196 THE BIOCHEMISTRY OF B VITAMINS 



enzyme, and transports the acetyl derivative to another system where it 

 is utilized. 



It is impossible to do more than speculate upon possible linkages that 

 may exist between coenzyme A and the phosphorylated acetate molecule. 

 Studies of the specificity of chemical structure for compounds that will 

 replace or inhibit pantothenic acid have been undertaken (p. 620), but 

 the information obtained is not sufficient to justify any conclusions con- 

 cerning the question posed in the previous sentence, or to decide what 

 groups of the coenzyme are essential for the formation of the enzyme- 

 coenzyme bond. 



Is there more than one pantothenic acid coenzyme? All enzymatic 

 phenomena in which pantothenic acid has been implicated can be ex- 

 plained on the basis of a single coenzyme. A fact offering corroborative 

 evidence for this statement is that all the intracellular pantothenic acid 

 is in a form which can serve as the active coenzyme for the aromatic 

 amine acetylation system. If the coenzyme requirement for this enzyme 

 is specific, then within cells there can be only one pantothenic acid coen- 

 zyme present in measurable quantities. 



Does the pantothenic acid coenzyme mediate reactions other than those 

 involving acetic acid derivatives? On the basis of information now avail- 

 able, there seems to be no justification for postulating other functions for 

 pantothenic acid. 



Coenzymes Involved in the Utilization of the Single Carbon Unit 



Formic acid is a common by-product of bacterial fermentation. Like 

 acetic acid, it had long been regarded as a waste product of inefficient 

 catabolic processes, but its role in essential synthetic reactions had gone 

 unrecognized. The first indication of its participation in biologically im- 

 portant synthetic reactions resulted from exploratory tracer studies de- 

 signed to establish the precursors of purines when they are synthesized 

 de novo by animal tissues. It was found that neither carbon dioxide nor 

 the carboxyl group of pyruvic acid could serve as a source of the carbon 

 atoms in the ureide portion of the purine molecule (positions 2- and 8-), 

 although it had been anticipated that they would be found to be pre- 

 cursors of the single carbon units. 276 When labelled formic acid was used, 

 however, the isotopic carbon was incorporated into the purine nucleus 

 in the 2 and 8 positions. 273 This discovery created considerable interest 

 in possible metabolic roles of formic acid and stimulated further investi- 

 gations, which have now disclosed other important reactions involving 

 the single carbon unit derived from formic acid. 



No statement can be made at this time as to the exact chemical nature 

 of this single carbon unit which is normally produced and utilized in 



