Enzymes 155 



acetone, alcohol, alkali (actually, the optimum pH of the 

 reaction is 12.5-13), very little sensitive to dichromate and 

 Lugol's solution and many other agents which will promptly 

 destroy practically all the known enzymes. However, it is sen- 

 sitive to acids. 



In 1933 Keilin^^ came forward with the theory that the 

 enzyme responsible for the G nadi reaction is identical with 

 cytochrome oxidase. According to this theory, dimethyl-p- 

 phenylene diamine is oxidized by cytochrome. Reduced 

 cytochrome is then reoxidized by cytochrome oxidase in the 

 presence of atmospheric oxygen. Keilin's ideas have become 

 generally accepted, and the designation "cytochrome oxi- 

 dase" has superseded the terms "labile" or "G" indophenol oxi- 

 dase. Since cytochrome oxidase is a rather sensitive enzyme, 

 readily destroyed even by drying and intolerant to formalin, 

 etc., it is obvious that it cannot be responsible for the effects 

 seen in the stable or M nadi reaction. 



First of all, by definition the stable enzyme is not an oxi- 

 dase, since it is effective in the absence of atmospheric oxy- 

 gen. In fact, it is altogether questionable whether enzymatic 

 action is involved in the stable reaction. It is difficult to con- 

 ceive of an enzyme which is resistant to formalin indefinitely, 

 to Lugol's solution, and to dichromate for over 30 minutes. 

 The writer was unable to confirm the sensitivity of the reac- 

 tion to HgCL, reported repeatedly. He could verify the find- 

 ings of Heringa and Ruyter:^^ blood smears after 30 minutes' 

 exposure to a 2 per cent solution of HgCb, followed by re- 

 moval of the metal with Lugol's solution, were indistinguish- 

 able from those given by untreated smears. The exceedingly 

 high pH optimum (around 13) is also an unlikely attribute 

 of an enzyme. Graeff^^ and Katsunuma^^ voiced early opin- 



36. Keilin, D.: Ergebn. d. Enzymforsch., 2:239, 1933. 



37. Heringa, G. C, and Ruyter, J. H. C: Acta brev. Neerland., 5:118, 

 1935. 



38. Katsunuma, S.: Intrazellulare Oxydation und Indophenolblausynthese 

 (Jena: G. Fischer, 1924). 



