90 PHYSIOLOGY 



These results show that all the proteins contain a very considerable 

 proportion of the total number of amino-acids which have as yet been 

 isolated from acid digests of proteins. The differences in various proteins 

 cannot therefore be determined by qualitative differences in their constituent 

 molecules, but must depend on the relative amounts of the amino-acids 

 which are present and on their arrangement in the whole molecule. As regards 

 relative amounts of amino-acids we find very striking differences, i Thus 

 glutamic acid, which forms 8 per cent, of egg albumin and only 1 '7 per cent, 

 of globin (derived from haemoglobin), amounts to 36 '5 per cent, in gliadin, 

 the protein extracted from wheat flour. Striking differences are also notice- 

 able in the relative proportions of the diamino-acids and bases, the so-called 

 hexone bases. Whereas in casein they form about 12 per cent, of the total 

 molecule, in globin they form about 20 per cent. ; and in the protamines, 

 salmine and sturine, about 85 per cent, of the total molecule consists of these 

 bodies. On this account the two last-named bodies have a strongly basic 

 character. From these figures it is evident also that certain of the amino- 

 acids must occur many times over in the protein molecule. Thus in globin, 

 if we assume the presence of one tyrosine molecule, there must be at least 

 thirty-two leucine and ten histidine molecules. On these data the molecular 

 weight of haemoglobin would come out at about 14,000, a figure which agrees 

 with that derived from a study of the amounts of sulphur and iron respec- 

 tively in its molecule. 



THE DISTRIBUTION OF NITROGEN IN THE PROTEIN 



MOLECULE 



Attempts have been made to differentiate among the proteins by a 

 method which, while less laborious than the isolation and recognition of the 

 individual amino-acids, may yet throw some light on the manner in which 

 the nitrogen is combined within the molecule, and on the relative amounts of 

 the different classes of nitrogen groups which may be present. One method, 

 which was devised by Hausmann, is carried out as follows. One gramme of 

 the protein is dissociated by boiling with strong hydrochloric acid. The 

 nitrogen, which has been split off as ammonia and is present in the solution 

 as ammonium chloride, is then distilled off with magnesia and received 

 into decinormal acid, where its amount can be determined by titration. This 

 nitrogen is variously designated as amide nitrogen, ammonia nitrogen, or 

 easily displaceable nitrogen. The remaining fluid, freed from ammonia, is 

 precipitated with phosphotungstic acid. By this means all the diamino-aci< I s 

 and bases are thrown down. The nitrogen in the precipitate is determined 

 by Kjeldahl's method and is called diamino- or basic nitrogen. In the 

 remaining fluid, which contains mono-amino-acids, the total nitrogen, the 

 mono-amino-nitrogen, is determined by Kjeldahl's method. Table I., p. 91, 

 gives some of the results obtained in this manner, and shows that there are 

 considerable differences in the distribution of the different kinds of nitrogen 

 among the various classes of proteins. The method is however only a 

 rough one as compared with the separation of the individual maino-acids. 



