80 



3. KINETICS OF ENZYME INHIBITION 



Inhibitors of this type, if specific, can be used to estimate the relative 

 amounts of enzymes in different preparations or tissues because using the 

 same concentration of inhibitor, various tissues exhibit different degrees 

 of inhibition if their enzyme content varies, according to Fig. 3-15. In fact, 

 the inhibition is inversely iDroportional to the amount of enzyme present. 

 The electron-transport inhibitor, antimycin A, has been used to estimate the 

 relative amounts of succinoxidase in different tissues (Potter and Reif, 

 1952). The antimycin titer was defined as the y of antimycin per gram of 

 tissue to give 50% inhibition: the values ran from around 0.5 for spleen, 

 lung, and tumor, through 1.7 for liver and 3.2 for kidney, to 8.8 for heart. 



INHIBITOR 

 TISSUE 



Cr/GM)- 



FiG. 3-16. Illustration of the determination of the 

 inhibitor titer for a tissue preparation. 



These values correspond quite well to those obtained manometrically on 

 homogenates of the various tissues. Similar results were obtained using 

 other cycle intermediates as substrates, since the step blocked by antimycin 

 is common to many electron-transport systems. If such experiments were 

 done on intact tissue and the inhibition of respiration determined, the anti- 

 mycin titer would not, of course, refer only to succinoxidase, but to the 

 amount of antimycin-sensitive component operating in the tissue. The 

 value of the titer for a particular preparation is arrived at by plotting the 

 inhibition against the ratio of inhibitor added to tissue weight; the midpoint 

 (50% inhibition) on the sigmoid curve represents the titer, just as in the 

 usual chemical titrations (Fig. 3-16). 



