50 ORIGIN AND MAINTEN. OF OPT. ACTIVITY 



cerning the oxidative deamiiiatioii do not contradict the 

 fact of a more rapid consumption by the whole organism 

 of natural isomers of amino-acids. Natural amino-acids 

 can evidently be consumed without deamination ; consump- 

 tion and deamination need not to coincide. 



From the standpoint of the problem of the maintenance 

 of optical purity in protoplasm, Krebs' results are very 

 significant. The study of catalysis has shown that in 

 protoplasm composed of optically pure left isomers of 

 amino-acids the appearance of small quantities of the 

 right forms is inevitable. Krebs' data suggest that the 

 organisms have developed a mechanism for removing the 

 isomers of unsuitable spatial configuration. This mecha- 

 nism would consist in a deamination of the inappropriate 

 right forms as soon as they appear. The right isomers 

 would be transformed into structurally inactive keto-acids 

 identical with those which can be obtained from the left 

 amino acids. In this manner, the organisms would by no 

 means be so helpless in regulating the optical purity of 

 their protoplasm as was assumed by Kuhn and they would 

 possess an active method of correction for securing the 

 fixity of their internal medium. 



It is to be noticed that Ritchie (1933), before any of the 

 researches that we mentioned on oxidative deamination 

 had been made, admitted a priori the possibility of the 

 existence of such a method of correction. He wrote that, 

 while one of the antipodes participates in cell metabolism, 

 the other, which is formed simultaneously but at a much 

 lower rate, almost certainly is removed as soon as it is 

 formed. Ages of evolution would be responsible, accord- 

 ing to him, for the development of such a physiological 

 regulating system. 



One might, at first, be inclined to consider the existence 

 of a special enzymatic system acting on unnatural isomers 

 of amino-acids, as a chance happening without particular 

 significance. But, then, what sense is there in talking of a 

 "specificity" of any enzymatic reactions, and in explain- 

 ing this specificity as a result of a long process of natural 

 selection? {Cf., Eric Holmes, 1937.) 



