28 PHYSIOLOGY CHAP. 



Keturning to the various amino- acids which represent the 

 products of the hydroly tic cleavage of proteins, we must note the 

 important fact that, with the exception of the protamines, all 

 proteins hitherto decomposed contain the] same units. One or 

 other of the amino-acids, e.g. glycine in egg albumin and serum 

 albumin, may be wanting, but these are rare exceptions. 



What differentiates the several proteins among themselves is, 

 on the other hand, the varying quantitative relations of the differ- 

 ent amino-acids which compose the protein molecule. In some 

 proteins, certain special amino-acids, e.g. leucine and more particu- 

 larly glutamic acid, occur in enormous quantities, as in the 

 proteins of plant seeds. There are great differences, again, in the 

 relative proportions of the mono- and di-amino acids ; the latter 

 are found in large quantities in the protamines, while they are 

 almost absent in some of the scleroproteins. 



The histones occupy an intermediate position between the pro- 

 tamines and the coagulable proteins (albumins and globulins). 



From these facts it may be anticipated that we shall before long 

 be able to classify the various groups of proteins on the basis of 

 similar end products. Indeed, from the fact that the same units 

 enter into their constitution, although in different proportions for 

 the different substances, we can even" now to a certain extent 

 perceive how the several alimentary proteins may be converted 

 into the other definite proteins of the animal body. 



It has been objected that the ultimate cleavage products of 

 the artificial hydrolysis of proteins are not really pre-formed as 

 so many units in the protein molecule : but the various data 

 recently acquired meet this objection. The following may be 

 briefly noted : 



(a) In whatever way the hydrolytic cleavage of any protein is 

 effected, whether by acids, by alkalies, or by proteolytic ferments, 

 the final products are approximately the same in quality and 

 quantity. Tryptophane is the sole exception, since it is largely 

 destroyed on hydrolysis by acids. 



(b) Fischer has succeeded in artificially combining two or 

 more molecules of amino-acid, and has thus obtained synthetically 

 the chemical compounds which he terms polypeptides, which in a 

 number of properties have affinity with the natural proteins. The 

 type on which this synthesis has been successfully carried out is 

 represented by the simplest dipeptide, which is known as glycyl- 

 glycine, and which results from the coupling together of two 

 molecules of glycine (or glycocoll) according to the following 

 equation : 



NH 2 .CH 2 .COOH + HNH.CH 2 .COOH = 



glycine glycine 



NH2.CH2.CO.NH.CH2.COOH + H20. ; 



glycyl-glycine 



