NUTRITION 255 



added would not be more than about 0'08 g. The experiment is referred to 

 at this point merely as an illustration and will be discussed further later on. We 

 may note, however, that the active constituent of milk is not one of the known 

 ones. Milk freed from protein and salts is equally effective and it was shown 

 some time ago by Lunin (1880, p. 37) that a "synthetic" milk, containing all the 

 known constituents, will not serve as a complete diet. 



We will now proceed to attempt some kind of analysis of the different sorts of 

 those special constituents of food, of which only a minute amount is required, but 

 which is essential. At the outset, it is clear that such substances cannot be 

 required for energy purposes directly, so that the part they play must be either 

 in growth or maintenance of cells, or else as hormones or catalysts, the function 

 of which will be explained presently. As regards growth and maintenance, there 

 is evidence that a particular substance may be necessary for the former but not 

 so for the latter. 



1. AS COMPONENTS OF TISSUES 



The tissue proteins, as we have seen (page 103), are composed of a considerable 

 number of different amino-acids, so that this question resolves itself into the 

 capability of the organism to synthesise for itself all these constituents, or whether 

 it has to depend upon the supply of some of them from the outside. The green 

 plant needs no further nitrogenous food than nitrates, so that the constituents of 

 its proteins, which are identical with those of animal protein, must be formed in 

 the organism itself. One of these plant proteins, gliadin from wheat, contains 

 alanine, valine, leucine, phenylalanine, tyrosine, serine, cystine, proline, aspartic 

 and glutamic acids, tryptophane, arginine and histidine. For the chemical 

 constitution of these, the reader is referred to the monographs by Plimmer 

 (1912, 1913). 



An interesting direct proof of synthesis of some of them has been given by Abderhalden 

 and Rona (1905), who grew the mould, Aspergillus, on a culture fluid containing only 

 potassium nitrate as source of nitrogen and cane-sugar as source of carbon. The amino-acids 

 were then separated by the esterification method of Emil Fischer (see Plimmer's monograph, 

 1912, pp. 22, etc.). Glycine, alanine, leucine, aspartic and glutamic acids were separated and 

 identified. The absence of aromatic derivatives is notable. 



Although the plant is capable of such varied synthetic processes, it is remark- 

 able that the power has been lost to a large extent by the animal organism. 

 A certain limited capacity is, however, known to exist, and it is possible that 

 further instances may be found in the future. Glycine can be formed in the 

 higher organism, as was shown by Magnus-Levy (1907). 



Two lines of evidence may be cited. The proteins of milk contain only about 0'3 per cent, 

 of glycine, but a sucking calf can build up 78 g. of tissue protein out of 100 g. of milk. Now 

 this animal tissue protein contains at least 2 '5 g. of glycine. Again, when benzoic acid is 

 given to an animal, it becomes conjugated with glycine to form hippuric acid (benzoyl-glycine), 

 which is excreted by the kidney. A rabbit which was estimated to contain 6*6 g. of glycine, 

 excreted 8 g. of this amino-acid in combination with benzoic acid, when the latter was 

 administered to it. This experiment is, perhaps, not altogether convincing, on account 

 of the uncertainty in the actual content of the rabbit in glycine, although it is improbable 

 that all the glycine-containing tissues should be decomposed in such a way as to give 

 up the whole of their glycine. 



We must admit the possibility of the formation of alanine also in the following 

 way. Embden and Kraus (1912) showed that lactic acid is formed by the liver 

 from glycogen, and Knoop (1910) that the liver can synthesise a-hydroxy-acids 

 with ammonia to form the corresponding u-amino-acids ; from lactic acid, alanine 

 is therefore obtained thus : 



CH 3 CH 3 



! ' I 



CHOH + NH : , = CH.NH 2 + H 2 O 



COOH COOH 



Moreover, Embden and Schmitz (1910) actually found that a liver rich in glycogen 

 showed a considerable formation of alanine when ammonium chloride was added to 

 the perfusion fluid. The liver can also use the ketonic acid for Synthesis of the 



