Zwischenferment Glucose-6-Phosphate Dehydrogenase

N4. Noltmann, E. A., Gubler, C. J., and Kuby, S. A., Glucose-6-phosphate dehydrogenase (Zwischenferment). I. Isolation of the crystalline enzyme from yeast. J. Biol. Chem. 236, 1225-1230 (1961). 

In spite of the difference in prosthetic groups, the catalytic activity of TPN-cytochrome c reductase is essentially identical with that of the yeast enzyme. In the presence of excess TPN, reduced with hexosemonophosphate and glucose-6-phosphate dehydrogenase (Zwischenferment) the turnover number was found to be 1140 moles of cytochrome per mole of flavoprotein per minute with the liver enzyme and 1300 with the yeast enzyme. Cytochrome c reductase from liver also resembles the enzyme from yeast in the behavior of the protein component with FAD and FMN (Table 3). In each case the enzyme reconstituted with FMN is more active than with FAD, despite the fact that the natural prosthetic group of the liver enzyme is FAD. 

The mechanism of action of the coenzymes was discovered by Warburg and his co-workers in 1935. They studied the oxidation of glucose-6-phosphate in yeast extracts. This oxidation is caused by the combination of two enzymes, glucose-6-phosphate dehydrogenase (Zwischenferment) and the old yellow enzyme. 

In our assay for TPNH-cytochrome c reductase, we used glucose 6-phosphate and Zwischenferment (yeast glucose 6-phosphate dehydrogenase) to generate reduced TPN, and thus began my interest in the oxidative pathway. This also introduced me to the powerful tool of absorption spectrophotometry, 3 years before the Beckman spectrophotometer. Warburg, who was probably the first to build an instrument for enzyme studies, had discovered the absorption bands of the reduced coenzymes, but he was devoted to his manometric techniques, and it remained for Haas and myself to apply the new technique to the routine assay of an enzyme-catalyzed reaction, which we measured by following the rate of appearance of the reduced band of cytochrome c. 

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