TPNH-cytochrome c reductase

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. 

In the first category are the metalloflavoproteins (Fe) such as DPN-cytochrome reductase and TPN-cytochrome reductase, enzymes which Hoffmann-Ostenhof suggests should be called DPNH cytochrome-c-transelectronase and TPNH cytochrome-c-transelectronase, respectively. 

Cytochrome Reductase of Yeast. A third yeast flavoprotein was isolated by Haas and co-workers. This was called cytochrome reductase because its rate of reaction with cytochrome c is over 150,000 times as great as the rate of cytochrome c reduction by old yellow enzyme. Its rate of reaction with oxygen is only 8 per cent that of old yellow enzyme. The prosthetic group is FMN. Reduction occurs only with TPNH. Another enzyme, DPN-cytochrome reductase, was partially purified in Hogness laboratory. 

The enzymatic composition of microsomes is still obscure. Nevertheless we know that they contain important amounts of cytodurome-reductases. They must therefore presumably play some part somewhere in the stage of electron transfer between DPNH or TPNH and cytochrome-c. 

DPN is the electron acceptor for three, and TPN for one of the five oxidases of the citric-acid cycle. The nature of the primary electron acceptor for succinate is still unknown. The reaction of DPNH and TPNH with one of the cytochromes (c) is known to be mediated by reductases containing riboflavin in their prosthetic groups. Both DPN and TPN cytochrome reductases have been isolated in homogeneous state from an-... 

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