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Query: EC:3.1.3.9 (glucose-6-phosphatase)
 3,081 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The concentrations of NAD and NADP have been determined in detergent extracts of washed rat liver microsomes. Precautions were taken during the preparation of the microsomes to remove nicotinamide nucleotides from their external surface both by hydrolysis by nucleotide pyrophosphatase (EC 3.6.1.9) and by washing them three times in 0.15 M-Tris/HCl, pH 8.0, to remove soluble proteins which bind these nucleotides. The mannose phosphatase was essentially completely latent, indicating that the microsomes were intact. Assuming these nucleotides are in the cisternae of the microsomes, the concentrations in the cisternae are 240 +/- 25 microM-NAD and 55 +/- 12 microM-NADP. These levels of nucleotides are compatible with both the glucose:NAD+ and the glucose 6-phosphate:NADP+ oxidoreductase activities of hexose phosphate dehydrogenase (EC 1.1.1.47). Since the organ and subcellular distributions of this dehydrogenase and glucose-6-phosphatase are similar, and Pi stimulates the glucose:NAD+ oxidoreductase activity, it is proposed that the combined action of these two enzymes leads to the reduction of both coenzymes in the lumen of the endoplasmic reticulum. A modification of the colorimetric method of Nisselbaum & Green [(1969) Anal. Biochem. 27, 212-217] for the determination of NADP+ is described. Colour formation is linear with the concentration of NADP+ and is sensitive to less than 0.3 nmol of NADP+.
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PMID:The levels of nicotinamide nucleotides in liver microsomes and their possible significance to the function of hexose phosphate dehydrogenase. 282 15

Rat liver microsomes are known to contain a 6-phosphogluconate dehydrogenase which differs from the 6-phosphogluconate dehydrogenase in the soluble fraction. Microsomes which were washed once bind the soluble phosphogluconate dehydrogenase more tightly than they do glucose-6-phosphate dehydrogenase. Microsomes washed three times in 0.15 M Tris-HCl, pH 8.0, contain only the microsomal 6-phosphogluconate dehydrogenase. Two observations show that this dehydrogenase is located in the cisternae. First, this dehydrogenase is inactive in intact, three times washed microsomes. Second, proteolytic inactivation of 6-phosphogluconate dehydrogenase like that of the cisternal enzyme glucose-6-phosphatase requires disruption of the membrane. Under the conditions used, detergent did not affect the proteolytic inactivation of NADPH-cytochrome c reductase, an enzyme located on the external surface. The excellent correspondence between the activations of hexose phosphate dehydrogenase and 6-phosphogluconate dehydrogenase in microsomes at various stages of disruption of the microsomal membrane produced by detergent supports the earlier contention that these two dehydrogenases are reducing NADP in the same region of the microsomes. A similar experiment which shows an exact correspondence between the activations of 6-phosphogluconate dehydrogenase and mannose-6-phosphatase with increasing concentrations of detergent indicates that the activation of the dehydrogenase can be explained solely by the penetration of the substrates to the active dehydrogenase within the microsomes and strongly suggests that the dehydrogenase is catalytically active in the cisternae.
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PMID:The topology of phosphogluconate dehydrogenases in rat liver microsomes. 282 99

The permeability of rat liver microsomes to glucose was investigated in relation to the hexose-6-phosphate dehydrogenase system (EC 1.1.1.47). It was found that glucose-6-phosphate dehydrogenase activity could be assayed with NADP as coenzyme in both untreated and detergent-treated microsomes. However, when glucose was used as substrate, activity was only measurable in detergent-treated microsomes. Moreover, radioactive glucose added to microsomes in a variety of experimental conditions was never taken up by the vesicles. Our results indicate that NADP (or NAD) availability is probably not the reason for the absence of glucose dehydrogenase activity in untreated microsomes but rather membrane impermeability to glucose would account for the complete latency observed. This finding calls for a reevaluation of glucose transport in relation to other enzymes of the endoplasmic reticulum, such as glucose-6-phosphatase.
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PMID:Absence of glucose uptake by liver microsomes: an explanation for the complete latency of glucose dehydrogenase. 818 4

An improved method has been developed for the assay of hexokinase (EC 2.7.1.1) levels in human tissue homogenates. The enzyme is quantitated by the spectrophotometric measurement, at 340 nm, of NADPH formed according to the reaction scheme: [formula: see text] In tissue homogenates a number of enzymes are present which can interfere with the assay by reacting with substrates or products of the assay reactions. In the described procedure hexokinase is assayed directly in homogenates under conditions in which the effect of possible contaminating enzymes (glucose dehydrogenase, EC 1.1.1.47; glucose 6-phosphatase, EC 3.1.3.9; glucose phosphate isomerase, EC 5.3.1.9; 6-phosphogluconate dehydrogenase EC 1.1.1.44; and NADP-reducing enzymes) are eliminated. Precision studies on the assay gave within-day reproducibility of 4.3% (CV) on a tissue having a mean activity of 1.68 U/g of tissue, and day-to-day variability of 15% (CV) for a tissue averaging 1.83 U/g of tissue.
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PMID:An improved assay for hexokinase activity in human tissue homogenates. 976 31