10 1. PERSPECTIVES OF METABOLIC INHIBITION 



riod in the history of enzyme inhibition, inasmuch as it was during this 

 interval that the actions of many of the most important inhibitors were 

 studied. It was also the period when inhibitors began to be used as tools 

 to elucidate metabolic pathways and to probe the dependence of cell func- 

 tions on metabolism. Before 1925 no exact sites and mechanisms of inhi- 

 bition were known and, indeed, the concepts of metabolic blockade were 

 vague. A cursory reading of the publications on enzymology and metabolism 

 before 1925 clearly shows that this branch of enzymology was relatively 

 undeveloped, despite the many reports of the actions of inhibiting sub- 

 stances on various systems. Actually, it required the isolation of enzymes 

 and the mapping of metabolic pathways before inhibitors could be rationally 

 studied. It is a well-known fact that the characterization of the compo- 

 nents of several metabolic pathways was aided by the use of inhibitors 

 whose sites of action were simultaneously established. Although there were 

 hundreds of publications before 1925 on the miscellaneous effects of what we 

 now recognize as enzyme inhibitors, the fundamental idea of a substance 

 producing a relatively specific block in metabolism was not conceived as 

 yet. Also the concept that inhibitors might be used to investigate the na- 

 ture of enzymes was in its infancy. 



Let us conclude this historical summary by outlining some of the major 

 contributions made during this decade. The establishment of competitive 

 inhibition, the importance of sulfhydryl groups, and the complexing of 

 cyanide with iron have already been mentioned. The localization of the 

 action of carbon monoxide to cytochrome oxidase and the demonstration 

 of the light sensitivity of the complex by Warburg and Negelein in 1928 

 led to a more accurate characterization of this enzyme whose existence 

 was known only from indirect evidence. Schwartz and Oschmann in 1925 

 reported that bromoacetate prevented the formation of lactate in muscle 

 and this was followed in 1930 by the work of Lundsgaard with iodoacetate. 

 These observations stimulated the subsequent work on glycolysis. Between 

 1931 and 1935 the glycolytic pathway was mapped and the sites of action 

 of iodoacetate, fluoride, and arsenate were determined through the work 

 of Dickens, Lipmann, Lohmann, Braunstein, Harden, and Meyerhof. The 

 mechanism by which phlorizin induces a " diabetic " state of glucosuria 

 was worked out in 1933 by Lundsgaard, who found that this inhibitor in- 

 terfered with phosphorylations in the renal tubular cells. During 1934 and 

 1935 the action of dinitrophenol in stimulating respiration while simulta- 

 neously depressing function was noted and the study of the mechanisms 

 involved was initiated. The localization of the site of arsenical action was 

 carried a step forward by Lipmann in 1933 when he showed that a block 

 was produced in the oxidation of the keto acids. The determination of in- 

 hibitor sites in the oxidative sequence by Quastel, using spectroscopic tech- 

 niques, and Keilin's work on the actions of cyanide and azide on the cyto- 



