THE PROTEINS 



103 



this section of the chemistry of the body by the close relationship which we 

 shall have to study later between the nuclein metabolism of the body and 

 the production and excretion of uric acid. 



The researches of Levene have thrown light on the manner in which these different 

 groups are bound together to form nucleic acid. In the acid obtained from the thynms 

 the carbohydrate group Hexose is joined to a nitrogenous ring compound, forming 

 what is termed a ' nucleoside.' Four of these nucleosides, in thymic acid, join 

 with four molecules of phosphoric acid to form a ' tetra-nucleotide.' The formula 

 provisionally assigned to thymic acid is therefore as follows : 



HO, 



HO 



= PO- - 



HO 



C 5 H 4 N 5 



guanine group 



O =PO -C 6 H 10 4 



(/ 



I 



C 6 H 8 2 -- C 5 H 5 N 2 2 



thymine group 



O 



HO 



C 6 H 8 2 C 4 H 4 N 3 



cytosine group 



O = PO C 6 H 10 4 -C 5 H 4 N 5 



adenine group 

 HO 



Other nucleic acids are simpler in constitution and may be composed of only one 

 or two nucleotide groups. Thus the inosinic acid of muscle is a mono-nucleotide, con- 

 sisting of phosphoric acid linked by a pentose group with hypoxanthine. The defi- 

 nition of a nucleotide would thus be a compound in which a carbohydrate group links 

 a phosphoric acid group with a purine or a pyrimidine group. Nucleic acids are simple 

 or compound nucleotides. The pentose in inosinic acid is d-Ribose. The same 

 pentose occurs in yeast nucleic acid. The nucleic acid of the pancreas, also called 

 Guanylic acid, consists of phosphoric acid linked with guanine by a molecule of d-Ribose. 



(c) THE GLYCOPROTEJNS. In the glycoproteins the prosthetic group is 

 represented by a carbohydrate radical, generally containing nitrogen, such 

 as glucosamine or galactosamine. They are split into their two constituents, 

 protein and carbohydrate radical, on prolonged boiling with dilute mineral 

 acids or by the action of alkalies. They may be divided into the two main 

 groups of mucins and mucoids. 



The mucins play a large part in the animal kingdom as protective agents. 

 They form the slimy secretion which covers the inner surface of the mucous 

 membranes and the outer surface of many marine animals, and is secreted 

 either by the goblet cells of the epithelium or by special groups of cells 

 collected together to form a mucous gland. They may be precipitated from 

 their solutions or semi-solutions by the addition of acfds, and after precipita- 

 tion need the addition of alkalies for their re-solution. They are not coagu- 



