Chemical Constituents of the Animal Body. 263 



ing again the reactions of glycyl glycine. This substance 

 reacts with formaldehyde according to the equation 



CH(NH 2 ) CONH-CH 2 COOH + HCHO 



= CH(N : CH) CO NH CH 2 COOH 



It yields on hydrolysis two molecules of glycine, which 

 react with formaldehyde to give two molecules of the 

 methyleneimino-derivative, CH(N : CH)-COOH, which con- 

 tain twice as many carboxyl groups as the glycyl glycine 

 from which it has been derived. They will require, therefore, 

 twice as much standard alkali to neutralize. If now a pro- 

 tein is submitted to hydrolysis, and aliquot parts of the 

 hydrolysis mixture are removed from time to time, treated 

 with formaldehyde in excess (half the volume of 40 per cent, 

 aldehyde solution) and titrated with standard alkali solu- 

 tion in the presence of phenolphthalein, it will be found 

 that the mixture becomes more and more acid as hydro- 

 lysis proceeds and the peptide linkages undergo scission. 

 This method, which is due to Sorensen, is a convenient 

 one for following the course of the digestion of a protein, 

 and is largely employed in the study of the action of the 

 so-called proteoclastic ferments, to which more detailed 

 reference will be made later. 



It will be seen from the above description that it is 

 possible to obtain hydrolysis products from the proteins 

 varying in complexity from simple amino-acids up to pro- 

 ducts of relatively high molecular weights, which still 

 possess many properties in common with the original 

 proteins, such as the non-diffusibility through parchment 

 membranes. These latter substances have been derived from 

 the original protein by the scission of a comparatively small 

 number of amino-acid groups, and yield on complete hydro- 

 lysis a relatively large number of amino-acids. Complex 

 polypeptides have been obtained synthetically containing 

 as many as sixteen amino-acids conjugated together, which 



