INTRODUCTION 9 



points to one isoraer being present in proteins and not the other, but 

 these data are of little value since other enzymes may hydrolyse those 

 combinations not attacked by trypsin. Definite information is given 

 by the isolation and identification of polypeptides resulting from in- 

 complete hydrolysis of protein. The separation of these compounds 

 also affords further proof of the acid amide structure of the protein 

 molecule. Several polypeptides have now been obtained from proteins, 

 and we know, therefore, the arrangement of some of these units in the 

 proteins from which they have been derived. The position of certain units 

 in a protein molecule is shown by the action of reagents on the protein 

 followed by complete hydrolysis ; nitroarginine has been prepared from 

 nitroclupeine ; the guanidine grouping in arginine is therefore probably 

 not used in the linking together of amino acids. An amino grouping 

 which is not in combination with another amino acid will be attacked 

 by nitrous acid ; subsequent hydrolysis and analysis will show the 

 absence of this amino acid. Proteins are readily racemised by treat- 

 ment with alkali, but the racemisation is not complete ; subsequent hy- 

 drolysis gives a mixture of optically active and optically inactive amino 

 acids : according to Dakin the optically active acids will stand at the 

 end of a chain (p. 87). 



There is no reason to suppose that the amino acids in a protein 

 are combined in any other way than as acid amides, for we have no 

 evidence of the presence of ester or ether linkages between oxyamino 

 acids nor of the imide form of combination. But on this supposition an 

 explanation is still required for the difference in action between the pro- 

 teoclastic enzymes. Pepsin does not hydrolyse any of the synthetical 

 polypeptides, trypsin hydrolyses the majority and the other enzymes 

 seem to hydrolyse all of them. The difference in action between trypsin 

 and the other enzymes is probably accounted for by a different arrange- 

 ment of the units. Fischer supposed that the chain of units in the syn- 

 thetical polypeptides was not long enough ; yet length of chain hardly 

 accounts for the difference in action between pepsin and trypsin on 

 proteins. Pepsin appears to have some special function. So far only 

 simple anhydrides of amino acids have been prepared and their 

 behaviour to enzymes has scarcely been investigated. The presence 

 of anhydride rings in the protein molecule may be the explanation of 

 the difference. Not only will they account for a difference in action, 

 but also they may give a reason for the stability of certain scleropro- 

 teins to hydrolysis by trypsin until they have been acted upon by 

 pepsin. We may imagine a protein to be built up as follows : 



