Histochemie 19, 355 362 (1969) Kinetics of Glucose 6-Phosphatase in Pancreatic Islets as Revealed by Staining Histochenfistry INGE-BERT TXLJEDAL Department of Histology, University of Ume~, Umes Sweden Received June 20, 1969 Summary. Glucose 6-phosphate hydrolysis in pancreatic islets of mice was visualized by the Gomori technique. Staining intensities were quantitatively assayed in a microscope photometer, and enzyme activities were expressed in arbitrary units, after correction of optical densities according to lead sulfide standards. Glucose 6-phosphate was most rapidly split at a pH of about 6.7. At this pH level there was a low rate of fl-glycerophosphate hydrolysis, the ratio between the activities toward the two substrates (40 raM) being 4.0 In contrast to glucose 6-phosphate, fl-glycerophosphate was more rapidly split at pH 5.0 than at pH 6.7. Preincubation of the cryostat sections at pH 5.0 for 15--30 min inactivated the glucose 6-phosphate-splitting activity. Inactivation of the enzyme activity toward glucose 6-phosphate also occurred during brief fixation of the sections in glutaraldehyde or formalin. The apparent K m for glucose 6-phosphate was 1--5 mM in the islets but in the order of 20 mM in the acinar tissue. Glucose was a potent inhibitor of glucose 6-phosphate hydrolysis, the apparent Km being strikingly increased by the sugar. These results support previous biochemical evidence for the presence of glucose 6-phosphatase in the pancreatic islets of mice. The kinetics of the enzyme in the cryostat section are furthermore consistent with the hypothesis that glucose 6-phosphatase is part of the fl-cell's glueoreceptor mechanism. Introduction Biochemical evidence has recently been presented for the existence of glucose 6-phosphatase in the pancreatic islets of some mammalian species (TXLJEDAL, 1969). The enzyme kinetics displayed by microsomes isolated from fi-cells of obese-hyperglycemic mice gave rise to the assumption that the enzyme is a physiological regulator of the glucose 6-phosphate level in these cells. In order to evaluate further the significance of glucose 6-phosphatase in pancreatic islets, information about its properties in the non-homogenized cell is desirable. The present histochemical study was undertaken with the aim of elucidating this matter against the background of previous biochemical results. The use of a microscope photometer permitted a quantitative evaluation of the histochemical staining intensities. Materials and Methods Animals. The animals used were adult mice from a stock that has been bred in our labora- tory since 1959. It originates from R. B. Jackson Memorial Laboratories, Bar Harbor Maine, USA, and carries a gene "ob", which in the homozygous state gives rise to a syndrome of obesity and hyperglycemia (INGALLS et al., 1950; HELLMAN, 1965). Free access tv food and water was given throughout the series of experiments. If not otherwise stated, animals of normal phenotype were used.