48 ORIGIN AND MAINTEN. OF OPT. ACTIVITY 



organisms are devoid of enzymes suitable for catalyzing 

 transformations of the unnatural isomers. 



Scliulze and Bosshard observed this selective action of 

 one isomer in lower organisms already in 1886 and their 

 data were later confirmed by a great number of authors 

 and especially by Pringsheim (1910). 



Many similar observations were made on mammals. A 

 dog which receives a racemic preparation of leucine 

 (Abderhalden and Samuely, 1906) or of alanine (Abder- 

 halden and Schittenhelm, 1907) consumes preferably the 

 natural isomers and excretes in its urine a large portion 

 of the unnatural amino-acids. The same was observed 

 later by Abderhalden and Tetzner (1935) in rats, rabbits 

 and dogs fed with racemic alanine. 



But another series of facts points out the possibility of 

 the presence, in living protoplasm, of an active mechanism 

 contributing, by a method of correction, toward maintain- 

 ing optical purity and thus toward securing the fixity of 

 the internal medium. 



(c) Krebs' Mechanism. Krebs (1933) who has 

 undertaken extensive investigations on the oxidative de- 

 amination of different amino-acids by tissue slices of liver 

 and kidney from rat, pig, cat, dog and rabbit, discovered 

 the very important fact that, while both optical isomers of 

 amino-acids are deaminated, the unnatural forms of the 

 right steric series are almost always deaminated much 

 more rapidly than the natural ones {cf. Table 8). 



These observations were soon confirmed by Kisch (1935) 

 and by Neber (1936). Some data of Kisch are given in 

 Table's. 



Krebs (1935) assumes that there are two different 

 enzymatic systems one of which catalyzes the deamination 

 of the right and the other that of the left amino-acids. 

 This assumption follows, in particular, from the fact that 

 the deamiiiation of the left amino-acids is inhibited by 

 octyl alcohol, Avhile that of the unnatural isomer of the 

 right series is not affected by octyl alcohol of the same 

 concentration. He further points out that the data con- 



