110 PHYSIOLOGY 



These hydrolytic changes proceed, however, by a series of stages, 

 so that the intermediate products still present many of the protein 

 reactions. The hydrated proteins are divided into two groups, pro- 

 teoses and peptones. The formation of these intermediate products 

 is especially marked with the proteolytic ferments. Pepsin with 

 hydrochloric acid, the ferment of the 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 peptones 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 mole- 

 cule determines a great similarity of composition between its various 

 disintegration 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 forma- 

 tion, and this heterogeneous character of the molecule renders possible 

 a much greater variety of intermediate 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 repre- 

 sented 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 polypeptides. 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 proteoses and pep- 

 tones are, however, ill-defined bodies. We have at present no satis- 

 factory means of isolating the different members of these groups and 

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

 therefore, like that of the proteins generally, a conventional one, 

 depending on their solubilities and their precipitability by neutral salts, 

 especially ammonium sulphate. Both proteoses and peptones give the 

 xanthoproteic and Millon's reactions common to all proteins, and, like 

 these, are precipitated by such reagents as mercuric chloride, potassio- 

 mercuric iodide, or phosphotungstic acid. On adding excess of 

 caustic potash and a drop of dilute copper sulphate to solutions of 

 either of these classes of bodies, a pink colour is produced which 



