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Multifunctional glucose-6-phosphatase: Cellular biology

Life Sciences, ISSN: 0024-3205, Vol: 24, Issue: 26, Page: 2397-2404
1979
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Glucose-6-phosphatase is a multifunctional enzyme, displaying potent ability to synthesize as well as hydrolyze Glc-6-P. These multifunctional characteristics have been exploited in studies of the extended distribution of the enzyme, and their physiological significance has been examined. The enzyme is considerably more widely distributed than previously suspected. It has been found in pancreas, adrenals, lung, testes, spleen, and brain as well as in liver, kidney, and mucosa of small intestine. Approximately 15–20% of total hepatic glucose-6-phosphatase-phosphotransferase is present in nuclear membrane, 75–80% is found in endoplasmic reticulum, and small amounts have been detected also in plasma membrane and repeatedly-washed mitochondria. Both hydrolytic and synthetic functions, in constant proportions, have been found in livers of 21 species of birds, amphibia, reptiles, crustacea, fishes, and mammals (including man) studied. With 5 mM phosphoryl donor and 100 mM D-glucose as substrates, carbamyl-P:glucose phosphotransferase activity of glucose-6-phosphatase exceeded that of glucokinase by 5–50 fold. While latencies of activities of isolated microsomal preparations are extensive, those of nuclear membranes are not. Latencies of activities of intact endoplasmic reticulum of permeable hepatocytes are 28% for Glc-6-P phosphohydrolase and 56% for carbamyl-P:glucose phosphotransferase. Studies with isolated perfused livers from fasted rats suggest rather convincingly that such phosphotransferase activities may function as an hepatic glucose-phosphorylating system supplemental to glucokinase and hexokinase. This conclusion is based both on comparisons of rates of glucose uptake with hepatic enzyme levels (glucokinase, hexokinase, phosphotransferase), and on observed inhibitibility of glucose uptake by ornithine and 3-0-methyl-D-glucose. The question of availability of adequate concentrations of suitable phosphoryl donor(s) in cytosol of the liver cell constitutes a principal focus for continuing studies regarding physiological functions of this enzyme.

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