II GLUCOSE, FATTY ACIDS CATABOLISM I5 



a Stepwise one electron transfer reaction in what would otherwise be a two electron 

 transfer reaction. 



A particulate form of succinic dehydrogenase obtained from beef heart mito- 

 chondria has been purified 10-15 ^"^""^ over that of the starting mitochondrial 

 suspension. Flavin, heme, and non-heme iron, are present in the succinic dehydro- 

 genase complex in a ratio of i :4:i6 respectively. 10 to 30% of the flavin is in 

 the form of flavin adenine dinucleotide. (Green et aL, 1955). A soluble form of 

 succinic dehydrogenase has been obtained (Singer et aL, 1956) by extracting rat 

 liver or beef heart mitochondria with an alkaline buffer (Kearney and Singer, 

 1956). Although the enzyme cannot interact with cytochrome c or ferricyanide, 

 the activity can be followed by means of the reduction of phenazine methosulfate. 

 The purified enzyme contained iron and flavin. 



As in the case of pyridine nucleotide dehydrogenases, many other FAD dehydro- 

 genases are known. Some of these are listed in Table 2. Of particular significance 

 in cellular metabolism is the enzyme, DPNH oxidase. It has been shown that 

 DPNH oxidase is a ferroflavoprotein. Several examples of molybdenum contain- 

 ing flavoprotein enzymes are also given in Table 2 (nitrate reductase, aldehyde 

 oxidase, xanthine oxidase). 



C. Terminal Hydrogen and Electron Transport System 



As indicated above, many cell metabolites are oxidized by pyridine nucleotide or 

 flavin dehydrogenases. As a result of these oxidations, DPNHj, TPNH, and 

 FADH2 are generated. If the oxidation of the substrate is to continue, the reduced 

 coenzymes must be reoxidized. A number of devices have been developed by 

 cells to accomplish this. 



I. ''Dismutatwn" reactions and pyridine nucleotide dehydrogenases 



Whenever the oxidation or reduction of two substrates is catalyzed by dehydro- 

 genases requiring the same coenzyme, the oxidation of the first substrate may be 

 coupled with the reduction of the second substrate. Muscle cells have recourse to 

 this mechanism under anaerobic conditions as do fermenting yeast cells (Fig. 6). 

 The DPNH2 generated in reaction i, Fig. 6, is presumably released from the 

 glyceraldehyde-phosphate dehydrogenase and is then bound to the alcohol or 

 lactic dehydrogenase. Following reduction of pyruvate or acetaldehyde (Fig. 6, 

 reactions 2 and 3), the oxidized DPN* is then released from the latter dehydro- 

 genases and may recombine with glyceraldehyde-phosphate dehydrogenase to 

 continue the cyxle. The "dismutation" mechanism for the reoxidation of TPNH2 

 also may take place under certain circumstances : 



gIucose-6-P 

 dehydrogenase 



a) Glucose-6-phosphate + TPN"^ » 6-phosphogIuconate + TPNH2 



malic 

 enzyme 



b) CO2 ^ pyruvate + TPNH, > malate + TPN^ 



Literature p. 124 



