TERMINAL H • ELECTRON TRANSPORT SYSTEM 21 



Ferroflavoprotein enzyme 



2e" le^ le" 

 DPNH2 > flavin y Fe""" > cyt. c (Fe""*) 



dyes 

 —>■ (2,6 dichlorophenolindophenol) 



To summarize, the DPNHj dehydrogenase exists in at least three different 

 forms: a) as the "closed" complex, DPNH: oxidase, which reacts with molecular 

 oxygen, b) as the form which contains a heme group and which is obtained by 

 tryptic digestion of the former enzyme, and c) as a soluble ferroflavoprotein. The 

 last named form of the enzyme can be further degraded to a simple flavoprotein 

 without iron which is similar to Straub's diaphorase. 



J. The oxidation of succinate, fatty acids, choline, and xanthine 



Electron transport from substrate to the cytochrome pigments need not necessarily 

 be mediated by pyridine nucleotides. Thus, some of the flavin enzymes shown in 

 Table 2 transfer electrons, directly from substrate to the cytochrome system. 

 Succinate is one of the substrates oxidized in this way. An abbreviated electron 

 transport system illustrating the oxidation of succinic acid looks as follows : 



Succinate -^ ferroflavin -^ cyt. b -^ cyt. Ci — ► cyt. c — > cyt. a -^ cyt. aj -^ O2 



The three known acyl-dehydrogenases are also flavoproteins. The transfer of 

 electrons to the cytochrome pigments in this case is mediated by a fourth flavo- 

 protein, the electron transferring flavoprotein (ETF) (Crane and Beinert, 1954; 

 Crane, 1956; Crane and Beinert, 1956). The ETF flavoprotein, in contrast to 

 the G, Yj and Y-, acyl dehydrogenases, interacts with cytochrome c : 



Y2 — > ETF -^ cytochromes -^ O2 



/g 



Flavoproteins are also involved in the oxidation of choline to betaine aldehyde and of 

 xanthine and various aldehydes (but not betaine aldehyde). The oxidation of choline by 

 rat liver mitochondria may be linked with the reduction of cytochrome c (Rothschild et al., 

 1954). The flavoprotein, aldehyde oxidase (Hurwitz and Cooperstein, 1955) of liver is 

 capable of reducing either cytochrome b or cytochrome c. However, in the system studied, 

 the evidence indicated that the flavoprotein interacted with cytochrome oxidase and 

 oxygen without the necessary participation of cytochrome c. A xanthine oxidase occurs in 

 milk, which is exceedingly unspecific as to substrate. It oxidizes aldehydes, DPNH2, 

 pterins, and hypoxanthine as well as xanthine (Mahler and Green, 1954). It is worth 

 noting that milk xanthine oxidase, liver aldehyde oxidase, and nitrate reductase require 

 molybdenum for their activity. 



^. Autooxidizable favoproteins 



Several flavoprotein enzymes have been isolated which lack a inetal component, 

 which do not interact with the cytochromes, but which instead mediate the trans- 

 fer of hydrogen between substrate and molecular oxygen. The amino acid oxidases, 



Literature p. 124 



