METABOLIC FUNCTIONS OF B VITAMINS 225 



postulation would be that the precursor of this compound was formed 

 by a condensation of a triose and an activated acetate unit. 



The Metabolism of Lipides 



A general scheme of lipide metabolism must account for fatty acid 

 synthesis and degradation, glycerol formation and utilization, and for 

 the reactions by which fat molecules are synthesized from their com- 

 ponent parts, and should also explain the origin of sterols. 



Fatty acid metabolism. The mechanism now postulated for the syn- 

 thesis and degradation of fatty acids 15 is very similar to that shown in 

 the scheme for the production of butyl alcohol, and the same combination 

 of vitamins — thiamine, pantothenic acid, nicotinic acid, and riboflavin — 

 is needed. When proteins are converted to fats a fifth vitamin, pyridoxal 

 is also undoubtedly required to catalyze the deamination of amino acids. 16 

 A route of synthesis by which butyryl phosphate can be formed from a 

 hexose unit has been diagrammed (p. 222) . The synthesis of higher fatty 

 acids presumably is carried out similarly by a series of reactions which 

 lengthen the carbon chain of a fatty acyl phosphate in some fashion 

 corresponding to the diagram on the following page. The net result of this 

 process is an anaerobic utilization of each hexose unit for increasing the 

 chain of a fatty acid by four carbon atoms. 



The discovery of the reactive phosphoryl acetyl compound cleared up 

 a number of points previously obscure concerning the synthesis and 

 degradation of fatty acids. This fundamental acetylating agent which 

 can be formed by a number of metabolic processes furnishes the units 

 from which fatty acids are constructed, and in turn these same acetyl 

 units are regenerated when a fat is metabolized. The mechanism of fatty 

 acid catabolism 13 is believed to be just the reverse of the synthetic proc- 

 ess; fatty acyl phosphates are degraded by dehydrogenations, hydra- 

 tions, and phosphoroclastic cleavage of the /3-keto acids to yield the 

 reactive phosphorylated acetyl units (associated with coenzyme A?) and 

 the hydrogenated coenzymes of nicotinic acid and riboflavin. If the 

 aerobic mechanisms for oxidation of the acetyl units (tricarboxylic acid 

 cycle) are inhibited from functioning or are overloaded by the inter- 

 mediates of carbohydrate or protein metabolism, then the accumulation 

 of the acetyl units will result in their condensation, and fats will be 

 formed. On the other hand, if there is insufficient carbohydrate or pro- 

 tein degradation to furnish sufficient substrate for the energy-producing 

 tricarboxylic acid cycle the fats will be degraded to supply the acetyl 

 units for the oxidative cycle. 



