752 A MANUAL OF VETERINARY PHYSIOLOGY 



These enter the body as salts of sodium and potassium, and are 

 oxidised, by which means the acid portion is converted into carbonic 

 acid and water, while the bases are set free. These unite with the 

 carbon dioxide of the blood or lymph, and form carbonates. 



The Nitrogenous Bodies. 



Proteins. — The term ' protein ' is applied to a group of organic 

 substances which form the essential bases of all animal and vegetable 

 tissues, both fluid and solid. Chemically, they are characterised 

 by the presence of nitrogen, together with carbon, hydrogen, oxygen, 

 and sulphur. The proportion in which these elements are found 

 is liable to variation. For example : 



Per Cent. 



Carbon ----- 515 to 545 



Hydrogen - - - - &g „ 73 



Oxygen ----- 20-9 „ 235 



Nitrogen ----- 15*2 „ 170 



Sulphur - 03 „ 20 



The protein group is chemically of a highly complex character ; 

 their constitution is unknown, for they have never been obtained 

 in a sufficiently pure condition for analysis. The molecule is known 

 to be large, probably containing not less than 700 atoms of carbon. 

 Some of the proteins have been crystallised, but neither the crystals 

 so obtained nor protein itself are capable of diffusion through an 

 animal membrane. 



There are three classes of organic radicles in the protein molecule : 

 one is purely basic, another is acid, and a third possesses both acid 

 and basic properties. To this latter belongs the important class of 

 amino acids. The characteristic feature of the amino acids is their 

 ability to unite with one another or other organic bodies, and so 

 form larger molecules. The complexity of the protein molecule 

 is due to the presence of amino acids. These play a part which has 

 been aptly compared by Garrod to that played by atoms in simpler 

 chemical molecules. 



A simple amino acid contains one organic acid radicle derived 

 from the carboxyl group (COOH), and one organic basic radicle 

 from the amidogen group (NH 2 ). Amino acids containing one 

 amidogen group are termed ' mon-amino acids.' Those possessing 

 two such groups are termed ' di-amino acids.' There is generally 

 only one acid group, but there may be more. Thus there are 

 mono- and di-basic mon-amino acids. The formation of amino 

 acids may be illustrated by an interesting one formed from the 

 simple fatty acid acetic acid CH 3 .COOH. If the CH 3 group loses 

 one of its atoms of Irydrogen and this is replaced by NH 2 , a substance 

 known as glycine, or the amido acid of acetic acid is formed, 

 CH 2 .NH 2 .COOH. Glycine may then conjugate with benzoic acid 

 to form hippuric acid, and with cholalic acid to form glycocholic 

 acid. 



All the fatty acids are capable of forming different mon-amino 

 acids, and the following series are then produced : 



Mono-basic mon-amino acids : Glycine, Alanine, Serine, Valine, 

 Leucine. 



A second group is formed from fatty acids containing two carboxyl 

 groups, i.e. — Di-basic mon-amino acids: Asparagin, Aspartic acid, 

 Glutamic acid, and others less important. 



