TERMINAL H ELECTRON TRANSPORT SYSTEM 



19 



enzyme lipid results in a 75-95% decrease in cytochrome c reductase activity. Enzymatic 

 activity can be completely and specifically restored upon the addition of the lipid cofactor 

 which is removed by the isooctane extraction or by the addition of tocopherol (Nason and 

 Lehman, 1956). Whether tocopherol (or the unknown lipid cofactor) serves in some 

 indirect fashion or undergoes alternate oxidation and reduction during the course of 

 electron transport — possibly to the free radical or to the tocopheroxide — has, however, not 

 as yet been established. The tocopherol activation of DPNH-cytochrome c reductase has 

 also been observed in yeast, Neurospora, soy bean leaves, rat liver, and beef heart preparations. ' 



Vitamin K has also been implicated in the electron transport scheme. It has been 

 reported that the oxidation of phosphogluconate, glucose-6-phosphate, and ribose phos- 

 phate by yeast autolysates is promoted by the vitamin K analog, menadione, (Strength 

 and Seibert, 1955a). Enzymes capable of reducing menadione are widely distributed in 

 nature (Wosilait and Nason, 1954a). Pyridine nucleotides function in the reduction of 

 menadione in E. colt while in S.faecalis, i o CI, either FAD or FMN is required for menadione 

 reductase activity. Menadione at a concentration of lo^'-io"-* M (Dolin, 1954) also causes 

 an appreciable stimulation of the nitrate reductase system of E. coil (Wainwright, 1955). 



Chance has shown by spectroscopic measurements that the cytochrome b content of several 

 tumors is very low (Chance and Castor, 1952). The addition of cytochrome b and menadione 

 to preparations of a lymphosarcoma markedly increases the rate of cytochrome c reduction 

 by DPNH while cytochrome b or menadione alone are ineffective (Strength and Siebert, 

 1955a, 1955b). The vitamin K antagonist, dicumarol, inhibits cytochrome c reduction by 

 the tumor preparations and the inhibition is reversed by the addition of menadione. The 

 site of the stimulation of cytochrome c reduction by menadione and the inhibition by 

 dicumarol has been localized between cytochrome b and c. The reduction of cytochrome c 

 is also inhibited by BAL (dimercaptopropanol), 2-alkyl-3-hydroxy napthoquinone, (Ball 

 et al., 1947) and antimycin A. Since the reduction of cytochrome c is enhanced by vitamin 

 K, or menadione, it does not appear unlikely that the various inhibitors already mentioned 

 exert their effect on the same component of the electron transport system. It may be noted 

 that the standard potential of the menadione couple lies between that of cytochrome b 

 and cytochrome c. 



Evidence for an additional cytochrome component between cytochrome b and c has 

 also been presented (Wainio and Cooperstein, 1956). This pigment has been referred to 

 as cytochrome Ci and as cytochrome e. It is characterized by an absorption maximum at 

 5530 A. Cytochrome Cj(e) is reduced during the oxidation of DPNH or succinate by 

 purified skeletal or heart muscle preparations. 



Electron transporting particle 



It was observed during the 1930's that sokible enzyme preparations from muscle 

 which catalyzed pyridine nucleotide linked dismutation reactions were unable 

 to react with cytochrome c, with O2, or with dyes such as methylene blue. The 

 addition of a particulate cell fraction restored the abihty of the so called '"mutases" 

 to reduce methylene blue or to consume oxygen. The active component of the 

 particulate fraction was termed the "coenzyme factor" by Green. In 1939, 

 Straub isolated a soluble flavoprotein from heart muscle which could interact 

 with dyes (diaphorase activity) and showed that the prosthetic group was a 

 flavin-adenine-dinucleotide, whileCorran^^a/., (Corran, Green and Straub, 1939), 

 tested the catalytic properties of the enzyme and concluded that it was identical 

 with the "coenzyme factor". 



The problem of the reoxidation of DPNHi has recently been reinvestigated by Green 

 and associates. A particulate enzyme system was extracted from beef heart mitochondria 

 which catalyzed the oxidation of DPNH 2 by molecular oxygen but which catalyzed at a 



^ See Addendum to this Chapter, Note i, p. 123. 



Lilerature p. 124 



