Vanadium (IV) formation in the reduction of vanadate by glutathione reductase/NADPH and the role of molecular oxygen

Experimental evidence documenting the formation of a relatively stable V(IV) species appears to be important with regard to the biochemical mechanism of reduction of vanadate by enzymatic systems. The present study demonstrates that a mixture of vanadate and glutathione reductase/nicotinamide-adenine-dinucleotide phosphate (NADPH), in phosphate (pH 7.2) buffer generates V(IV) under ambient conditions. Once formed, V(IV) does not rapidly autoxidize so as to defy detection by electron spin resonance (ESR) spectroscopy. The aerobic environment was guaranteed by preparing reaction mixtures in well stirred, wide mouth, standard test tubes in air over a period of 50 minutes, and by making ESR measurements in nuclear magnetic resonance (NMR) sample tubes as well as oxygen-permeable Teflon tubes. The V(IV) ESR signal intensity was found to increase linearly with time elapsed after reaction initiation. The linear growth of the V(IV) species also shows that this species is fairly stable, over a period of at least 50 minutes. Similar V(IV) stability data were obtained from VOSO₄, a model compound as a source of V(IV). The results obtained in the present study demonstrated that V(IV) can be generated in the reduction of V(IV) by glutathione reductase in the presence of NADPH under aerobic condition.