Glucose-6-phosphatase phosphotransferase activity

N3. Nordlie, R. C., Some properties and possible physiological functions of phosphotransferase activities of microsomal glucose-6-phosphatase. Ann. N.Y. Acod. Sci. 166, 699-718 (1969). 

Fig. 3. Proposed mechanism involving hydrolytic and synthetic activities of glucose-6-phosphatase in the transport of glucose between intracellular and extracellular compartments. The shaded area represents the cross-sectional view of endoplasmic reticulum. E and E" are modified forms of glucose-6-phosphatase displaying principally phosphohydrolase and principally phosphotransferase activities, respectively. Differential influences of the intra- and extracellular milieu are postulated to maintain molecules of the enzyme selectively as E or E". Additional details are given in Section II,D.

A wide variety of compounds and treatments have been reported to activate microsomal glucose-6-phosphatase phosphotransferase. These are described briefly below the possible relevance of the action of a number of these factors in the biological regulation of activities of the enzyme has been considered in Section II,C,3. 

Inhibitors of Hydrolase and Phosphotransferase Activities of Glucose-6-Phosphatase°... 

Recent studies indicate that the various phosphohydrolase and phosphotransferase activities of glucose-6-phosphatase are affected by numerous metabolites (see Table X and Sections II,C and III,D,4). The possible significance of observed activation or inhibition by a number of these compounds in vitro relative to regulation of both types of activity of the enzyme in vivo has been considered in a number of instances. Possible modes of control of net glucose release, involving the regulation by a variety of factors, of both hydrolytic and synthetic activities of the enzyme, have been discussed in considerable detail in earlier reviews by the author (9, 10). 

It previously has been pointed out that, potentially, PP glucose phosphotransferase and ATP-glucose phosphotransferase activities of glucose-6-phosphatase are the most potent glucose phosphorylating systems which have been characterized for liver (9, 10, 41, 118). Such a conclusion appears to have possible validity principally at and below pH 7 however (see Fig. 9) because of the nature of the pH-activity profiles of the phosphatase-associated phosphotransferase activities. 

In addition to altering the maximum potential activity of glucose-6-phosphatase, treatment with phospholipase A leads to a reversible increase in the affinity of this enzyme for glucose-6-phosphate (G-6-P), and to differential stabilities in the PPj-glucose phosphotransferase activity and in the PPi- and glucose-6-P-phosphohydrolase activities of the enzyme. All of these activities (equations la, lb, 2, and 3, below) have been shown to be common to one enzyme (cf., Nordlie and Arion, 1964). 

Subcellular Distribution of Inorganic Pyrophosphatase, PPi-GLUcosE Phosphotransferase, and Glucose-6-P Phosphohydrolase Activities of Rat Liver Microsomal Glucose-6-Phosphatase ... 

Comparison of Levels of Phosphotransferase and Phosphohydrolase Activities of Glucose-6-Phosphatase in Liver, Kidney, and Small Intestine of Rabbit"... 

As discussed briefly in Section I,A, glucose-6-phosphatase is now known to be a multifunctional enzyme capable of catalyzing potent phosphotransferase as well as phosphohydrolase reactions [see Eqs. (1)—(4) ]. Compounds demonstrated to function as effective phosphoryl donors include fructose-6-P (30), mannose-6-P (40), PPi (35-38), a variety of nucleosidetriphosphates and nucleosidediphosphates—most effectively CTP, CDP, deoxy-CTP, ATP, ADP, GTP, GDP, and ITP (41, 45)— carbamyl-P (43), phosphoramidate (44), phosphopyruvate (42, 43) and glucose-6-P itself (30, 31). The various phosphoryl donors are also hydrolyzed by action of the enzyme (see preceding references). Eqqa-tions (1)—(4), which describe these various activities, are given in Section I,A. 

Stetten, M. R., and Burnett, F. R., 1966, Activation of rat liver microsomal glucose-6-phosphatase, inorganic pyrophosphatase and inorganic pyrophosphate-glucose phosphotransferase by hydroxyl ion, Biochim. Biophys. Acta 128 344. 

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