ENZYME-SUBSTRATE COMPOUNDS 



it must have a higher flux than the flavoprotein pathway. 

 Thus blockage of the vitamin K pathway would cause respira- 

 tion to fall by about Vs- But uncoupling effects generally cause 

 increases of respiration. Thus the shunt mechanism is at pres- 

 ent inadequate. 



Another question is, of course, whether the P : O value falls 

 by one or whether thyroxin uncoupling gives a continuous 

 decrease of phosphorylative activity to zero (40,44). In the 

 latter case the split respiratory chain is of no great aid in ex- 

 plaining the uncoupling effect. 



The third alternative respiratory chain is proposed by 

 Green et al. (38), on the basis of purified nonphosphorylating 

 DPNH oxidase particles. Their contribution is of unusual 

 interest because it states that Keilin's reduced cytochrome c 

 oxidase "must be considered, . .to be artifactual activities. . . ." 

 and that cytochrome c is "not a part of this electron transfer 

 sequence." These statements must now be regarded as incor- 

 rect in view of recent work in this laboratory by Estabrook and 

 Mackler (339). The closed DPNH oxidase preparation has 

 cytochrome content, steady-state oxidation reduction levels, 

 and turnover numbers of the components that are so similar to 

 those previously reported for a Keilin and Hartree preparation 

 (12) that the mechanism of electron transport from cytochrome 

 c to oxygen in the two preparations can be considered to be 

 identical and in accord with equations (19) and (20). On the 

 other hand, this material is less turbid than Keilin and Hartree's 

 preparations, thus it has been possible to use faster flow appara- 

 tuses than that of Figure 6 in the study of the time sequence of 

 oxidation and reduction of the cytochrome components as well 

 as in the reaction of ferrocytochrome az with oxygen. These 

 results support the chemical representations of the electron 

 transfer processes discussed here and obviate the necessity of 

 formulating alternative respiratory chains or concepts of elec- 

 tron transport. 



335 



