194 THE BIOCHEMISTRY OF B VITAMINS 



almost exclusively in the form of its coenzyme. As is the case with several 

 of the other B vitamins, the plasma pantothenic acid is in the form of the 

 free vitamin, whereas the vitamin within the red blood cells has been 

 converted quantitatively to the coenzyme. 



Biosynthesis. The chemical routes by which pantothenic acid is con- 

 verted to coenzyme A are not known. Before coenzyme A was discovered, 

 it had been shown that the addition of pantothenic acid quickly activated 

 the pyruvate metabolism of bacterial cells which were deficient in panto- 

 thenic acid. 262 Later it was shown that incubation of pantothenic acid- 

 deficient yeast and bacteria with pantothenic acid resulted in a rapid 

 synthesis of coenzyme A. 263, 264 This indicates that these cells possess 

 adequate mechanisms for the rapid synthesis of the coenzyme. Since the 

 tissues of animals receive their pantothenic acid in an uncombined form 

 from the blood stream, they too must be able to carry out this conversion. 

 Strangely enough, however, the addition of pantothenic acid to surviving 

 deficient tissues of ducks and rats in vitro does not result in any demon- 

 strable synthesis of the coenzyme. 265 



Incubating resting yeast cells with glutamic acid and pantothenic acid 

 (or /3-alanine) results in the production of a conjugate which occasionally 

 shows a thousand times the activity of an unincubated control containing 

 these same substances. 259 That the formation of the conjugate may not 

 be direct from the vitamin itself was indicated by the fact that the 

 incubation product of /^-alanine was consistently more active than that 

 obtained from pantothenic acid. 



Reactions Catalyzed by Coenzyme A. Although all the reactions cata- 

 lyzed by coenzyme A may involve a common substrate, they result in 

 the formation of a variety of chemical compounds: amides, esters, acid 

 anhydrides, and compounds produced by the condensation of an acetate 

 radical with keto acids or acid phosphates. 



The equations for enzymatic reactions in which coenzyme A is a cata- 

 lyst are tabulated on the following page. Following this list is a summary 

 of the enzyme reactions in which pantothenic acid has been definitely 

 implicated, but which have not been sufficiently well characterized to en- 

 able one to say with certainty that coenzyme A is the coenzyme. 



It should be noted that the reaction in which the acetyl derivative 

 condenses with oxalacetic acid to form a tricarboxylic acid is the one 

 which initiates the cycles by which both carbohydrates and fatty acids 

 are metabolized aerobically, and by which they are converted to the 

 dicarboxylic acids, a-ketoglutaric, glutamic, fumaric, tartaric, malic, 

 oxalacetic, and aspartic acids (p. 223). When carbohydrate metabolism 

 provides the acetyl molecule, thiamine and pantothenic acid are required; 

 when fatty acids or ethanol are the source of the reactive phosphoryl- 



