Specificity and Ultrastructural Localization of Pancreatic B Cell Glucose-6-Phosphatase

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Abstract
Light microscopic studies demonstrate that rabbit pancreatic B-cell glucose-6-phosphate hydrolyzes pentoses and hexoses phosphorylated at the terminal carbon. There is weak hydrolysis of α glycerophosphate, while fructose, 1–6 diphosphate, 6-phosphogluconic acid, triose phosphates, as well as a variety of other phosphorylated substrates were not hydrolyzed. For fine structural localization of the enzyme, gluteraldehyde fixed tissue was found satisfactory. Reaction product was visualized within the cisternas of the endoplasmic reticulum (E.R.), and nuclear membrane internal to the membranes. These findings parallel the specificity and localizations reported for glucose-6-phosphatase in liver and epididymis. The association of this enzyme with E.R. suggests that it is involved in the mechanism for insulin synthesis while its absence from granules implies that it does not play a role in their release.