal/ NAD(P)H system is endowed with the (O₂^--independent) vanadium(V) reductase property.