NUTRITION 285 



It is a remarkable fact that the structures of living organisms are composed 

 only of the one series of optical isomers, when there is the possibility of both. 

 The amino-acids are all of the c?-series, the starches and sugars also of the 

 c?-series, and so on. Not only so, but in the use of food-stuffs even for energy 

 purposes, there is a decided preference for the same series. 



Much has been made of this fact in connection with that of the inability 

 of pure chemical methods alone to produce anything but optically inactive 

 mixtures of the two optical isomers. It is true that all the ordinary means 

 used, for convenience, to separate the two components involve the use of 

 vital agency at some stage or other, but there are several considerations which 

 seem to me to indicate that it is possible to lay too much stress on the argument. 

 We will mention them briefly : 



1. When an optically active substance is synthesised under the agency of 

 an optically inactive catalyst, such as hydrochloric acid, a mixture of both 

 isomers is formed. On the other hand, if the catalyst is an optically active 

 one, only the one isomer is formed ; or, at all events, it is in great preponderance. 



For example, from glucose and methyl alcohol, by the action of hydrochloric acid, the two 

 optically isomeric a- and (3-methyl-glucosides are produced. When the enzyme, emulsin, is 

 used, the /3-glucoside is formed, and by another enzyme, maltase, the a-glucoside. The reader 

 is recommended to consult the paper by Fajans (1910) with respect to the question of 

 asymmetrical catalysis, both by enzymes and other optically active substances. Of course, 

 it may be said that all optically active catalysts were originally produced by vital agency, 

 but the point here is that a chemical substance, not actually living, is able to form new 

 optically active material, provided that it is itself optically active. 



In connection with the production of other asymmetrical compounds by the aid of those 

 already existent, the work of Erlenmeyer (1914) is of importance. By the action of rf-tartarip 

 acid on benzaldehyde in alcoholic solution, a Itevo-benzaldehyde was obtained. Other papers 

 by the same worker may be found in Biochemische Zeitschrift (Band 64). 



2. Although organisms show a preference for one isomer, they are not incapable 

 of using the other one. In the classical experiment of Pasteur (1858) of 

 separating the two tartaric acids by the action of moulds, the o?-acid is used up 

 first, so that the 1-a.cid can be separated from the culture after the rf-acid is used 

 up. But, if the experiment be allowed to continue, it was found that the rotation 

 began again to diminish, owing to the consumption of the 1-a.cid. This fact is 

 sometimes forgotten. Similar cases have been described in the utilisation of 

 amino-acids by fungi. 



These and other cases will be found given in my monograph on Enzyme Action (1913, 2, 

 pp. 152 and 153). It may be mentioned here that this capacity of utilising both isomers is not 

 confined to fungi. Parnas (1911) has shown that the rabbit can utilise /-lactic acid when the 

 inactive mixture is given, although, given alone, this acid itself is toxic and is excreted. 



3. In such cases, the question arises as to how far the use of what we may 

 call the " foreign " isomer is merely for energy purposes, or whether new tissue 

 is formed from it. If the latter, it must obviously be converted into the 

 opposite isomer in some way. This is not an impossible occurrence. If /-leucine 

 be heated with baryta water at 180, it is converted into cK-leucine, or racemised. 

 That is, half of it is changed from the I- to the d- form. As we shall see in 

 the next chapter, there are present in organisms, in the form of enzymes, 

 more active catalytic agents than alkalies or acids. 



4. When we produce artificially, in the laboratory, an inactive mixture, 

 it is not that the chemical reaction is unable to produce the substance that 

 the " vital " reaction does, but that the former produces the opposite isomer in 

 addition. 



5. There is considerable evidence to show that, when an enzyme appears to 

 deal with one optical isomer alone, it is not absolutely inactive with respect to 

 the opposite one ; there are many differences of degree in this respect (see the 

 paper by Fajans, 1910, and the monograph by myself, 1913, 2, pp. 151-155). 



6. It is quite conceivable that optically active products might be obtained 

 by allowing a reaction to proceed under the influence of some asymmetrical 

 external force, say, for example, a photo-chemical reaction under polarised light ; 

 although attempts to do so have not yet met with success, 



