Interaction of trace levels of vanadium(IV) and vanadium(V) in biological systems

Enzymes kinetics have been used to study interactions of trace-level concentrations of vanadate (V(V)) and vanadyl cation (V(IV)) in biological systems. A quantitative method based on the inhibition of alkaline or acid phosphatase by monomeric vanadate or vanadyl cation has been developed to determine the concentration of free monomeric vanadate or vanadyl cation at 10 super(-5)-10 super(-7) M vanadium concentrations. Interactions of vanadate and vanadyl cation with potential ligands including buffers, chelating agents, enzyme substrates, cofactors, amino acids, peptides, and proteins were examined. Seven out of 26 commonly used buffers were found to strongly complex vanadate, and an additional 11 buffers were found to complex vanadate to various degrees. The vanadyl cation generally interacts more strongly with these buffers than does vanadate. In contrast, the Tricine super(1)-vanadate complex was 8 times more stable than the Tricine-vanadyl complex. The formation constant (K super(eff) sub(pH 8.0)) for the EDTA-vanadate complex was determined to 1.4 x 10 super(4) L/mol in agreement with previous estimations in this pH range. The interaction of vanadate with enzyme substrates such as glycerol and glucose or with cofactors such as NAD was not observable at 10 super(-6) M vanadate concentrations.