612 METABOLISM 



tains in adequate proportion all of the necessary units to meet the pro- 

 tein demands of growing tissues. In the case of casein, for example, 

 one of the units, namely, glycocoll, which is the simplest of all the 

 amino acids, is entirely missing, and another, cystine, which is a sul- 

 phur-containing amino acid, is present only in small amount. The ab- 

 sence of glycocoll, however, is not of importance, because the organism 

 can manufacture it for itself (see page 663). In the case of cystine, 

 which the tissues can not manufacture themselves, the deficiency has to 

 be made up for by feeding an excess of casein so as to cover the needs 

 of the tissues for this amino acid. By so doing a superabundance of 

 most of the other units will be ingested, and this superabundance will 

 entail the destruction and excretion of the useless amino acids, a process, 

 however, which is conducted in such a way as to permit of the utilization, 

 by the organism, of a part of the energy which the cast-off amino acids 

 contain (see page 699). It is, therefore, not entirely a wasteful process. 



When the supply of casein is limited, on the other hand, the curve 

 of growth becomes subnormal, because an insufficient supply of cystine 

 is thereby offered (Fig. 184). Similar results have been obtained in the 

 case of edestin, a protein from hempseed. This contains an insufficiency 

 of the diamino acid, lysine. Fed in abundance, edestin gave a normal 

 curve of growth, but when fed in insufficient amount the curve failed to 

 ascend properly (which, however, it could be made to do by adding some 

 lysine to the edestin). 



There is a large group of proteins which fail to permit of any growth 

 no matter in what amounts they may be added to the basal ration. These 

 include: legumelin (soy bean), vignin (vetch), gliadin (wheat or rye), 

 legumin (pea), legumin (vetch), hordein (barley), conglutin (lupine), 

 gelatine (horn), zein (maize), phaseolin (kidney bean). The adequacy 

 to maintain growth of any of these pure proteins varies according to 

 the deficiency in their amino acids. In the case of gliadin of wheat or 

 rye, glycocoll is -lacking, and lysine is present only in small amount (see 

 table). The absence of glycocoll can not, however, as we have already 

 seen in the case of casein, explain the inadequacy of gliadin as a foodstuff 

 for growth (Curve II in Fig. 183) . It must be the lysine that is at fault. A 

 still more deficient protein is the zein of maize. With this as the only 

 protein added to the basal diet, the curve of growth actually descends 

 (Curve III of Fig. 183), thus indicating that the animal is starving and 

 must soon succumb. The missing units in this protein are glycocoll, 

 lysine and tryptophane (see table on page 609), and it is very signifi- 

 cant that if the latter two amino acids are supplied along with zein, an 

 almost normal curve of growth will result. Some improvement can 

 even be brought about by giving tryptophane alone ; that is to say, the 



