METABOLIC FUNCTIONS OF B VITAMINS 227 



It is interesting to note that fatty acids have an appreciable sparing 

 effect upon the pantothenic acid 17 and riboflavin 1S requirements of lactic 

 acid bacteria, indicating that a considerable fraction of each of these 

 vitamins is being used by these organisms in the fat synthesis systems, 

 but that lipides have only a slight effect upon the nicotinic acid response. 19 

 (A reduction in utilization of an enzyme system is reflected in the lowered 

 demands of the organism for the cofactors.) The sparing effect of fats 

 upon the thiamine requirement of mammals (p. 000) can be attributed 

 to the fact that the body is oxidizing acetyl units primarily derived from 

 fatty acids rather than those arising from the decarboxylation of pyruvic 

 acid. 



The reverse process, in which a deficiency of an enzyme system alters 

 the dietary requirements, can also be of physiological importance. For 

 example, in lactic acid bacteria where the thiamine system is inherently 

 deficient, the organisms need an exogenous supply of acetate or fatty 

 acids as a source of acetyl units to supplement the sub-optimal quantities 

 furnished by the thiamine enzymes. 20 



An additional problem encountered in the synthesis of fats is that of 

 the formation of unsaturated fatty acids. The requirement for specific 

 unsaturated fatty acids in both bacteria and mammals would indicate 

 that these organisms are deficient in enzymes designed for this purpose. 

 Enzymes that dehydrogenate fatty acids are known, 21 but have never 

 been shown to be dependent upon any B vitamin coenzymes for activation. 

 However, the biotin-sparing activity of oleic acid and other unsaturated 

 fatty acids for certain microorganisms (p. 173) is suggestive of a function 

 of biotin related in some way to the formation of a double bond by dehy- 

 drogenation of saturated fatty acids. 



Glycerol Formation and Utilization. The glycerol needed for lipide 

 synthesis is a by-product of carbohydrate metabolism. The phosphoryl- 

 ated triose, dihydroxy acetone phosphate (formed by the hexose diphos- 

 phate cleavage), is reduced by the corresponding dehydrogenasi, the 

 specific hydrogen donor required is the hydrogenated diphosphopyridine 

 nucleotide. The phosphorylated glycerol is then available for reactions 

 producing phospholipides — compounds which are assumed to be necessary 

 intermediates in fat synthesis. 



Nicotinic acid, in the form of its coenzyme, is also essential for the 

 utilization of the glycerol liberated from fat, since the glycerol (after 

 phosphorylation) must be dehydrogenated by DPN to a triosephosphate 

 before it can be metabolized in the carbohydrate system. 22 After the 

 initial dehydrogenation, the substrate can be utilized during anabolic 

 phases of cell activity for synthesis of glycogen or starch; during periods 

 in which the catabolic activity of the cells predominate, it will be utilized 



