Acyl coenzyme A inhibition of Leuconostoc mesenteroides glucose-6-phosphate dehydrogenase: a comparison of the TPN and DPN linked reactions

Phosphorylation of glucose by a guanosine-5'-triphosphate (GTP)-dependent glucokinase in Fibrobacter succinogenes subsp. succinogenes S85

Cell extracts of Fibrobacter succinogenes subsp. succinogenes S85 phosphorylated glucose with a GTP-dependent glucokinase. The enzyme showed little activity with ATP (12% of that with GTP). Of other phosphate donors tested, only dGTP and ITP gave high glucokinase activities. Dialyzed extracts required Mg+2 and K+ for maximal activity. In potassium phosphate buffer, glucokinase showed maximum activity at pH 7.5 with glucose-6-phosphate dehydrogenase as the coupling enzyme. In this assay, glucokinase was active with glucose (100%), 2-deoxy-D-glucose (40%), and mannose (20%). Partially purified glucokinase had a molecular weight of 82,000 and a pI of 4.82. Double-reciprocal plots of substrate concentration versus velocity were linear and the enzyme had apparent Km values of 55 microM for glucose and 72 microM for GTP. Dialyzed cell extracts of Fibrobacter intestinalis C1A also contained a GTP-dependent glucokinase that showed little activity with ATP. Potassium also stimulated the activity of this enzyme. These results suggest that this unusual glucokinase may be characteristic of the genus Fibrobacter.

Glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides: revised kinetic mechanism and kinetics of ATP inhibition

The kinetic mechanisms of the NAD- and NADP-linked reactions catalyzed by glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides were examined using product inhibition, dead-end inhibition and alternate substrate experiments. The results are consistent with a steady-state random mechanism for the NAD-linked and an ordered, sequential mechanism with NADP+ binding first for the NADP-linked reaction. Thus, the enzyme can bind NADP+, NAD+, and glucose 6-phosphate, but the enzyme-glucose 6-phosphate complex can react only with NAD+, not with NADP+. This affects the rate equation for the NADP-linked reaction by introducing a term for a dead-end enzyme-glucose 6-phosphate complex. The kinetic mechanisms represent revisions of those proposed previously (C. Olive, M.E. Geroch, and H.R. Levy, 1971, J. Biol. Chem. 246, 2047-2057) and provide a kinetic basis for the regulation of coenzyme utilization of the enzyme by glucose 6-phosphate concentration (H.R. Levy, and G.H. Daouk, 1979, J. Biol. Chem. 254, 4843-4847) and NADPH/NADP+ concentration ratios (H.R. Levy, G.H. Daouk, and M.A. Katopes, 1979, Arch, Biochem. Biophys. 198, 406-413). The kinetic mechanisms were found to be the same at pH 6.2 and pH 7.8. The kinetics of ATP inhibition of the NAD- and NADP-linked reactions were examined at pH 6.2 and pH 7.8. The results are interpreted in terms of ATP addition to binary enzyme-coenzyme and enzyme-glucose 6-phosphate complexes.

Kinetic determination of serum glucose by use of the hexokinase/glucose-6-phosphate dehydrogenase method

We have developed a kinetic fixed-time approach for the quantitative determination of serum glucose by use of the hexokinase/glucose-6-phosphate dehydrogenase method. To achieve a large measuring range, we have apparently increased the Michaelis constant of glucose-6-phosphate dehydrogenase through addition of the competitive inhibitor ATP. By this means, serum samples with glucose concentrations up to 55.5 mmol/l could be analyzed without pre-dilution. The method has been adapted to the ENI GEMSAEC analyzer and to the LKB 2086 Mark II analyzer. It yielded satisfactory results with regard to precision. A comparison of the kinetic procedure with the manual end-point method showed good agreement. No interferences from hemoglobin, bilirubin, or lipemia have been observed.

Selective denaturation of several yeast enzymes by free fatty acids

The denaturation of eight purified yeast enzymes, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, 3-phosphoglycerate kinase, alcohol dehydrogenase, beta-fructosidase, hexokinase and glucose-6-phosphate isomerase, promoted under controlled conditions by the free fatty acids myristic and oleic, is selective. Glucose-6-phosphate dehydrogenase (D-glucose-6-phosphate:NADP+ 1 oxidoreductase, EC 1.1.1.49) is extremely sensitive to destabilization and was studied in greater detail. Results show that chain length and degree of unsaturation of fatty acids are important to their destabilizing effect, and that ligands of the enzyme can afford protection. The denaturation process results in more than one altered form. These results can be viewed in the perspective of the possibility that amphipathic substances, and in particular free fatty acids, may play a role for enzyme degradation in vivo, by initiating steps of selective denaturation.

Characterization of the fatty acid-sensitive glucose 6-phosphate dehydrogenase from Pseudomonas cepacia

The adenosone 5'-triphosphate-insensitive glucose 6-phosphate dehydrogenase from Pseudomonas cepacia has been found to be strongly inhibited by long-chain fatty acids and their acyl coenzyme A esters, suggesting that an important role of this isoenzyme might be to provide reduced nicotinamide adenine dinucleotide phosphate for reductive steps in fatty acid synthesis. The enzyme, which has been redesignated the fatty acid-sensitive glucose 6-phosphate dehydrogenase, has been purified to homogeneity using affinity chromatography with nicotinamide adenine dinulceotide phosphate-substituted Sepharose as a key step in the purification. The purified preparations were used to study the immunological properties and subunit composition of the enzyme and its relationship to the adenosine 5'-triphosphate-sensitive glucose 6-phosphate dehydrogenase present in extracts of P. cepacia. Although both enzymes were found to be composed of similar size subunits of about 60,000 daltons, immunological studies failed to demonstrate any antigenic similarity between them. Studies of the sedimentation behavior of the fatty acid-sensitive enzyme in sucrose gradients indicated that its apparent molecular weight is increased in the presence of glucose 6-phosphate and suggest that it may exist in an aggregated state in vivo. Palmitoyl coenzyme A, which strongly inhibited the enzyme, failed to influence its sedimentation behavior.