798 15. EFFECTS OF VARIOUS FACTORS ON INHIBITION 



iting reaction to another. They also i)oint out, with justification, that the 

 experimental accuracy must be a good deal greater than it usually is in 

 the literature in order to establish sharp inflections and true critical tem- 

 peratures. 



The // value for beef heart succinoxidase was found to be changed by 

 certain inhibitors by Hadiclian and Hoagland (1939, 1941). Succinoxidase 

 is, of course, a multienzyme system and the results were interpreted in 

 terms of shifts in the limicing reaction step. Cyanide at concentrations 

 between 1.8xl0~^ and 2.4xl0~^ mM (at which the inhibition is around 

 50-70%) raised fi from a normal value of 11,200 to 16,100. They assumed 

 that in the normal sequence the dehydrogenase is the limiting step and is 

 characterized by a // of 11,200, whereas in the presence of cyanide the cyto- 

 chrome oxidase with a // of 16,100 becomes limiting. Selenite, which was 

 believed to inhibit the dehydrogenase component, thus should not change 

 the normal // value but should bring the // of the cyanide-poisoned system 

 back towards the normal value; this was found to occur. Pyrophosphate 

 was found to increase // to 17,500 and it was concluded that it inhibited 

 some step between the dehydrogenase and the cytochrome oxidase. The 

 kinetics of regenerative electron-transport chains are complex and it is 

 difficult in the present state of our knowledge to evaluate accurately such 

 results. For example, it is not easy to understand how an inhibition of only 

 50-70% could completely shift the limiting reaction step from the dehydro- 

 genase to cytochrome oxidase, and it is even more difficult to interpret 

 the return of the // value to normal brought about by selenite when the 

 inhibition produced was not much greater than with cyanide alone. Much 

 more accurate experimental work must be done with such systems before 

 the sites of inhibition can be localized by temperature studies and the 

 mechanisms involved be established. It is quite possible that this approach 

 might become very useful in the study of cellular inhibitions once a firm 

 foundation is laid. 



Before leaving the subject of the temperature effects on inhibition in 

 multienzyme systems it is well to point out that the j)ossibility of critical 

 temperatures and their modification by inhibitors depends on the type of 

 multienzyme system that is being studied. For example, in either conver- 

 gent or divergent branched chains, the chance of observing a sudden in- 

 flection in the Arrhenius plots is very small because one would predict 

 that a gradual change from the // value of one l^ranch to the /i value of the 

 other branch would occur, and that the effect of inliibitors would depend 

 on the degree of inhibition exerted so that any // value between the ex- 

 tremes would be obtained. On the other hand, in irreversible (or jjractically 

 irreversible) monolinear sequences it is possible to envision rather sudden 

 changes in the // values and the appearance of true critical temperatures. 

 In the simi)le chain, A -^ B -> C, the over-all rate of the formation of C is 

 determined either by the rate of reaction 1 or the rate of reaction 2. If the 



