EFFECTS ON ELECTRON TRANSPORT SYSTEMS 473 



Ways in Which Quinones May Modify Electron Transport 



Quinones may either stimulate or depress the rate of electron flow in 

 the total sequence, or differentially alter the rates in different segments 

 of the sequence, even occasionally increasing the flow rate in one region 

 and decreasing it in another. There is probably no group of so-called in- 

 hibitors so apt to stimulate metabolism as the quinones. 



(A) Directly inhibit enzymes of the sequence. The inhibition of some 

 dehydrogenases has been discussed and other examples may be found in 

 Table 5-2, and quite possibly more distal enzymes are occasionally at- 

 tacked, even the cytochromes not being immune. The mechanism of the 

 inhibition might be reaction with SH groups, but other mechanisms must 

 also be considered (see page 445). 



(B) Compete ivith or displace natural or endogenous quinones. Such an ac- 

 tion would not necessarily lead to inhibition, inasmuch as the exogenous 

 quinone may function in electron transfer even more effectively than the 

 endogenous quinones. The best way to demonstrate such a mechanism is 

 to deplete the system of its natural quinones, and then add back both the 

 natural and the test quinones at different concentrations, as in any exper- 

 iment designed to establish whether an effect is competitive or not. 



(C) Serve as simple electron donors or acceptors. Hydroquinones can feed 

 electrons into an electron transport sequence, while quinones can drain 

 them out of the normal channel of flow: 



^Q -> O, 



/ 

 QH, 



The hydroquinones formed in the latter case might pile up or be directly 

 oxidized by oxygen. The rates of electron flow in different segments of the 

 chain may thus be influenced quite differently. If one is measuring total 

 oxygen uptake, the final result will depend on a variety of factors. If one 

 is measuring the rate of reduction of some acceptor, such as a dye, quinones 

 might exert an apparent inhibition by diverting the flow away from the 

 dye: 



dye 



/ 



SH2 -^ Xi 



\ 



Q 



Indeed, the quinones may accept electrons directly from a substrate, as 

 we have noted for NAD(P)IT, and it is often important to correct for non- 



