D-Glucose 6-phosphatase

D-Glucosamine 6-phosphate is hydrolyzed by the D-glucose-6-phosphatase of rat-liver mitochondria. The rate of this hydrolysis is about 8 % of that of D-glucose 6-phosphate hydrolysis. A phosphatase which preferentially catalyzes the hydrolysis of D-glucosamine 6-phosphate has been prepared from Neurospora crassa. This enzyme is not stimulated by magnesium ions and has an optimum activity between pH 6 and 7.5. It appears to be distinct from acid, alkaline, and other specific phosphatases. 

Fig. 6.—Interconversion of Glycogen and n-Glucose by way of Glycogen Cycle. (Key (1), hexokinase-adenosine 5-triphospliate (2), phosphoglucomutaae (3),UDPG-pyrophoaphorylase (4), glycogen-UDPglucosyl transferase and branching enzyme (5), phosphorylase and amylo-l,6-glucosidase (6), D-glucose 6-phosphatase.)...

The synthesis of glycogen in rat adipose tissue is catalysed by multiple D-glucose 6-phosphatase-dependent forms of glycogen synthetase. Hormones... 

Recently, two examples of the separation of enantiomers using CCC have been published (Fig. 1-2). The complete enantiomeric separation of commercial d,l-kynurenine (2) with bovine serum albumin (BSA) as a chiral selector in an aqueous-aqueous polymer phase system was achieved within 3.5 h [128]. Moreover, the chiral resolution of 100 mg of an estrogen receptor partial agonist (7-DMO, 3) was performed using a sulfated (3-cyclodextrin [129, 130], while previous attempts with unsubstituted cyclodextrin were not successful [124]. The same authors described the partial resolution of a glucose-6-phosphatase inhibitor (4) with a Whelk-0 derivative as chiral selector (5) [129]. 

Zl. Zakim, D., Regulation of microsomal enzymes by phospholipids. I. The effect of phospholipases and phospholipids on glucose 6-phosphatase. J. Biol. Chem. 245, 4953-4961 (1970). 

It had been known from at least the time of Pasteur that the presence of sodium or potassium phosphate aided the progress of a yeast fermentation. Later intensive study showed that a complex group of enzymes (phosphatases and phosphorylases) was responsible for the phosphorylation, dephosphorylation and interconversion of D-glucose 6-phosphate, D-fructose 6-phosphate, D-fructose 1,6-diphosphate and similar substances in various types of cells and muscle tissue. Detailed reviews of the field are available. - A further advance was made in 1936, when Cori and Cori noted that in certain circumstances well-washed frog muscle immersed in a sodium phosphate buffer utilized the inorganic phosphate to produce a new hexose phosphate (the Cori ester). This compound was later shown to be a-D-glucopyranose-l-phosphate and yielded crystalline dipotassium and brucine salts. The Cori ester arose because... 

Hepatio Renal Endoor 100 mg/kg/d 100 mg/kg/d 100 mg/kg/d (Increased Mg ATPase, acid phosphatase, and glucose-6-phosphatase activities) (Increased Mg ATPase activity) (Increased blood glucose) sulfate... 

The release of glucose from the glycogen stores in the liver is mediated by glucose 6-phosphatase, which is apparently embedded within the membranes of the endoplasmic reticulum. A labile enzyme, it consists of a 357-residue catalytic subrmit,251/252 which may be associated with other subunits that participate in transport.252 253 A deficiency of this enzyme causes the very severe type la glycogen storage disease (see Box 20-D).251 253 Only hepatocytes have significant glucose 6-phosphatase activity. 

Glucose-6-phosphatase (D-glucose-6-phosphate phosphohydrolase, EC 3.1.3.9) is a unique enzyme in several respects. It is the only principal... 

D. Possible Significance of the Membranous Nature of Glucose-6-phosphatase... 

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.

Fig. 8. Ionic species of some phosphate compounds serving as substrates and/or inhibitors of glucose-6-phosphatase (157). See Sections III,D,4 and 5 for details.

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). 

The third enzyme in the pathway, KD0-8-phosphate phosphatase, has been purified to homogeneity (26). Because of its abosolute specificity, it should be a focal point for chemotherapeutic studies. jThe apparent for KD0-8-phosp te was+ etermined to be 5.8 x 10 M in the presence of 1.0 mM Co or Mg. This specific KD0-8-phosphate phosphatase was separated from enzymes, present in crude extracts, having phosphatase activity on other phosphorylated compounds by column chromatography on DGAE-Sephadex (26). Three distinct peaks of activity were detected. Fractions from each peak were pooled and the rates for the hydrolysis of five compounds were measured. Peak A possessed phosphatase activity for D-glucose-6-phosphate, D-arabinose-5-phosphate, D-ribose-5-phosphate and j-nitrophenylphosphate Peak B dephosphorylated D-arabinose-5-phosphate, D-ribose-5-phosphate and D-glucose-6-phos-phate. Peak C, which was well separated from the other two peaks, could only utilize KD0-8-phosphate as a substrate. KD0-8-phos-phate was not hydrolyzed by the phosphatases present in peaks A and B. 

D-Glucose-6-phosphate dehydrogenase, D-gluconate-6-phosphate dehydrogenase, KDO-8-phosphate synthase, KDO-8-phosphate phosphatase and CMP-KDO synthetase have been purified to homogeneity and characterized. Munson, Rasmussen and Osborn (15) have partially purified one KDO transferase. D-Arabinose-5-phosphate isomerase is very unstable and has only been purified about 100 fold. D-Ribose-5-phosphate isomerase activity is approximately 20x that... 

Massillon, D., Barzilai, N., Chen, W., Elu, M. and Rossetti, L. (1996) Glucose regulates in vivo glucose-6-phosphatase gene expression in the liver of diabetic rats.J. Biol. Chem., 271, 9871-9874. 

Lochhead, P. A., Salt, I. P., Walker, K. S., Hardie, D. G., and Sutherland, C. 2000. 5-aminoimidazole-4-carboxamide riboside mimics the effects of insulin on the expression of the 2 key gluconeogenic genes PEPCK and glucose-6-phosphatase. Diabetes 49 896-903. 

Herling AW, Burger HJ, Schwab D et al. (1998) Pharmacodynamic profile of a novel inhibitor of the hepatic glucose-6-phosphatase system. Am J Physiol 274 G1087-G1093... 

Zakin, D., and Vessey, D. A., Techniques for the Characterization of UDP-Glucuronyltransferase, Glucose 6-Phosphatase, and Other Tightly-Bound... 

Inositol is synthesized by the same route in all cells. D-Glucose-6-phosphate (d-G-6-P) is converted to L-mvo-inositol-1 -phosphate (l-I- 1 -P), which can also be termed D-myo-inositol-3-phosphate. The L-I-l-P is then hydrolyzed to myoinositol by a relatively specific phosphatase. As we will see, enzymes that do these reactions in bacteria and archaea can have quite different characteristics from the eukaryotic enzymes. 

Chou JY, Matern D, Mansfield BC, Chen YT. Type 1 glycogen storage diseases disorders of the glucose-6-phosphatase complex. 

A. Burchell and I.D. Waddell. 1991. The molecular basis of the hepatic microsomal glucose-6-phosphatase system Biochim. Biophys. Acta 1092 129-137. (PuhMed)... 

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