Vanadate-stimulated NADH oxidation in microsomes

Addition of vanadate, stimulated oxidation of NADH by rat liver microsomes. The products were NAD + and $H_2O _2$. High rates of this reaction were obtained in the presence of phosphate buffer and at low pH values. The yellow-orange colored polymeric form of vanadate appears to be the active species and both ortho- and meta-vanadate gave poor activities even at mM concentrations. The activity as measured by oxygen uptake was inhibited by cyanide, EDTA, mannitol, histidine, ascorbate, noradrenaline, adriamycin, cytochrome c, $Mn^{ 2+}$ , superoxide dismutase, horseradish peroxidase and catalase. Mitochondrial outer membranes possess a similar activity of vanadate-stimulated NADH oxidation. But addition of mitochondria and some of its derivative particles abolished the microsomal activity. In the absence of oxygen, disappearance of NADH measured by decrease in absorbance at 340 nm continued at nearly the same rate since vanadate served as an electron acceptor in the microsomal system. Addition of excess catalase or SOD abolished the oxygen uptake while retaining significant rates of NADH disappearance indicating that the two activities are delinked. A mechanism is proposed wherein oxygen receives the first electron from NAD' radical generated by oxidation of NADH by phosphovanadate and the consequent reduced species of vanadate $(V ^{iv})$ gives the second electron to superoxide to reduce it $H_2O _2$. This is applicable to all membranes whereas microsomes have the additional capability of reducing vanadate