| Autor: |
Carolyn L. Orthner, Lewis I. Pizer |
| Rok vydání: |
1974 |
| Předmět: |
|
| Zdroj: |
Journal of Biological Chemistry. 249:3750-3755 |
| ISSN: |
0021-9258 |
| DOI: |
10.1016/s0021-9258(19)42537-4 |
| Popis: |
Evidence based upon measurements of (a) the rates of radioactive CO2 production from specifically labeled [14C]glucose, and (b) the intracellular concentrations of key intermediates of the hexose monophosphate shunt indicates that the rate of glucose oxidation by glucose 6-phosphate dehydrogenase is only 40% higher in an Escherichia coli mutant having 10-fold the normal amount of enzyme. These results not only provide evidence for regulation of the first step of the shunt but, more importantly, indicate the limits under which a regulatory mechanism must operate. A comparison of the kinetic constants of partially purified glucose-6-P dehydrogenase with the measured intracellular levels of metabolites indicates that glucose-6-P and NADP+ are saturating and that the rate of the enzyme in the mutant cannot be explained by substrate or coenzyme limitations. Furthermore, the concentration of NADPH in the mutant is not sufficient for the 90% inhibition required to reduce the activity of glucose-6-P dehydrogenase down to a level only 40% higher than that of the wild type strain. A limited survey of possible inhibitors was unsuccessful, and it is concluded that an unknown metabolite must be involved in regulation of glucose-6-P dehydrogenase or that the regulatory properties of the enzyme are altered upon removal from the cell and purification. A comparison of the in vivo and in vitro rates of gluconate-6-P dehydrogenase suggests that this enzyme is also not operating at its maximal velocity in the cell and is regulated. The activity of the third shunt enzyme likewise does not appear to be limited by the availability of substrate or coenzyme. |
| Databáze: |
OpenAIRE |
| Externí odkaz: |
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