Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UNIPROT:P56851 (epididymal)
 11,273 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

A study was made of the effect of alimentary deficiency of niacin and of exogenous nicotinamide (500 mg/kg) on the activity of the key enzymes of the pentose phosphate pathway and NADP-dependent malate and isocitric dehydrogenase in the epididymal fatty tissue of rats. It is established that vitamin depletion in the animals' body brings about a 3-fold decrease in the content of NADP+ and a 1.7-fold decrease in the content of NADPH, a 43-percent inhibition of the activity of glucose 6-phosphate dehydrogenase and a 39-percent reduction with respect to transketolase. Nicotinamide suppresses the activity of glucose 6-phosphate dehydrogenase by 35% and that of isocitric dehydrogenase by 40% 12 hours after intraperitoneal injection. It is suggested that NADPH production in the fatty tissue of rats undergoes appreciable changes under the effect of niacin.
 ...
PMID:[The role of niacin in regulating the pentosophosphate pathway and production of NADP-H in fatty tissue]. 253 4

1. The distributions and rates of transfer of carbon isotopes from a selection of specifically labelled ketosugar-phosphate substrates by exchange reactions catalyzed by the pentose and photosynthetic carbon-reduction-pathway group-transferring enzymes transketolase, transaldolase and aldolase have been measured using 13C-NMR spectroscopy. 2. The rates of these exchange reactions were 5, 4 and 1.5 mumol min-1 mg-1 for transketolase exchange, transaldolase exchange and aldolase exchange, respectively. 3. A comparison of the exchange capacities contributed by the activities of these enzymes in three in vitro liver preparations with the maximum non-oxidative pentose pathway flux rates of the preparations shows that transketolase and aldolase exchanges exceeded flux by 9-19 times in liver cytosol and acetone powder enzyme preparations and by 5 times in hepatocytes. Transaldolase was less effective in the comparison of exchange versus flux rates: transaldolase exchange exceeded flux by 1.6 and 5 in catalysis by liver cytosol and acetone powder preparations, respectively, but was only 0.6 times the flux in hepatocytes. 4. Values of group enzyme exchange and pathway flux rates in the above three preparations are important because of the feature role of liver and of these particular preparations in the establishment, elucidation and measurement of a proposed reaction scheme for the fat-cell-type pentose pathway in biochemistry. 5. It is the claim of this paper that the excess of exchange rate activity (particularly transketolase exchange) over pathway flux will overturn attempts to unravel, using isotopically labelled sugar substrates, the identity, reaction sequence and quantitative contribution of the pentose pathway to glucose metabolism. 6. The transketolase exchange reactions relative to the pentose pathway flux rates in normal, regenerating and foetal liver, Morris hepatomas, mammary carcinoma, melanoma, colonic epithelium, spinach chloroplasts and epididymal fat tissue show that transketolase exchange may exceed flux in these tissues by factors ranging over 5-600 times. 7. The confusion of pentose pathway theory by the effects of transketolase exchange action is illustrated by the 13C-NMR spectrum of the hexose 6-phosphate products of ribose 5-phosphate dissimilation, formed after 30 min of liver enzyme action, and shows 13C-labelling in carbons 1 and 3 of glucose 6-phosphate with ratios which range over 2.1-6.4 rather than the mandatory value of 2 which is imposed by the theoretical mechanism of the pathway.
 ...
PMID:Exchange reactions catalyzed by group-transferring enzymes oppose the quantitation and the unravelling of the identify of the pentose pathway. 847 19

The pentose phosphate pathway plays several key roles in metabolism including supply of biosynthetic carbon skeletons and reducing power. Previous research has focused on determining the fluxes through the reactions of this pathway using carbon-labeled substrates and models that make certain assumptions about the reversibility of the transketolase and transaldolase reactions in the nonoxidative pathway. These assumptions, however, have resulted in inconsistencies between the predicted carbon label distributions using these models and those determined experimentally. A general metabolic reaction network model developed in this paper and applied to the pentose phosphate pathway not only incorporates reaction reversibility but also accounts for the effect of individually varying extents of reaction reversibility on labeled carbon fractional enrichment values for intermediate metabolites. In addition, an algorithm is presented that can be used to calculate the three individual transaldolase and transketolase extents of reversibility. The results of this method show that varying extents of reaction reversibility have an observable effect on the metabolite carbon label distributions which can in turn affect flux calculation for other parts of the metabolic network such as the tricarboxylic acid cycle. In addition, the observability of reversibility extent and accuracy of flux calculations depend on the particular choice of metabolite carbon enrichments measured. In particular, [6-13C]hexose 6-phosphate and [4-13C]erythrose 4-phosphate carbon enrichment values resulting from [1-13C]glucose feeding contained more information as compared to those from ribose 5-phosphate. This analysis was applied to literature data of metabolite carbon labeling that resulted from supplying either 13C- or 14C-enriched substrates to several cell types growing under various conditions. The specific activities of metabolite carbon atoms taken from rat epididymal adipose tissue, goosefish islet cells, Corynebacterium glutamicum, and Escherichia coli supplied with either [2-14C]glucose or [1-13C]glucose demonstrate how reversibility is present in the pentose phosphate pathway and the extents of reversibility can be estimated from labeled carbon data sets.
 ...
PMID:Effect of reversible reactions on isotope label redistribution--analysis of the pentose phosphate pathway. 954 50