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      Enzymatic characterization of the pancreatic islet-specific glucose-6-phosphatase-related protein (IGRP).

      The Journal of Biological Chemistry

      analysis, Animals, RNA, Messenger, metabolism, genetics, Proteins, Mice, Obese, Mice, Inbred C57BL, Mice, pharmacology, Metals, Male, enzymology, Liver, Islets of Langerhans, Insects, physiopathology, Hyperglycemia, Hydrogen-Ion Concentration, Humans, antagonists & inhibitors, Glucose-6-Phosphatase, Gene Expression, Free Radical Scavengers, drug effects, Enzyme Activation, Dimethyl Sulfoxide, Diabetes Mellitus, Type 2, Coloring Agents, COS Cells, Buffers, Baculoviridae, Rosaniline Dyes

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          Glucose is the main physiological stimulus for insulin biosynthesis and secretion by pancreatic beta-cells. Glucose-6-phosphatase (G-6-Pase) catalyzes the dephosphorylation of glucose-6-phosphate to glucose, an opposite process to glucose utilization. G-6-Pase activity in pancreatic islets could therefore be an important factor in the control of glucose metabolism and, consequently, of glucose-dependent insulin secretion. While G-6-Pase activity has been shown to be present in pancreatic islets, the gene responsible for this activity has not been conclusively identified. A homolog of liver glucose-6-phosphatase (LG-6-Pase) specifically expressed in islets was described earlier; however, the authors could not demonstrate enzymatic activity for this protein. Here we present evidence that the previously identified islet-specific glucose-6-phosphatase-related protein (IGRP) is indeed the major islet glucose-6-phosphatase. IGRP overexpressed in insect cells possesses enzymatic activity comparable to the previously described G-6-Pase activity in islets. The K(m) and V(max) values determined using glucose-6-phosphate as the substrate were 0.45 mm and 32 nmol/mg/min by malachite green assay, and 0.29 mm and 77 nmol/mg/min by glucose oxidase/peroxidase coupling assay, respectively. High-throughput screening of a small molecule library led to the identification of an active compound that specifically inhibits IGRP enzymatic activity. Interestingly, this inhibitor did not affect LG-6-Pase activity, while conversely LG-6-Pase inhibitors did not affect IGRP activity. These data demonstrate that IGRP is likely the authentic islet-specific glucose-6-phosphatase catalytic subunit, and selective inhibitors to this molecule can be obtained. IGRP inhibitors may be an attractive new approach for the treatment of insulin secretion defects in type 2 diabetes.

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