Effect of Boric Acid on the Phosphoglucomutase of Pea Seeds

Abstract

IT has been suggested that a possible role of boron in higher plants is to activate the processes involved in the uptake and movement of sugars, and the manner of operation has been attributed to the facility with which boric acid forms complexes with the cis-hydroxyl groups of the sugar molecule. Dugger and Humphrey¹ suggest, as the result of assay of a comprehensive range of enzymes from pea seedlings, that boron inhibits sucrose synthesis from uridinetriphosphate, glucose-1-phosphate and fructose, and stabilizes the level of uridine diphospho-glucose as well as increasing the rate of its formation. They consider that boron inhibits starch phosphoryl-ase, thus increasing the availability of glucose-1-phosphate for the formation of uridine diphospho-glucose. The transfer of the former through the normal glycolytic cycle is considered to be unaffected on the basis of assay of specific enzymes, including phosphoglucomutase. The phosphoglucomutase used was apparently prepared from muscle, and, in fact, it has been shown that low concentrations of boric acid specifically inhibit the phosphoglucomutase of pea seeds². The enzyme can be purified by alcohol fractionation at − 5° C. and obtained free of phos-phorylase and phosphatases which utilize the substrate and products. The phosphoglucomutase activity is always associated with that of phospho-hexoisomerase, but this enzyme has been found to be unaffected by boric acid. Fig. 1 shows the extent of the inhibition by a range of concentration in a preparation of this type. The fall in inorganic phosphorus + phosphorus released by hydrolysis at 100° C. in 1 N hydrochloric acid (free + 7′P) is used as a measure of formation of glucose-6-phosphate. When the purified system is used, no increase in inorganic phosphorus occurs during the time required for the system to reach equilibrium, and at this time chromatographic assay indicates the presence of only glucose-1-phosphate, glucose-6-phosphate and fructose-6-phosphate.