228 THE BIOCHEMISTRY OF B VITAMINS 



in one of the previously described processes in which pyruvic acid is an 

 intermediate. 



Synthesis and Hydrolysis of Fats. A number of lipases from both 

 plant and animal sources have been thoroughly investigated. 23 These 

 esterases catalyze the hydrolysis of fats, liberating free fatty acids, 

 glycerol, and the other components found in complex lipides. In no case 

 has a coenzyme requirement been demonstrated for this hydrolysis. 



The synthesis of fats by the reverse reaction can be accomplished in 

 vitro by these same enzymes if the molar ratio of fatty acids to fat is 

 greater than two to one. 24 Since the intracellular concentration of fatty 

 acids is very low, the synthesis of fats here must be by a different process. , 

 The mechanism probably is similar to that involved in the formation of 

 polysaccharides from simple sugars and involves phosphorylated inter- 

 mediates. In the condensations and reductions by which the phosphoryl- 

 ated acetyl units are elaborated into the fatty acids, it was indicated that 

 the fatty acid derivatives actually produced are undoubtedly fatty acyl 

 phosphates. (This supposition is in line with evidence concerning the 

 reverse process, wherein it has been demonstrated that before a fatty acid 

 can be degraded enzymatically it must be first converted to the corre- 

 sponding acyl phosphate. 25 ) The fatty acyl phosphates react directly 

 with the hydroxyl groups of glycerol phosphate or other alcohols to yield 

 the esters which constitute the saponifiable lipides. These reactions arc 

 analogous to those employed for the nonenzymatic synthesis of esters 

 wherein acyl halides are used. The equilibrium reached in this reaction 

 is so greatly in favor of ester formation that the reaction can be assumed 

 for practical purposes to have "gone to completion." Both in the synthesis 

 of fats and in the utilization of the fatty acids from fats the individual 

 fatty acids of different carbon chain length appear to be used in a non- 

 specific and random fashion. 20 



The vitamin requirements for fat metabolism are summarized in the 

 diagram on the following page. 



Sterol Metabolism. Very little is known about the reactions by which 

 sterols are formed. Tracer studies using isotopes of carbon first showed 

 the importance of acetate in the synthesis of sterols. 27 An independent 

 demonstration was provided by the study of the acetate requirements of 

 organisms which produce lactic acid. 20 These bacteria are very limited in 

 their ability to convert pyruvic acid to the reactive two-carbon unit. 

 Consequently, they require an exogenous supply of acetate in order to 

 function normally. Sterols as well as fatty acids were found to have a 

 significant sparing action upon this acetate requirement of these or- 

 ganisms. 



