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SCIENCE 



[N. S. Vol. XXVII. No. 685 



cally in some respects at least. There is 

 harmony in the fact that all three yield no 

 lysine by hydrolysis, but zein yields only 

 11 per cent, of glutaminie acid, while the 

 other two furnish over 30 per cent, of this 

 amino acid. Further, gliadin and hordein 

 yield only about 2 per cent, of tyrosine, 

 while zein furnishes 10 per cent, of this 

 substance. To any one familiar with the 

 kaown relationship between chemical con- 

 stitution and physiological action as dem- 

 onstrated in the study of drugs, the ques- 

 tion as to what these differences in chem- 

 ical constitution signify in the physio- 

 logical action of three food proteins so 

 closely related otherwise must be a perti- 

 nent one. 



The present-day conception of the pro- 

 tein molecule is that it is a complex of 

 different amino acids variously .joined 

 together. By energetic hydrolysis of the 

 protein the latter is naturally broken apart 

 into simple fragments represented by the 

 monoamino and diamino acids. When the 

 hydrolysis is carefully conducted as by a 

 weak solution of trypsin, various proteoses 

 result as the primary products, i. e., high 

 molecular polypeptides, which by further 

 action of the enzymes may be successively 

 broken down into simpler polypeptides, 

 such as tetra-, tri- and dipeptides. Fischer 

 and Abderhalden* a few years ago obtained 

 a polypeptide in the pancreatic proteolysis 

 of several proteins which was characterized 

 by being composed solely of glycocoll, pro- 

 line and phenylalanine. More recently Os- 

 borne, and Clapp'' obtained in the hydrol- 

 ysis o"f gliadin what appeared to be a 

 crystalline dipeptide composed of proline 

 and phenylalanine. Further, Fischer and 

 Abderhalden" have just described several 



* Zeitsohrift fiir pUysiologischen Chemie, Band 

 XXXIX., p. 81, 1903. 



' Americaii Journal of Physiology, Vol. XVIII., 

 p. 123, 1907. 



' Zentralblatt fiir Physiologic, No. 15, p. 472, 

 1907. 



dipeptides obtained in the partial hydrol- 

 ysis of proteins; notably, glycocoll and 

 1-tyrosine from silk, giycocoU and 1-leucine 

 from elastin and 1-leueyl-d-glutaminic acid 

 from gliadin; all characteristic dipeptides. 

 In these resiilts we see suggested the pos- 

 sibility of a primary cleavage of proteins 

 into dissimilar polypeptides and dipeptides 

 with distinct chemical make-up. If such 

 reactions as these do occur, under the in- 

 fluence, for example, of pepsin or trypsin 

 proteolysis, or even through the agency of 

 the duodenal enzyme erepsin, then it is 

 certainly reasonable to consider whether 

 the individual proteoses or polypeptides 

 formed during gastric and pancreatic di- 

 gestion may not be endowed with different 

 physiological properties. It raises the 

 question whether in the digestion of pro- 

 tein in the gastro-intestinal tract by the 

 enzymes naturally present there a kind of 

 selective cleavage may not occur, in which 

 the various amino acids contained in the 

 protein are split off in special combinations 

 representative of particular lines of at- 

 traction or union. Further, a tendency 

 toward the formation of di- and poly- 

 peptides having a definite composition, 

 assuming it to exist, may furnish a clue 

 to the way in which the synthesis of pro- 

 tein may be accomplished. Obviously, 

 however, there remains to be discovered 

 first the nature of the 29-46 per cent, of 

 the protein not yet represented by known 

 decomposition products. 



The well-known lability of the protein 

 molecule accounts for the ease with which 

 it undergoes hydrolysis, but it is, I think, 

 quite plain that the ordinary cleavage of 

 protein is not the result of a promiscuous 

 breaking down of the molecule. On the 

 contrary, there is satisfactory evidence that 

 certain of the building stones are easily 

 split off, while others are separated with 

 greater difficulty and still others remain 

 combined in large groups or masses. 



