422 



THE CHEMICAL COMPOSITION OF THE BODY [CH. XXVIII. 



estimations having yet been made. Where the sign + occurs, the 

 substance in question has been proved to be present, but not yet 

 determined quantitatively. Among the more striking points brought 

 out are : 



1. The absence of glycine from albumins. 



2. The high percentage of glycine in gelatin. 



3. The absence of tyrosine and tryptophane in gelatin. 



4. The high percentage of the sulphur-containing substance 

 (cystine) in keratin. 



5. The high percentage of glutamic acid in vegetable proteins. 



Emil Fischer, Abderhalden, T. B. Osborne, Levene, van Slyke, and 

 others are attempting to make the list complete, and are devising 

 new and more accurate methods of analysis. Fischer has also 

 discovered the way in which the amino-acids are linked together into 

 groups ; and the culmination of his work will be the discovery of the 

 way in which such groups are linked together to form the protein 

 molecule. The last stage has not yet been reached, but it will be 

 interesting to see how the amino-acids are linked together into 

 groups. 



The groups are termed peptides or polypeptides ; many of these 

 have been made synthetically in his laboratory, and so the synthesis 

 of the protein molecule is foreshadowed. 



We may take as our examples of the peptides some of the simplest, 

 and may write the formulae of a few amino-acids as follows : 

 NH . CH 2 . COOH Glycine 

 ^ . C 2 H 4 . COOH Alaiiine 



NH . C C H 1A . COOH Leucine 



or in general terms 



HNH.R.COOH. 



