The Inhibition by Physiological Orthophosphate Concentrations of Hydrolytic and Synthetic Activities of Liver Glucose 6-Phosphatase

Abstract

Abstract The inhibition by orthophosphate of glucose 6-phosphate phosphohydrolase and inorganic pyrophosphate-glucose phosphotransferase activities of rat liver glucose 6-phosphatase (EC 3.1.3.9) has been studied in detail. Significant inhibitions of both activities were noted over the entire range of pH values studied—pH 5.0 to 7.5 (hydrolase) or pH 5.0 to 6.5 (phosphotransferase). The detergents cetyltrimethylammonium bromide (Cetrimide) and lysolecithin significantly lowered Ki values for Pi with both activities. For example, at pH 7.5, a Ki value of 18 mm was noted in the absence of detergents, while preliminary treatment of microsomes with Cetrimide, to 0.05% (w/v), reduced this value to 2.0 mm, and inclusion of 0.4 mm lysolecithin in assay mixtures led to a decrease in Ki to 4.6 mm. Because of differences in the extents of accompanying modifying effects of detergents on Km values for substrates, net inhibition by Pi of glucose 6-phosphate phosphohydrolase activity was considerably more extensively potentiated by detergents than was that of phosphotransferase activity. Both in the absence and presence of detergents, inhibitions were kinetically competitive with respect to glucose 6-phosphate and pyrophosphate and non-competitive with respect to glucose in the phosphotransferase reaction. The mutually competitive nature of interactions of orthophosphate with adenosine triphosphate, bicarbonate, and pyrophosphate noted in inhibition studies carried out with phosphohydrolase activity also was shown. A physiologically significant regulatory role for Pi, through its effects on activities of this enzyme, is suggested on the basis of these experimental observations.