1. PERSPECTIVES OF METABOLIC INHIBITION 9 



Onaka in 1911. Although other inhibitions had been observed earlier, these 

 respiratory inhibitions stimulated much further work on the mechanisms 

 by which these substances acted. At this time the enzymes upon which such 

 compounds acted were unknown and the components of the respiratory 

 system were formulated only in very vague terms. 



The kinetics of inhibition produced by a substance binding reversibly 

 with the enzyme were developed in the original publication of Michaelis 

 and Menten in 1913 but it required many years before this type of inhibi- 

 tion was characterized. Quastel and his co-workers in their studies of the 

 inhibition of bacterial dehydrogenases by malonate between 1925 and 1928 

 were able to show that the inhibition was rather specific for succinate oxi- 

 dation. The idea that there was a reversible competition between succinate 

 and malonate for the enzyme site was mentioned but it was not until 1930 

 that Cook stated definitely that a competitive mechanism had been es- 

 tablished. Michaelis with Rona and Pechstein in 1914 while studying maltase 

 and invertase had recognized a second type of inhibition which they re- 

 ferred to as " inhibition without affinity," the inhibitor being said to de- 

 crease the rate of breakdown! of the enzyme-substrate complex, but they 

 did not develop a mathematical expression for this. Haldane in 1930 was 

 the first to derive an equation for this type of inhibition and he gave it the 

 name of "noncompetitive inhibition." Thus the formulation and experi- 

 mental establishment of competitive and non-competitive inhibitions really 

 must be dated as recently as 1930. 



The early theory of Warburg that certain inhibitors, such as cyanide, 

 combined with iron, or other metals involved in metabolism, was not es- 

 tablished until around 1927 when the complexes of cyanide and carbon 

 monoxide with the " Atmungsferment " (cytochrome oxidase) were demon- 

 strated. The mechanism by which the arsenicals act was suggested by Voegt- 

 lin and his group in 1923 from their discovery that the inhibitions were 

 combatted by sulfhydryl compounds. This, coupled with the contemporary 

 interest in sulfhydryl substances (such as glutathione), stimulated by the 

 work of Hopkins, led to the postulate that arsenicals inactivated necessary 

 sulfhydryl compounds in the cell, but it was over 10 years later that enzyme 

 inhibition on the basis of reaction with protein sulfhydryl groups was re- 

 cognized by Hellerman through his study of heavy metals. It is interesting 

 to note that in the " Allgemeine Chemie der Enzyme " of Haldane and Stern, 

 published in 1932, sections are devoted to the following enzyme inhibitors: 

 heavy metals, oxidants (hydrogen peroxide, iodine, and permanganate), 

 amino group reagents, carbonyl reagents (phenylhydrazine), cyanide, sul- 

 fide, pyrophosphate, metal chelators, surfactants, fluoride, arsenicals, and 

 antienzymes. Thus by this time the study of inhibitors had been recognized 

 as a part of enzymology. 



The decade of 1925-1935 might be appropriately termed the classic pe- 



