614 METABOLISM, NUTRITION AND DIETETICS 



suitable quantities of carbo-hydrate and fat in addition, the glycin 

 contained in certain of the body proteins must therefore have been 

 produced in the body itself. We have already- seen (p. 580) that 

 for the synthesis of hippuric acid after the administration of benzoic 

 acid, glycin is necessary, and the quantity of hippuric acid which 

 can be thus produced is so great that it is impossible to suppose 

 that it all comes from glycin preformed in the body or from glycin 

 in the food substances. It may accordingly be taken as proved 

 that the tissues have the power of synthesizing at least this one of 

 the amino-acids (amino-acetic acid), reckoned among the protein 

 ' building stones. ' It is said that if the casein has been hydrolysed 

 by acid, the products will not preserve nitrogen equilibrium, per- 

 haps because the acid has broken up all the polypeptides (p. 2), 

 some of which the cells may need as the starting-point of protein 

 synthesis. This, however, is uncertain. 



Lysin also appears to be capable of being synthesized in the body, 

 and protein foods free from lysin, or containing only a trace of it, 

 may yet be adequate for nutrition and growth. Prolin, too, is not 

 indispensable, and this is of special interest, for the amino-acids 

 hitherto mentioned as capable of being built up in the tissues are 

 all more or less directly related to each other, being derivatives of 

 the series of saturated fatty acids. The task of changing one of 

 these into another in which the food is deficient may, therefore, be 

 considered a comparatively easy one. But prolin has no obvious 

 relation to most of the other amino-acids. 



It is a-pyrrolidin carboxylic acid, 



CHg CHj CH 2 CHg 



CH 8 CH.COOH i.e., pyrrolidin, CH 2 CH 2 



NH NH 



in which H in one of the CH 2 groups is replaced by carboxyi (COOH) . 

 It has been suggested that prolin may be formed in the body from 

 glutaminic (amino-glutaric) acid, which by loss of a molecule of water 

 can be made to yield a-pyrrolidon carboxylic acid. Thus, 



CH 8 .CH 2 .CH.COOH CH 2 CH t 



COOH NH 2 -H 2 O = CO CH.COOH 



NH 



Glutaminic acid. a-pyrrolidon carboxylic acid. 



By reduction the latter compound might be changed into prolin. 



With proteins deficient in certain other amino-acids a totally 

 different result has been obtained. Gelatin, for example, contains 

 most of the amino-acids and other groups which compose the body 

 proteins, but tyrosin, cystin, 'and tryptopha*ne are lacking in the 



