The catalytic effect of molybdate on the hydrolysis of organic phosphates

Abstract

Lutwak and Sacks (1953) (Jour. Biol. Chem. 200, 565) found that, when solns. of organic phosphates were heated with molybdate, estimations of inorganic phosphate by direct colorimetry after previous precipitation did not agree. It has now been shown that this effect is observed only when a blue color develops during exposure to heat. The effect is similar whether the soln. contains, e.g., fructose phosphate or a mixture of fructose and phosphate, and is therefore unlikely to be due to the formation of organo-phosphomolybdate complexes. The effect may be due to 3 factors. (a) When magnesia mixture is added to the reduced phosphomolybdate complex and the resulting precipitate is analysed colorimetrically, recoveries of inorganic phosphate are low. (b) Free molybdic acid is reduced, giving molybdenum blue. Contrary to prevailing opinions molybdenum blue is extracted from acid solns. by isobutanol. (c) The specific extinction coefficient is about 4-5 times higher when the color of the reduced phosphomolybdate complex is developed at 100[degree] than when it is developed at room temp. This is true for most reducing agents except stannous chloride, with which a higher extinction is obtained at room temp. The effect is not observed under the milder exptl. conditions of Weil-Malherbe and Green (Biochem. Jour 49: 286, 1952). If it has occurred, a stannous chloride method of phosphate estimation, such as was used by these authors, would have been less subject to error than a method using amino-naphtholsulfonic acid.