Investigations into the properties of 6-PG dehydrogenase in cell free extracts ofEscherichia coli revealed a pit optimum at pH 9.5 with a sharp decline on both sides of the optimum. The addition of 1.0 β’ 10 -3 ~ MgCI 2 produced maximal activity, whereas higher concentrations caused inhibition. The Km values were 2.5 β’ 10 -4 ~ for 6-phosphogluconate and 2.5 β’ 10 -~ ~ for NADP+ as substrate. The enzyme was extremely stable for at least 5 hours if stored at 4 ~ C in Tris-NaC1-MgC12 buffer at pH 7.5.6-PG dehydrogenase activity was shown to be proportional to cell free extract concentration over the range 0--0.3 mg protein. An assay method based on the new optimal conditions has been established and has been shown to be 33o/o more sensitive than a number of commonly used methods.
In a previous paper (SLY and DOV.LL]~, 1968) a comparison of the properties of ghicose-6-phosphate dehydrogenase (G-6-P dehydrogenase, E.C. 1.1.1.49) in cell free extracts showed that the enzyme of Zymomonas mobilis exhibited a three-fold higher specific activity than the corresponding enzyme of Escherichia coll. In contrast to Escherichia coli which possesses both EMP and HMP pathways, Zymomonas mobilis depends solely on an Entner-Doudoroff (ED) mechanism for the dissimilation of glucose, and thus enhanced G-6-P dehydrogenase activity would be expected. This observation has stimulated some speculation on the possibility of predicting the presence of glucose degradative pathways by the detection of certain enzymes. 6-phosphogluconate dehydrogenase (6-PG dehydrogenase, EC 1.1.1.44) is an important key enzyme in glucose metabolism (Do~LL~, 1968) and the possibility exists that the determination of this enzyme in conjunction with the determination of G-6-P dehydrogenase activity may provide a means of distinguishing between organisms using the HMP and/or ED pathways. This paper deals with an investigation of the properties of 6-PG dehydrogenase in cell free extracts of Escherichia coli.
* l~einem hochverehrten Lehrer I-Ierrn Professor A. Rn~r~L zum 80. Geburtstage.