PROTEINS 177 



than the nucleoproteins and the chromoproteins, the composition 

 and properties of which have been discussed in previous chapters. 

 The nucleoproteins undoubtedly occur in the embryos of many, if 

 not all, seeds. 



3. Derived Proteins. Representatives of the various types of 

 derived proteins are undoubtedly found as temporary inter- 

 mediate products in plants, both as products of hydrolysis pro- 

 duced during the germination of seeds and as intermediate forms 

 in the synthesis of proteins. So far as is known, however, they 

 do not occur as permanent forms in any plant tissues. They have 

 been prepared in large numbers and quantities, by the hydrolysis 

 of the natural proteins and the artificial synthesis of polypeptides. 



In the present state of our knowledge concerning the func- 

 tioning of the proteins, no significance in the physiology of plant 

 life, or metabolism, is to be attached to the particular type of 

 protein material which it contains, at least so far as the simple 

 proteins of the cytoplasm are concerned, 



DIFFERENCES BETWEEN PLANT AND ANIMAL PROTEINS 



A much larger variety of protein materials is found in animal 

 tissues than in plants. This is undoubtedly because different 

 animal organs perform so much more varied physiological func- 

 tions than do those of plants. Three groups of simple proteins, 

 the histones, the protamines, and the albuminoids, which are quite 

 common in animal tissues, are entirely unknown in plants. Fur- 

 ther, conjugated proteins of greater complexity and more varied 

 structure are found in animal tissues, especially in the brain, 

 nerve-cells, etc., than in plants. 



Plant proteins, in general, usually contain larger proportions 

 of proline and of glutamic acid than are found in animal proteins; 

 also more arginine than is found in any of the animal proteins 

 except the protamines, which contain as high as 85 per cent of this 

 amino-acid. 



Of the twenty-five plant proteins which have thus far been 

 hydrolyzed and studied from this standpoint, all contained leucine, 

 proline, phenylalanine, aspartic acid, glutamic acid, tyrosine, 

 histidine, and arginine; two gave no glycine; two others, no 

 alanine; four contained no lysine; and one, no tryptophane. 

 Zein, the principal protein of corn contains no glycine, lysine, or 



