Kinetic properties of human placental glucose-6-phosphate dehydrogenase

The kinetic properties of placental glucose-6-phosphate dehydrogenase were studied, since this enzyme is expected to be an important component of the placental protection system. In this capacity it is also very important for the health of the fetus. The placental enzyme obeyed “Rapid Equilibrium Or...

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Bibliographic Details
Published in:The international journal of biochemistry & cell biology 2001-03, Vol.33 (3), p.221-226
Main Authors: Özer, Nazmi, Aksoy, Yasemin, Ögüs, I.Hamdi
Format: Article
Language:English
Subjects:
2,3-Diphosphoglycerate - antagonists & inhibitors
2,3-Diphosphoglycerate - metabolism
Coenzymes - metabolism
Glucose-6-phosphate dehydrogenase
Glucosephosphate Dehydrogenase - metabolism
Human placenta
Humans
Inhibited by 2,3DPGA and NADPH
Kinetics
NADP - analogs & derivatives
NADP - antagonists & inhibitors
NADP - metabolism
Placenta - enzymology
Substrate and coenzyme specificity
Substrate Specificity
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Summary:The kinetic properties of placental glucose-6-phosphate dehydrogenase were studied, since this enzyme is expected to be an important component of the placental protection system. In this capacity it is also very important for the health of the fetus. The placental enzyme obeyed “Rapid Equilibrium Ordered Bi Bi” sequential kinetics with K m values of 40±8 μM for glucose-6-phosphate and 20±10 μM for NADP. Glucose-6-phosphate, 2-deoxyglucose-6-phosphate and galactose-6-phosphate were used with catalytic efficiencies ( k cat/ K m) of 7.4×10 6, 4.89×10 4 and 1.57×10 4 M −1.s −1, respectively. The K mapp values for galactose-6-phosphate and for 2-deoxyglucose-6-phosphate were 10±2 and 0.87±0.06 mM. With galactose-6-phosphate as substrate, the same K m value for NADP as glucose-6-phosphate was obtained and it was independent of galactose-6-phosphate concentration. On the other hand, when 2-deoxyglucose-6-phosphate used as substrate, the K m for NADP decreased from 30±6 to 10±2 μM as the substrate concentration was increased from 0.3 to 1.5 mM. Deamino-NADP, but not NAD, was a coenzyme for placental glucose-6-phosphate dehydrogenase. The catalytic efficiencies of NADP and deamino-NADP (glucose-6-phosphate as substrate) were 1.48×10 7 and 4.80×10 6 M −1s −1, respectively. With both coenzymes, a hyperbolic saturation and an inhibition above 300 μM coenzyme concentration, was observed. Human placental glucose-6-phosphate dehydrogenase was inhibited competitively by 2,3-diphosphoglycerate ( K i=15±3 mM) and NADPH ( K i=17.1±3.2 μM). The small dissociation constant for the G6PD:NADPH complex pointed to tight enzyme:NADPH binding and the important role of NADPH in the regulation of the pentose phosphate pathway.
ISSN:1357-2725
1878-5875
DOI:10.1016/S1357-2725(01)00011-5