Purification and characterization of a cofactor that controls the oxidative phase of the pentose phosphate cycle in liver and other tissues of rat

The regulation of the pentose phosphate pathway: Remember Krebs

The changes in gene expression and posttranslational modifications of enzymes are comprised in the concept of "coarse control" of the oxidative phase of the pentose phosphate pathway. However, these changes are slow in its implementation. The defensive mechanism against oxidative stress requires a most rapid response, impossible to achieve with coarse regulation systems. Recently, it has been suggested that a quick acceleration mechanism of G6PD activity could be produced by the reduction of NADPH-inhibition of G6PD. The hypothesis opens new ways on possible mechanisms for rapid modulation that could be in accordance with results obtained in the 70s by Krebs. These results seemed outdated in view of the subsequent research. However, they deserve to be re-assessed at present.

The regulation of the oxidative phase of the pentose phosphate pathway: new answers to old problems

There is a paradox in the oxidizing phase of the phosphate pentose pathway that has not yet been solved. The flow through the pathway is reduced in basal conditions; however, it must rise notably when a NADPH supplement is required. The paradox consists of the strong inhibition that the NADPH exerts on the both dehydrogenases of the pathway, especially on the regulating enzyme glucose-6-phosphate dehydrogenase (G6PD). Theoretically, in anabolic situations, the increase of gene expression of G6PD and 6-phosphogluconate dehydrogenase can induce a rise in the production of NADPH, which would cause the immediate inhibition of the enzyme and a drastic flow reduction. However, increasing the flow through oxidative phase of the pentose phosphate pathway (OPPP) has been experimentally demonstrated in many physiological states. However, this situation will be resolved if the NADPH metabolized or otherwise sufficient NADPH is sequestered to relax the inhibition of the dehydrogenases of OPPP and to maintain high ratio of NADPH/NADP(+) needed to ensure the reducing environment of the cell cytoplasm and the contribution of NADPH for anabolic processes. In 1974, the presence of a protein capable of reversing the inhibition of G6PD by NADPH was detected; however, to date, this paradox remains undisclosed. This review deals with the possibility that such reverting action might be similar to the activity of a protein named HSCARG, which is responsible for the abduction of NADPH, thus keeping a portion of the coenzyme away from the catalytic action and, simultaneously, the immune response through the NF-κB (nuclear factor kappa light-chain enhancer of activated B cells) system. The model has many similarities with the hypothesis proposed some 40 years back on the reversion of G6PD inhibition by NADPH.

The modulation of the oxidative phase of the pentose phosphate pathway in mouse liver

The glucose-6-phosphate dehydrogenase from mouse liver is fully inhibited in vitro by physiological concentrations of NADPH. This suggests that the oxidative phase of the pentose phosphate pathway requires some deinhibitory system. In order to investigate regulation of the pentose phosphate pathway, various parameters (intermediate concentrations, mass-action ratios of reactions, etc.) were measured in liver from control mice and from meal-fed mice. Assays were also carried out to detect any molecules causing the reverse of glucose-6-phosphate dehydrogenase inhibition by NADPH. The liver of meal-fed mice show greater glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase activities. They also had greater concentrations of several metabolic intermediates and triglycerides than the control animals (P < 0.001). These results prove that the diet increases the flow of the pentose phosphate pathway in a lipogenic sense. The glutathione reductase does not change with the diet, suggesting that this enzyme does not participate in the modulating process. Unlike rat liver, no molecules causing the reverse of glucose-6-phosphate dehydrogenase inhibition by NADPH were detected. These data suggest that the increase of flow of the pentose phosphate pathway during lipogenesis is obtained by an increase in enzyme synthesis.

