614 METABOLISM 



had not, however, been lost, for when the gliadin was replaced by milk, 

 the animals resumed growth at a very great rate. The capacity to grow 

 had only been inhibited by the inadequate diet, and there was nothing 

 really abnormal about the stunted animals. For example, the reproduc- 

 tive function developed normally, as was shown in the case of a young 

 female rat which, after being fed with gliadin as the sole protein sup- 

 ply for 154 days, was mated and produced four young. Although the 

 mother was still maintained on the gliadin diet, the young rats pre- 

 sented normal growth, for they were living on the milk supplied by the 

 mother, and this milk, because it contained either casein or some other 

 necessary accessory factor (vide infra), was an adequate food. 



After removal from the mother, three of these rats were fed on an arti- 

 ficial diet of casein, edestin and the basal ration, and continued the nor- 

 mal course of growth, but when one of them was placed on the gliadin 

 food mixture it immediately failed to grow properly. It would appear 

 from these experiments that, of the two amino acids that are missing or 

 deficient in gliadin namely, glycocoll and lysine it must be the lysine 

 that is essential for growth. This very important conclusion was fully 

 corroborated by finding that, in young rats stunted by previous gliadin 

 feeding, growth immediately started when lysine was added to the diet 

 and ceased again when the lysine was removed, and so on, the experi- 

 ments being often repeated in various modifications. Mendel and Os- 

 borne call attention to the relatively high percentage of lysine in all 

 those proteins that are concerned in nature with the growth of young 

 animals; thus, it is present in large amounts in casein, lactalbumin and 

 egg vitellin. 



It is particularly in protein of vegetable origin that indispensable units 

 are likely to be missing, the best known of these units being the aromatic 

 amino acids, tyrosiiie and tryptophane; the diamino acid, lysine; and 

 the sulphur-containing acid, cystine. Some animal proteins, such as 

 gelatine, also fail to contain aromatic groups, and are therefore utterly 

 inadequate as foods. 



That the absence of one or two units should render a protein in- 

 capable of maintaining life suggests that a specific role may be taken 

 by certain amino acids in the maintenance of nutritional rhythm; thus, 

 they may be necessary for the elaboration of some hormone or other in- 

 ternal secretion essential to life, such as epinephrine, the active principle 

 of the suprarenal gland. This is an aromatic substance not far removed 

 in its chemical structure from tyrosine (see page 773). It is therefore 

 natural to suppose that the absence of the tryptophane unit in zein is the 

 reason that this protein is incapable of maintaining the initial body weight. 



