516 11. LOCALIZATION OF THE SITE OF INHIBITION 



most thoroughly in cardiac tissue. The depression of the rate and contrac- 

 tility in rabbit atria produced by iodoacetate can be partially overcome 

 by the addition of pyruvate (Webb, 1950 a). However, this occurred only 

 at lower concentrations of iodoacetate (0.2 mM) and following relatively 

 brief exposures before the depression had advanced too far, while at higher 

 concentrations (1 mM) or after longer exposures, pyruvate had no effect. 

 This can be correlated with the observation that the oxidation of pyruvate 

 by heart slices is completely prevented by 1 mM iodoacetate (Webb et at., 

 1949 b) and that cardiac mitochondrial oxidation of pyruvate is inhibited 

 very strongly by this concentration of iodoacetate (Yang, 1957). The 

 most complete differentiation of inhibition was achieved in rat ventricle 

 in which iodoacetate (0.2 mM for 30 min) completely blocked the 

 contractile stimulation produced by glucose and had little or no effect 

 on the stimulation by pyruvate (Masuoka et al., 1952). The comparison of 

 the effects of glucose and pyruvate in iodoacetate-inhibited preparations 

 will thus give some measure of the selective action of this inhibitor on gly- 

 colysis. 



When one branch of a convergent chain is blocked, the response to the 

 respective substrates will, of course, be different. Metabolism of A and C 



^ > 



B -1^ D -^ E (11-2) 



^ 



will be unaffected and that of B will be inhibited. In such a situation, 

 some care must be taken not to interpret the overcoming of the inhibition 

 by A or C to be a competitive effect with the inhibitor. In a simple cyclic 



A 



X^/ VI 



/ \ (11-3) 



C ^— r B 



Y 



system a block in the step A -> B will l)e circumvented by addition of B but 

 not by the addition of A or C if the formation of Y is being determined. 

 These simple principles may be applied to other and more complex systems 

 and form the basis for this method of localization. 



Spectroscopic Analysis 



The electron transport systems lend themselves readily to spectroscopic 

 study because of the characteristic absorptions of the nucleotides and cy- 

 tochromes, and the differences in absorption exhibited by the oxidized and 

 reduced states of these components. An inhibitor blocking the flow of 



