26 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



acids is against this supposition. Further, the principal diamino acid 

 is histidine, whereas in the other histones it is arginine. It should be 

 noted that haemoglobin contains a considerably greater amount of histi- 

 dine than the other proteins. 



Albumins contain no glycine, whereas globulins contain this amino 

 acid. Their differentiation on physical grounds is thus borne out by 

 chemical analysis. 



No great difference is to be noted between the crystalline vegetable 

 globulins and the vegetable proteins soluble in dilute alkali, but there 

 is a marked difference between these proteins and the alcohol-soluble 

 vegetable proteins. These contain scarcely any glycine, the small 

 amount obtained is probably due to impurity; except in zein the 

 amount of leucine in them in comparison with other proteins is small. 

 The most distinct features of these proteins are the absence of lysine 

 and the presence of an enormous quantity of glutamic acid (except 

 zein) ; the amount of proline is also high, and the amount of arginine 

 is low. It is curious that those amino acids which are absent in zein are 

 present in the protein of maize, which is soluble in dilute alkali. The 

 mixture of these proteins in the grain, therefore, gives all the amino 

 acids present in other proteins. As the gliadins are all so much alike, it 

 would seem that they are the same protein. 



There is no striking peculiarity noticeable with the phosphoproteins ; 

 the two caseinogens examined appear to have the same composition. 

 Vitellin, which has been hydrolysed by three sets of investigators, has 

 given very different results ; the most detailed examination is that by 

 Hugounenq, whose values are lower than those of Abderhalden and 

 Hunter and of Levene and Alsberg ; the latter observers used a purified 

 product, whereas the former used the commercial article. The amount 

 of glycine is small in vitellin and it corresponds in this particular with 

 caseinogen. 



The scleroproteins, which represent a heterogeneous collection of 

 proteins in their physical properties give on hydrolysis, as would be 

 expected, results which support their classification. 



Gelatin contains no tryptophane, cystine or tyrosine, but it contains 

 a large amount of glycine and a considerable amount of proline. It 

 appears to have no similarity to silk-gelatin, which contains so much 

 serine. 



Silk-fibroin is composed of practically only three amino acids, 

 glycine, alanine and tyrosine, and is probably the simplest protein 

 known. It resembles the fibroin of spider's silk very closely, but here 

 the presence of so much glutamic acid is one of the characteristics. 



