Vanadium(IV) inhibition of protein tyrosine phosphatases may be responsible for the insulin mimetic effect of vanadium. Electron paramagnetic resonance spectral data for vanadyl complexes of VHCSAG-NH2 (an active-site peptide of PTP-1B) between pH 5 and 9, as well as the derivatives VFCSAG-NH2, VHMSAG-NH2, and VHCGAG-NH2 suggests that the histidine or cysteine side-chain heteroatoms coordinate to the vanadyl ion while the serine hydroxyl remains uncoordinated.