230 THE BIOCHEMISTRY OF B VITAMINS 



When the function of pantothenic acid was shown to be that of a 

 mediator of reactions involving active forms of acetate, this vitamin was 

 naturally implicated in sterol synthesis. It was shown that sterols had a 

 sparing action upon the pantothenic acid requirement of Lactobacilli, 17 

 and that a sterol could partially reverse the toxicity of pantothenic acid 

 inhibitors. 28 The reversal was noncompetitive, indicating that sterols are 

 products of a series of reactions, part of which are catalyzed by panto- 

 thenic acid enzymes. 



The mechanism for the formation of both the polynuclear cholane 

 nucleus and the hydrocarbon side chains is completely unknown, but it 

 appears impossible to postulate any mechanism involving acetate con- 

 densations which does not include the hydrogenation of keto groups and 

 of ethylenic bonds. For these reactions nicotinic acid and riboflavin are 

 undoubtedly required. 



Pantothenic acid, riboflavin, and nicotinic acid may be the only vita- 

 mins required for sterol synthesis if a cell has a potential source of acetate 

 in the form of fatty acids or acetate itself. If carbohydrates must be used 

 as the initial source of carbon and hydrogen, then thiamine, of course, 

 will be required to form the active acetyl units from pyruvic acid. 



Biotin administration at one time was believed to cause the formation 

 of excessive amounts of cholesterol in the liver. 29 Critical reexamination 

 of this phenomenon has cast doubts upon this role of biotin. 30 Biotin. 

 however, could function in some fashion in the process responsible for 

 the formation of the unsaturated linkages in the sterol molecules in a 

 manner comparable to its possible function in the formation of the 

 ethylenic linkages in oleic acid (p. 227) . 



The Metabolism of Nitrogen Compounds 



From a chemical standpoint, the metabolism of proteins is more com- 

 plicated than that of carbohydrates and fats. The presentation of the 

 basic reactions is likewise more difficult. Unlike the simple sugars or the 

 fatty acids, the amino acids — the component units of proteins — vary con- 

 siderably in their chemical structure, and many reactions are necessary to 

 account for the synthesis of the individual amino acids. Polysaccharides 

 are usually polymers of a single hexose, and in fats the arrangement of 

 fatty acids is a random one; but in the formation of the proteins, poly- 

 merization of the different amino acids must take place according to a 

 highly specific pattern. 



Synthesis and Hydrolysis of Proteins. The proteolytic enzymes cata- 

 lyzing the direct hydrolysis of the amide bonds require no coenzymes. 

 Almost nothing is known of the enzymes responsible for the intracellular 

 synthesis and hydrolysis of proteins. Several enzymatic processes in which 



