The mode of reduction of vanadate(+V) to oxovanadium(+IV) by glutathione and cysteine.

Vanadate(+V) is completely reduced to oxovanadium(+IV) in vitro by thiols. A high transient absorbance at 750 nm is observed (intense blue) in solutions of a pH below 5 immediately after starting the reduction. EPR-measurements and determinations of the consumption of free thiol groups prove that the transient absorbance is caused by an intermediate compound not containing vanadium in its reduced state (+IV). The way of reduction of vanadate(+V) by thiols is discussed. We propose the formation of an intermediate vanadate(+V)-thioester which is formed before reduction in a preequilibrium step.