OXIDATIVE MECHANISMS IN ANIMAL TISSUES 25 



sidered to be transferred to oxygen in an unbroken chain by the 

 various acceptors. Hydrogen as hydrogen ion may enter or be 

 withdrawn from the acid-base continuum at several places in the 

 chain. Also, the various components of the chain may be classified 

 in two groups according to their ability to transport electrons. The 

 cytochromes can transport only one electron for each cycle of 

 oxidation and reduction of their prosthetic groups. The functional 

 groups of the flavoproteins and the pyridine nucleotides are capable, 

 however, of transporting two electrons for each cycle. Thus the 

 possible interaction of the cytochrome system with a flavoprotein 

 would be one where a two-step, one-electron transfer, with the 

 formation of a semiquinone flavoprotein intermediate, might play 

 an important biological role. The ability of the free flavins to under- 

 go such a stepwise oxidation has been amply demonstrated by the 

 work of Michaelis and Schwarzenbach (21). 



Now a consideration of the pathway just outlined might suggest 

 that tissues contain each of these constituents in somewhat similar 

 amounts. That such is not the case can be seen from Figure 3. 



The cytochrome c, flavin-adenine dinucleotide, and diphospho- 

 pyridine nucleotide content of four tissues from the rat are here 

 plotted in terms of millimoles per gram of wet tissue. The concentra- 

 tion of diphosphopyridine nucleotide in all four tissues is far 

 greater than that of the other two constituents. Cytochrome c is 

 present in lowest concentration in all these tissues. In liver, for 

 example, the concentration of the pyridine compound, expressed on 

 a millimolar basis, is 340 times that of cytochrome c. Since cyto- 

 chrome c transports only one electron per mole, this ratio becomes 

 680:1 when expressed in terms of equivalents. In the other tissues 

 the ratio is lower. From such relationships one might conclude that 

 the cytochrome system is far more efficient in the transport of elec- 

 trons than the other systems. Such indeed may be the case. A dif- 

 ferent explanation, however, is supported by more experimental 

 proof, namely, that the pyridine nucleotides and the flavoprotein 

 systems are involved in reactions other than those concerned in the 

 main oxidative pathway. The known role of the pyridine nucleo- 

 tides in certain anaerobic cycles is discussed elsewhere. With re- 

 spect to the flavoproteins, recent studies have indicated that some of 

 them are concerned in oxidative reactions which do not require the 

 cytochrome system. Substrates such as the d-ammo acids, hypo- 

 xanthine, xanthine, and certain aldehydes are so oxidized. The flavo- 

 proteins concerned in these reactions are unusual in that their 



