24 OPTICAL ACTIVITY OF BIOL. MATERIAL 



very rare but it is particularly significant. Since sugars 

 do not racemise when boiled in water, it seems that the 

 racemic state does not result from the process of isolation 

 but that the optically inactive forms actually enter into the 

 composition of living systems. The origin of racemic 

 sugars in living organisms is by no means clear. Neuberg 

 (see Fiirber, Nord and Neuberg, 1920) remarks that it 

 might not be a mere accidental fact that the two racemic 

 sugars found are just arabinose and galactose. 



To conclude, among the primary substances, all the 

 amino-acicls, the lecithins and the majority of important 

 sugars such as glucose, fructose and many others are 

 always present in protoplasm in the optically pure state. 



4. Asymmetry of Secondary Constituents of Proto- 

 plasm. As one passes from primary to secondary sub- 

 stances, the optical purity loses its obligatory character. 

 This is particularly evident in organic acids which repre- 

 sent intermediate products of metabolism. Their origin 

 and their signification is still a source of controversy, 

 especially in plants. Whether, in the latter, the forma- 

 tion of organic acids is related to the metabolism of the 

 amino-acids, or whether they represent a stage in the 

 carbohydrate cycle cannot be decided. When the organic 

 acids begin to appear, they are optically pure, as if bear- 

 ing some birth marks from the primary substances, but as 

 soon as they separate from the primary asymmetric sys- 

 tem, beginning perhaps to play the role of storage material, 

 they assume the character of racemic compounds. 



The experimental data on which these conclusions are 

 based are principally those of Ruhland and of his school.^ 

 Ruhland and Wetzel (1929), and later Schwarze (1932) 

 observed that, in the leaves of different plants, malic acid 

 is found especially in the two forms: laevorotatory and 

 racemic (Table 3). 



1 The data of the Leipzig school and, particularly, the analytical part of 

 the work were severely criticized by Bennet-Clark (1937). But, as far as 

 optical activity is concerned, Euhland's data are reliable. Enzymatic race- 

 misation of malic acid in plants, according to Bennet-Clark, was observed also 

 by Naylor (unpublished Thesis, Manchester University, 1935). 



