62 



PHYSIOLOGICAL CHEMISTRY. 



when present. When iron, copper, iodine, manganese, or. zinc are 

 present in the protein molecule they are practically without excep- 

 tion present only in traces^ 



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

 by far the most important. The human body needs nitrogery for 

 the continuation 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 pres- 

 ent in a form which is utilizable by the body. The nitrogen in 

 the protein molecule occurs in at least jour 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 approximately 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: 



Serum albumin = 4572-5100-5135 

 Egg albumin = 4900-6542 

 Globin = 15000-16086 



Oxyhsemoglobin = 14800-15000-16655-16730 



Of these figures, those given for oxyhaemoglobin 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 oxyhaemoglobin, namely, 

 Q}58H 1181 N 2 o 7 S 2 FeO 21 o, serves to show the great complexity of this 

 substance. The following formulas which have been proposed for 

 typical protein substances may serve to further impress the fact of 

 the great size of the protein molecule : 



Egg albumin = C 239 H 380 N 58 S 2 O 78 

 Serum albumin = C 450 H 720 N 11G S 6 O 140 



The decomposition 2 of protein substances may be brought about 



1 Some investigators regard these elements as contaminations, or constituents 

 of some non-protein substance combined with the protein. 



2 The terms "degradation," "dissociation," and "cleavage," are often used 

 in this connection. 



