The inhibition constants for vanadate, chromate, molybdate, and tungstate have been determined with Escherichia coli alkaline phosphatase, potato acid phosphatase, and Helix pomatia aryl sulfatase. Vanadate was a potent inhibitor of all three enzymes. Inhibition of both phosphatases followed the order WO4(2-) greater than MoO4(2-) greater than CrO4(2-). The Ki values for potato acid phosphatase were about 3 orders of magnitude lower than those for alkaline phosphatase. Aryl sulfatase followed the reverse order of inhibition by group VI oxyanions. Phenol enhanced inhibition of alkaline phosphatase by vanadate and chromate but did not affect inhibition of acid phosphatase. Phenol enhanced inhibition of aryl sulfatase by metal oxyanions in all cases following the order H2VO4- greater than CrO4(2-) greater than MoO4(2-) greater than WO4(2-), and N-acetyltyrosine ethyl ester enhanced inhibition of aryl sulfatase by H2VO4- and CrO4(2-) more strongly than did phenol. It is apparent that the effectiveness of metal oxyanions as inhibitors of phosphatases and sulfatases can be selectively enhanced in the presence of other solutes. The relevance of these observations to the effects of transition metal oxyanions on protein phosphatases in vivo is discussed.

The binding of inorganic vanadate (Vi) to rabbit muscle phosphoglycerate mutase (PGM), studied by using 51V nuclear magnetic resonance spectroscopy, shows a sigmoidal dependence on vanadate concentration with a stoichiometry of four vanadium atoms per PGM molecule at saturating [Vi]. The data are consistent with binding of one divanadate ion to each of the two subunits of PGM in a noncooperative manner with an intrinsic dissociation constant of 4 X 10(-6) M. The relevance of this result to other studies which have shown that the Vi-stimulated 2,3-diphosphoglycerate (2,3-DPG) phosphatase activity of PGM has a sigmoidal dependence on [Vi] with a Hill coefficient of 2.0 is discussed. At pH 7.0, inorganic phosphate has little effect on the 2,3-DPG phosphatase activity of PGM, even at concentrations as high as 50 mM. Similarly, 25 microM Vi has little effect on the phosphatase activity. However, in the presence of 25 microM Vi, a phosphate concentration of 20 mM increases the phosphatase activity by more than 3-fold. This behavior is rationalized in terms of activation of the phosphatase activity by a phosphate/vanadate mixed anhydride. This interpretation is supported by the observation of strong activation of the phosphatase activity by inorganic pyrophosphate. A molecular mechanism for the observed effects of vanadate is proposed, and the relevance of this study to the possible use of vanadate as a therapeutic agent for the treatment of sickle cell anemia is discussed.

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