Glucose-6-phosphatase mechanism

When C -formaldehyde was incubated anaerobically with dihydroxy-acetone phosphate and a protein fraction from rat liver, the isotope was found in three different compounds, which were separated. After removal of the phosphate with phosphatase, the major constituent, accounting for 80% of the isotope, was found to be a tetrose. About 10% was found in material behaving like ribose, and 7 % was found in glucose. The mechanisms of these reactions are unknown but are clearly of great interest. 

Metformin restrains hepatic glucose production principally by suppression of gluconeogenesis. The mechanisms involve potentiation of insulin action and decreased hepatic extraction of certain gluconeogenic substrates such as lactate. In addition, metformin reduces the rate of hepatic glycogenolysis and decreases the activity of hepatic glucose-6-phosphatase. Insulin-stimulated glucose uptake and glycogenesis by skeletal muscle is increased by metformin mainly by increased... 

Di-tt-octylphthalate has been shown to be a mild liver toxin at high doses in acute- and intermediate-duration studies in rodents. While the mechanism of action for these hepatic effects is not known, di-w-octylphthalate does not appear to behave like other phthalate esters such as di(2-ethylhexyl)phthalate, which have been shown to be hypolipidemic peroxisome proliferators. Instead, the liver changes associated with exposure to di- -octylphthalate are characterized by marked centrilobular accumulation of fat and loss of glycogen, accompanied by reduced glucose-6-phosphatase activity and some centrilobular necrosis. 

The amount of total enzymatic activity that becomes manifest only after disruption of membranous barriers between enzyme and substrate or upon removal of some otherwise inhibitory factor. Membrane disruption is often achieved by treatment with detergent to solubihze the enzyme. One example is the so-called microsomal glucose-6-phosphatase, an enzymatic activity that is located in the lumen of the endoplasmic reticulum but becomes trapped as a latent activity in microsome vesicles upon mechanical disruption of cells. 

Heavy metals stimulate or inhibit a wide variety of enzyme systems (16, 71, 72), sometimes for protracted periods (71, 73). These effects may be so sensitive as to precede overt toxicity as in the case of lead-induced inhibition of 8 ALA dehydrase activity with consequential interference of heme and porphyrin synthesis (15, 16). Urinary excretion of 8 ALA is also a sensitive indicator of lead absorption (74). Another erythrocytic enzyme, glucose-6-phosphatase, when present in abnormally low amounts, may increase susceptibility to lead intoxication (75), and for this reason, screens to detect such affected persons in lead-related injuries have been suggested (76). Biochemical bases for trace element toxicity have been described for the heavy metals (16), selenium (77), fluoride (78), and cobalt (79). Heavy metal metabolic injury, in addition to producing primary toxicity, can adversely alter drug detoxification mechanisms (80, 81), with possible secondary consequences for that portion of the population on medication. 

More recently, isotopic labeling experiments have assumed a major role in establishing the detailed mechanism of enzymic action. It was shown that alkaline phosphatase possesses transferase activity whereby a phos-phoryl residue is transferred directly from a phosphate ester to an acceptor alcohol (18). Later it was found that the enzyme could be specifically labeled at a serine residue with 32P-Pi (19) and that 32P-phosphoserine could also be isolated after incubation with 32P-glucose 6-phosphate (20), providing strong evidence that a phosphoryl enzyme is an intermediate in the hydrolysis of phosphomonoesters. The metal-ion status of alkaline phosphatase is now reasonably well resolved (21-23). Like E. coli phosphatase it is a zinc metalloenzyme with 2-3 g-atom of Zn2+ per mole of enzyme. The metal is essential for catalytic activity and possibly also for maintenance of native enzyme structure. 

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.

Tsiani, E., E. Bogdanovic, A. Sorisky, L. Nagy, and I.G. Fantus. 1998. Tyrosine phosphatase inhibitors, vanadate and pervanadate, stimulate glucose transport and GLUT translocation in muscle cells by a mechanism independent of phosphatidyli-nositol 3-kinase and protein kinase C. Diabetes 47 1676-1686. 

If beta cells are incubated in media containing 2 mM glucose and then treated with forskolin and/or tolbutamide, there is a small transient increase in insulin secretion. The subsequent addition of CCK8S leads to a very marked first phase of insulin secretion, but causes no sustained increase or second phase of insulin secretion. These results mean that an increase in cAMP alters the Ca2+ sensitivity of the response elements underlying the first phase of secretion. These elements, presumed to be Ca2+-calmodulin-dependent processes including CaM-dependent protein kinases, become more sensitive to activation by Ca2+ either because cAMP acts to enhance the sensitivity of CaM-dependent kinases to Ca2+, or because cAMP inhibits, by an unknown mechanism, the activity of phosphoprotein phosphatases. 

Hyperuricemia. Many patients with glucose 6-phosphatase deficiency have high serum levels of urate. Hyperuricemia can be induced in normal people by the ingestion of alcohol or by strenuous exercise. Propose a common mechanism that accounts for these findings. 

Vanadate (VOj or H2VO4 ) was first recognized in 1979 as having insulin mimetic properties [258]. Since then, vanadate and vanadyl (V ) have been shown to mimic most but not all biological actions of insulin in vitro and to lower blood glucose in streptozotocin-treated rats [259, 260]. Vanadate is a potent inhibitor of phosphotyrosine phosphatases, an interesting activity since the insulin receptor is a tyrosine kinase, and some of the actions of insulin have been proposed to take place via autophosphorylation of the insulin receptor and phosphorylation of cellular substrates on tyrosine residues [261]. Some recent developments on the mechanism and the in vivo activity of vanadate and its derivatives are presented here. 

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