EFFECTS OF ENZYME OR METABOLIC RATE 857 



Althougti quite a different phenomenon, it may be mentioned in connec- 

 tion with the presence of impurities, that insufficient attention has often 

 been paid to the possibility of interfering substances being introduced 

 with the inhibitor. Many commercial preparations of inhibitors are not 

 pure and the effects upon enzyme or metabolic systems may be partially 

 or totally dependent on these impurities. An excellent example of this has 

 been provided recently in a study of yeast alcohol dehydrogenase (Hoch 

 et al., 1960). The inhibition of this enzyme by iV'-methylnicotinamide was 

 shown to be due to a contamination of this substance with Ag+ ions. The 

 inhibition by semicarbazide could also be attributed to its metal ion con- 

 tent. Complete purification of each substance to remove these metal ions 

 led to an elimination of the inhibition. It was pointed out that in using 

 inhibitors that are not so potent, a dependency of the inhibition on the 

 enzyme concentration should lead one to suspect adventitious inhibition 

 by a potently-acting impurity. 



EFFECTS OF ENZYME OR METABOLIC RATE 



The question of w^hether the degree of inhibition in a particular system 

 will depend on the rate at which the system is operating often arises, and 

 is especially important in considerations of the relative effects of inhibitors 

 on tissues with different levels of activity. There are many ways in which 

 the metabolic rate could be altered: changes in the concentrations of sub- 

 strate, coenzymes, cofactors, or acceptors such as oxygen; change in the 

 total enzyme concentration; changes in temperature, pH, or ionic strength; 

 elimination of a limiting step, as in the uncoupling of oxidative phosphor- 

 ylation; the activation of an enzyme, as in the reduction of — S — S— groups 

 to — SH groups; the removal of some product that is slowing the forward 

 reaction; change in the spatial configuration in structurally dependent 

 multienzyme systems; and, in the case of many tissues, a modification of 

 the level of function, as in the stimulation of nerve or muscle. It is, there- 

 fore, difficult to formulate any general laws relating inhibition to meta- 

 bolic rate because in most cases the effect, if any, will depend on the way 

 in which the rate is altered. In every case in which the rate is changed, 

 the direct effect of the rate-altering procedure on the inhibition must be 

 eliminated if the effect of rate alone is to be determined. 



Single Enzymes 



An enzyme for which the Michaelis constant is given by the generalized 

 expression, K^„ = (^2 + ^-i)/^i5 may be inhibited to different degrees by 

 a constant concentration of an inhibitor as the rate is altered. This can 

 occur when some procedure changes ^2 specifically. If, for example, ^2 is 



