Inhibition by saccharin of glucose-6-phosphatase: effects of alloxan \u3ci\u3ein vivo\u3c/i\u3e and deoxycholate \u3ci\u3ein vitro\u3c/i\u3e

Inhibition by saccharin of rat liver glucose-6-phosphatase (EC 3.1.3.9) generally decreased as the pH increased in the range pH 4–8. This pattern was exhibited by homogenates from control and alloxan-treated animals assayed each in the absence and presence of 0.2% (w/v) deoxycholate. Saccharin inhibited in competitive fashion with respect to glucose-6-phosphate (glucose-6-P). There was a small increase in Km (glucose-6-P) but not Ki (saccharin) values in alloxan-treated rats when assays were conducted in the absence of deoxycholate. In the presence of this detergent there was no significant difference in these kinetic parameters between the alloxan-treated and control groups. Deoxycholate decreased Km (glucose-6-P) and increased Ki (saccharin) values. Calculations using these kinetic parameters indicate that, under usual hepatic glucose-6-P concentrations and relatively high levels of saccharin in liver, the inhibition by saccharin of glucose-6-phosphatase is unlikely to be of major significance in vivo

Inhibition by orthophosphate esters of glucose-6-phosphatase

Inhibition of rat liver microsomal glucose-6-phosphatase (D-glucose-6-phosphate phosphohydrolase, EC 3.1.3.9) by orthophosphate and organophosphate esters was examined at pH 6.0 and 7.5 with and without enzyme pretreatment with 0.2% (w/v) deoxycholate. Inhibition by orthophosphate and monoethyl phosphate was competitive with respect to glucose-6-P while inhibition by mono- and di-phenyl phosphate was of the mixed type. Monoalkyl phosphates were more effective inhibitors than the analogous di- and tri-alkyl phosphates and deoxycholate potentiated the inhibitory effects. Mono- and di-phenyl phosphates were more effective inhibitors than triphenyl phosphate, and deoxycholate decreased these inhibitory effects. The results are interpreted in terms of inhibitor and deoxycholate interactions with the enzyme

Glucose-6-phosphatase activity in a soluble fraction from cotyledon tissue of \u3ci\u3eBrassica nigra\u3c/i\u3e

A soluble fraction from cotyledon tissue of black mustard (Brassica nigra) was found to catalyze the hydrolysis of glucose 6-phosphate. In an attempt to determine whether this reaction was catalyzed by a distinct glucose-6-phosphate (d-glucose 6-phosphate phosphohydrolase, EC 3.1.3.9) or by an acid phosphatase (orthophosphoric monoester phosphohydrolase, EC 3.1.3.2), various characteristics of glucose 6-phosphate and p-nitrophenyl phosphate hydrolysis were compared. Both phosphatase activities exhibited a similar distribution pattern in subcellular fractions and in fractions obtained by precipitation with (NH4)2SO4. The activities also were compared with respect to the effects of reaction mixture pH in the absence and presence of 1 mM KF-4 mM EDTA and the effect of partial inactivation by various enzyme pretreatments. Glucose 6-phosphate and p-nitrophenyl phosphate were mutually competitive inhibitors for the hydrolysis of the other compound. The Km and Kf values were, respectively, 4.6 mM and 4.6 mM for glucose 6-phosphate and 0.72 mM and 0.79 mM for p-nitrophenyl phosphate. A substrate-specificity study indicated that p-nitrophenyl phosphate and phenyl phosphate were hydrolyzed more rapidly than glucose 6-phosphate. The soluble enzyme preparation did not exhibit PP1-glucose phosphotransferase activity. Glucose did not inhibit the hydrolysis of glucose 6-phosphate. It is concluded that the observed hydrolysis of glucose 6-phosphate was catalyzed by an acid phosphatase. The possible existence in plants of a distinct glucose-6-phosphatase is discussed