SPECIFICITY or INHIBITION 877 



advisable to use the lowest inhibitor concentrations commensurate with 

 adequate effects on this enzyme or metabolic process. If another enzyme or 

 sequence is inhibited only 10-20%, it makes any interpretation unreliable. 



Examples of Specificity and Lack of Specificity 



Certain inhibitors are really not specific at all in the general sense: 

 mercurials, arsenate, fluoride, chelating agents, heavy metal ions, protein 

 reactants such as diisopropylfluorophosphate, iodoacetate, and others 

 inhibit many systems. Nevertheless, under some cenditions, especially in 

 enzyme extracts where the number of significant enzymes is not great, such 

 inhibitors may be used to exert a specific effect. In work with cells or tis- 

 sues, good specificity is difficult or impossible to achieve. Other inhibitors 

 may be reasonably specific: the monoamine oxidase inhibitors, certain 

 types of cholinesterase inhibitors, fluoracetate, anti-enzym.es, and some 

 substrate analogs if used properly can be specific even in cellular prepa- 

 rations. Inasmuch as there are very few complete summaries of inhibition 

 spectra available, every effort will be made in the succeeding volumes to 

 present all the known actions of each inhibitor so that evaluations of spe- 

 cificity may be more readily made. 



It is frequently assumed, for example, that iodoacetate inhibits specifi- 

 cally the glyceraldehyde dehydrogenase and ~ can block, the glycolytic 

 pathway selectively. Yet a survey of the literature reveals that iodoacetate 

 can significantly inhibit at least 55 enzymes at a concentration of 1 mM 

 or below and, of course, not all the enzymes have been tested. Likewise 

 trivalent arsenicals are often considered to block the oxidation of keto 

 acids selectively, but again the literature shows at least 52 enzymes inhi- 

 bited by 1 n\M or less. This does not mean that these two inhibitors are 

 useless. These figures are presented to emphasize the wide range of possible 

 action in cellular preparations where hundreds of enzymes are operating, 

 and to point out the necessity of using the lowest concentrations possible. 

 Iodoacetate is often used at 1 mM when greater specificity could be achieved 

 at a lower concentration. The oxidation of various cycle substrates and 

 intermediates is inhibited from 35% to 85% by 1 mM iodoacetate (Yang, 

 1957) and even 0.1 mM inhibits the oxidation of the keto acids quite 

 strongly. Inasmuch -aa. 0.2 jto_0.5 mM iodoacetate must usually be used 

 to inhibit glycolysis completely, it is evident that a truly specific effect is 

 difficult to obtain. The oxidation of pyruvate by cardiac tissue is almost 

 completely inhibited by 1 mM iodoacetate (Webb et al., 1949 b) so that 

 similar relations apparently hold in intact cells. Some effort has been 

 made to determine the most specific concentration of iodoacetate for 

 cardiac tissue. It was found that rabbit atria whose rates and contractile 

 amplitudes were depressed with 1 mM iodoacetate could not be stimulated 

 by pyruvate (Webb, 1950 a). A concentration of 0.2 mM exerts a pro- 



