THE PROTEINS 



. 



molecules of water to the protein molecule (hydrolysis). This actio 

 earned to its end results m the production of the amino-acids which we ha' 



already dealt with. 



These hydrolytic changes proceed by a series of stages - > thai 

 intermediate products still present many of the protein reactions Th, 

 hydrated proteins are divided into two groups, proteoses and peptones. 

 The formation of these intermediate products is especially marked with the 

 proteolytic ferments. Pepsin with hydrochloric acid, the ferment of t 

 gastric juice, for example, only breaks down the protein molecule as far as the 

 proteoses and peptones. Trypsin also gives rise to both proteoses and pep 

 tones as intermediate products. The action of these ferments on proteins is 

 in fact closely analogous to the action of diastase on the great polysaccharide 

 molecule of starch. In this case, as intermediate products we have first 

 dextrins of various complexity, secondly maltose, and finally, if the ferment 

 maltase be also present, dextrose. The monotony of the starch molecule 

 determines a great similarity of composition between its various disintegra- 

 tion products. It may be regarded as an anhydride of many (100 or more) 

 molecules of a hexose, and the intermediate stages in this hydrolysis are also 

 hexoses and their anhydrides. The protein molecule is distinguished by the 

 variety of the groups which enter into its formation, and this heterogeneous 

 character of the molecule renders possible a much greater variety of inter- 

 mediate products than we find in the starches. Thus a protein molecule may 

 consist of the groups A, B, C, D, E, F, G, H, &c. When hydrolysis occurs 

 it may result in the immediate splitting off, say, of part of group A, while 

 the residue breaks up into a series of proteoses whose composition may be 

 represented as ABF, ABC, DFG, BDEF, &c. With further hydrolysis these 

 groups are broken into still smaller ones, and the penultimate stages of the 

 hydrolysis will be polypeptides similar to those which have been synthetised 

 by Fischer from the ultimate products of protein hydrolysis. No sharp 

 dividing line can be drawn between the proteoses, peptones, and poly- 

 peptides. Of the last group we have already seen that the higher members 

 give the biuret reaction as well as the other protein reactions, if the necessary 

 groups, e.g. tyrosine, tryptophane, are present in the molecule. The prote- 

 oses and peptones are, however, ill-defined bodies. We have at present no 

 satisfactory means of isolating the different members of these groups and 

 obtaining them in a state of chemical purity. Their classification is there- 

 fore, like that of the proteins generally, a conventional one, depending on 

 their solubilities and their precipitability by neutral salts, especially ammo- 

 nium sulphate. Both proteoses and peptones give the xanthoproteic and 

 Millon's reactions common to all proteins, and, like these, are procipitjit.-il 

 by such reagents as mercuric chloride, potassio- mercuric iodide, or phospho- 

 tungstic acid. On adding excess of caustic potash and a drop of dilute 



