64 PHYSIOLOGICAL CHEMISTRY 



exception present only in traces and with the exception of iodine are 

 probably not constituents of the protein molecule. 1 



Of all the various elements of the protein molecule, nitrogen is by far 

 the most important. The human body needs nitrogen for the continua- 

 tion of life, but it cannot use the nitrogen of the air or. that in various 

 other combinations as we find it in nitrates, nitrites, etc. However, in 

 the protein molecule the nitrogen is present in a form which is utilizable 

 by the body. The nitrogen in the protein molecule occurs in at least 

 four different forms as follows: 



I. Monamino acid nitrogen. 



II. Diamino acid nitrogen or basic nitrogen. 



III. Amide nitrogen. 



IV. A guanidine residue. 



The actual structure of the protein molecule is still unknown, and we 

 have as yet no means by which its molecular weight can be even approxi- 

 mately established. The many attempts which have been made to 

 determine this have led to very different results, some of which are given 

 in the following table: 



Globin =1500016086 



Oxyhemoglobin =1480015000 16655 16730 



Of these figures, those given for oxyhemoglobin deserve the most 

 consideration, for these are based on the atomic ratios of the sulphur 

 and iron contained in this substance. The simplest formula that can 

 be calculated from analyses of oxyhemoglobin, namely, 



serves to show the great complexity of this substance. 



The decomposition 2 of protein substances may be brought about by 

 oxidation or hydrolysis, but inasmuch as the hydrolytic procedure has 

 been productive of the more satisfactory results, that type of decomposi- 

 tion procedure alone is used at present. This hydrolysis of the protein 

 molecule may be accomplished by acids, alkalis, or superheated steam, 

 and in digestion by the action of the proteolytic enzymes. The char- 

 acter of the decomposition products varies according to the method 

 utilized in tearing the molecule apart. Bearing this in mind, we may 

 say that the decomposition products of proteins include proteases, pep- 

 tones, peptides, carbon dioxide, ammonia, hydrogen sulphide, and amino 



1 Some investigators regard these elements as contaminations, or constituents of some 

 protein substance combi 



2 The terms "degradation," "dissociation," and "cleavage," are often used in this con- 



non-protein substance combined with the protein. 



2 Thi 

 nection. 



