Modifications of Hydrolytic and Synthetic Activities of Liver Microsomal Glucose 6-Phosphatase

Abstract 1. The effects of deoxycholate, cholate, taurocholate, Triton X-100, cetyltrimethylammonium chloride (cetrimide), and NH4OH on liver microsomal glucose 6-phosphatase have been investigated. Treatment of microsomal preparations with detergents activate and inhibit (not inactivate) the phosphohydrolase and phosphotransferase activities of the enzyme. The inhibitory action of detergents was substantially reversed by the inclusion of bovine plasma albumin in the assay media. The glucose-binding site on the enzyme was implicated as a principal site of interaction of detergents with enzyme by the following observations. (a) The extent of inhibition by detergents was considerably more pronounced with the phosphotransferase activity, and (b) in the absence of albumin a substantial fraction of the PPi-phosphohydrolase activity of detergent-treated microsomes was insensitive to inhibition by glucose. Detergent treatment significantly decreased the Michaelis constant for glucose-6-P, and this decrease was potentiated by albumin. The Michaelis constants for PPi in both the phosphohydrolase and phosphotransferase reactions also appeared to be decreased by treatments with detergents, particularly the anionic compounds. However, in the presence of albumin the values for Kppppi were equal to those of the untreated microsomes. The kinetic constants determined for NH4OH-treated microsomes were not altered by albumin and were identical with those obtained in the presence of albumin with detergent-treated microsomes. 2. In confirmation of earlier studies by others, cetrimide effected marked reductions in Km and Vmax values. These effects were found to be the result of uncompetitive inhibition by this cationic detergent. 3. The extent of inhibition by deoxycholate as well as the effectiveness of albumin in reversing the inhibition were identical with enzyme preparations from adrenalectomized, normal, and cortisol-treated animals. This finding strengthens the earlier conclusion that glucocorticoids in vivo stimulate activities of glucose 6-phosphatase by activating existing, but latent, enzyme. 4. An automated method for determining glucose-6-P is described.