Kinetic properties of hexose-monophosphate dehydrogenases. II. Isolation and partial purification of 6-phosphogluconate dehydrogenase from rat liver and kidney cortex

6-Phosphogluconate dehydrogenase (6PGDH) from rat-liver and kidney-cortex cytosol has been partially purified and almost completely isolated (more than 95%) from glucose-6-phosphate dehydrogenase activity. The purification and isolation procedures included high-speed centrifugation, 60-75% ammonium-sulphate fractionation, by which both hexose-monophosphate dehydrogenases activities were separated, and finally the protein fraction was applied to a chromatographic column of Sephadex G-25 equilibrated with 10 mM Tris-EDTA-NADP buffer, pH 7.6, to eliminate any contaminating metabolites. The kinetic properties of the isolated partially purified liver and renal 6PGDH were examined. The saturation curves of this enzyme in both rat tissues showed a typical Michaelis-Menten kinetic, with no evidence of co-operativity. The optimum pH for both liver and kidney-cortex 6PGDH was 8.0. The Km values of liver 6PGDH for 6-phosphogluconate (6PG) and for NADP were 157 microM and 258 microM respectively, while the specific activity measured at optimum conditions (pH 8.0 and 37 degrees C) was 424.2 mU/mg of protein. NADPH caused a competitive inhibition against NADP with an inhibition constant (Ki) of 21 microM. The Km values for 6PG and NADP from kidney-cortex 6PGDH were 49 microM and 56 microM respectively. The specific activity at pH 8.0 and 37 degrees C was 120.7 mU/mg of protein. NADPH also competitively inhibited 6PGDH activity, with a Ki of 41 microM. This paper describes a quick, easy and reliable method for the separation of the two dehydrogenases present in the oxidative segment of the pentose-phosphate pathway in animal tissues, eliminating interference in the measurements of their activities.

Purification, characterization and kinetic mechanism of glucose-6-phosphate dehydrogenase from mouse liver

1. Glucose-6-phosphate dehydrogenase (G6PDH EC 1.1.1.49) from mouse liver has been purified 1100-fold by extraction, ion-exchange chromatography on DE-52, absorption chromatography on Bio-Gel HTP and gel filtration through sepharose 6 HR 10/30. The purified enzyme showed a single band in silver stained SDS-PAGE. 2. The native and subunit molecular weight were 117 and 31 kDa respectively. 3. The kinetic studies and the patterns obtained from the inhibition by-products suggest that the enzyme follows an ordered sequential kinetic mechanism. 4. The reduced Km values for the substrates favour the operativity of the enzyme. The "fine control" of the enzymatic activity was exerted by the NADPH, whose Ki is several fold lower than the in vivo concentration.

Primed pentose cycle activity supports production and elimination of superoxide anion in Kupffer cells from rats treated with endotoxin in vivo

Glucose use and pentose cycle activity were determined in freshly isolated rat Kupffer cells 3 h after an i.v. injection of Escherichia coli endotoxin (0.1 mg/kg body weight), by using [1-14C], [6-14C] and [2-3H]glucose. Endotoxin treatment in vivo caused a 5-fold increase in the basal glucose uptake in Kupffer cells. Pentose cycle activity was elevated from 8.7 to 13.6 nmol/h per 10(7) cells after endotoxin. In vitro treatment of the cells from saline- and endotoxin-treated animals with phorbol ester (10(-6) M) increased pentose cycle activity 2-fold and 8-fold, respectively. Phorbol ester caused a 50% increase in glucose uptake in both groups. t-Butyl hydroperoxide (0.5 mM) caused a similar increase in pentose cycle activity as phorbol ester. Glucose oxidation in the Krebs cycle was also doubled after endotoxin. KC from endotoxin-treated animals produced O2- spontaneously, and were primed to produce additional large amounts of O2- upon phorbol ester treatment. Addition of t-butyl hydroperoxide inhibited O2- production by Kupffer cells. Depletion of glutathione by N-ethylmaleimide (0.1 mM), or inhibition of NADPH oxidase by diphenyliodonium (0.1 mM) inhibited both the pentose cycle activity and the O2- production. Increasing the concentration of exogenous glucose in the cell medium elevated the glycolytic rate, while pentose cycle flux was not affected either under basal conditions or following subsequent challenges by phorbol ester or t-butyl hydroperoxide. Our data suggest that the endotoxin-induced elevated glucose use in Kupffer cells is accompanied by a primed state of the pentose cycle. This condition supports superoxide and macromolecule synthesis and could also represent a potentiated protective mechanism against oxidative cellular injury during bacterial infections.