Enzymes of Pentose Biosynthesis

Several recent studies propose that DPNH exerts a modulating effect on a variety of enzymes, thus constituting a metabolic control system. We have reinvestigated the reported control of glucose 6-phosphate dehydrogenase (EC 1.1.1.49) of Escherichia coli B by a study of steady state kinetics of the reaction and find no evidence of such control. The reported apparent modulation is explained by the presence of stray light in single beam spectrophotometers which are used for assaying enzyme activity by change in optical density. It is suggested that the proposed control of several other enzymes by DPNH also needs to be reinvestigated.