480 CHEMICAL AGENTS 



in cell-free preparations. The poisoning of certain iron enzymes, e.g., 

 cytochrome oxidase, by cyanide or azide presumably involves chelate 

 bonds with the iron atom. Enzymes known to depend on metals for 

 their activity are inhibited by chelating agents: polyphenoloxidases by 

 diethyldithiocarbamate, azide, and other compounds able to complex 

 with copper (184, 319, 405), zinc metalloenzymes by oxine, a,«'-dipyri- 

 dyl, and o-phenanthroline (420). The specificity of chelating agents 

 is, however, so low at biological pH levels that inhibition by them does 

 not as a rule suffice to identify the metal active for a given enzyme or 

 enzymatic system. 



In the systems just mentioned, it is not known whether the chelating 

 agent combines with the metal in situ on an enzyme or coenzyme or 

 competes with the catalyst for the limited amount of metal present. 

 Inhibition of dialkylfluorophosphatase by 8-hydroxyquinoline and 

 ethylenediaminetetraacetic acid (EDTA) appears to arise from removal 

 of calcium from the enzyme (299); other enzymes dependent upon cal- 

 cium for activity or stability are also inhibited by EDTA (86, 139). 



Finally, a few examples of in vivo effects of chelating agents may be 

 mentioned: inhibition of synthesis of phenolic compounds by Phyco- 

 myces blakesleeanus (48), prevention of the transformation of Brucella 

 abortus from the smooth to the non-smooth form (68), and inhibition 

 of glutamic acid assimilation by Staphylococcus aureus (117). In the 

 two last-mentioned instances, it is believed that the biologically active 

 metal is manganese, but the locus of the manganese chelate — within 

 the cell or outside it — cannot be determined. Several metal complex- 

 ing agents, including cyanide, sodium dimethyldithiocarbamate, and 

 ethylenediaminetetraacetic acid, cause the release of subcellular par- 

 ticulates from cells of Candida albicans (284). 



Although it is usually found that in any series of chelating agents 

 many compounds are toxic (174, 180), and many empirically discov- 

 ered toxicants turn out to be chelating agents (7, 60, 433), the associa- 

 tion is not uniform, presumably because both the stability of the com- 

 plex and its molecular architecture are factors in toxicity or in the 

 ability of the molecule to reach the site of toxicity. Chelation alone, 

 in other words, does not necessarily confer toxicity (8, 264). The best 

 example of a close association between toxicity and chelation is in the 

 series of compounds related to oxine: only those that complex metals 

 are bacteriostatic (10, 11). 



The association of toxicity and ability to chelate was at first thought 

 to signify only that the chelating agent deprives the cell of essential 

 metals (461, 462). Several subsequent developments, however, make 

 it seem unlikely that metal starvation per se is the primary mechanism 



