High Vanadate Interferes with the Fiske-Subbarow Determination of Inorganic Phosphate

Abstract

We evaluated the possibility that oxyions of vanadium might react with molybdate and, in that manner, interfere with the Fiske-Subbarow colorimetric determination of inorganic phosphate. Phosphate (P _i) standard curves were prepared (0.03-0.30 μmole/ml) in the presence and absence of oxyvanadium solutions (2 × 10^-4 M) prepared from ortho- and metavanadate. Molybdate prepared in 5 N sulfuric acid was added to each standard. Upon addition of a reducing agent to develop color of the phosphomolybdate complex, a less intense color was observed at any given P _i concentration in the presence of oxyvanadium species. The slope of the regression line for the P _i standard curve in the presence of 2 × 10^-4 M oxyvanadium species was markedly depressed. The effect of oxyvanadium was similar when solutions were prepared from ortho- and metavanadate, despite differences in pH of these solutions. In addition, in the final reaction the pH was similar in the presence and absence of oxyvanadium, independent of the source of vanadate used to prepare solutions. Thus, interference by oxyvanadium did not appear to be related to changes in pH of samples containing vanadium oxyions. Interference was concentration dependent and the minimal concentration of vanadium oxyions that interfered was 5 × 10^-5 M. The effects of oxyvanadium (2 × 10^-4 M) on Mg⁺²-dependent and Na⁺-K⁺-ATPase activities in a renal microsomal preparation were then evaluated through the measurement of inorganic phosphate generation. Enzyme activities were determined with and without correction for interference by oxyvanadium with the method of Fiske and Subbarow. A significant artifactual depression of Mg⁺² ATPase activity, but not Na⁺-K⁺-ATPase activity, was consistently observed when enzyme activities were not corrected for interference by oxyvanadium with the measurement of inorganic phosphate. These data indicate that when effects of high vanadate concentrations (5 × 10^-5 M) on ATP hydrolyzing enzymes are evaluated through changes in P _i generation, artifactual depression of enzyme activity may occur.