190 THE BIOCHEMISTRY OF B VITAMINS 



preceding divisions of this chapter are so decidedly in favor of the cleav- 

 age of carbon-to-carbon bonds rather than the reverse carboxylation, 

 these reactions have sometimes been ignored when effective enzymatic 

 mechanisms for increasing the number of carbon atoms in a molecule are 

 considered. However, carboxylations, both of the a and B types, catalyzed 

 by the coenzymes of thiamine and biotin (?), may be utilized to a greater 

 extent in biological syntheses than was once realized. 241 



The type of condensation for which the greatest number of reactions 

 are recognized resembles the Claisen type (second in the listing above) . 

 The many processes in which it is employed were not understood until 

 the recent discovery of an essential metabolite — a derivative of acetic 

 acid. Acetic acid had long been recognized as the principal end product 

 in certain types of fermentations, but its importance as an intermediate 

 compound in metabolism was not fully realized until it was shown that 

 acetic acid (labelled with isotopic atoms), when introduced into organ- 

 isms, was converted into some activated derivative which participated in 

 a number of metabolic processes. 242 Although the concentration of acetic 

 acid in normal animal tissues is too low to be measured by conventional 

 methods, it was shown by the use of isotopes that in a 24-hour period an 

 adult rat produces a quantity of acetic acid (a phosphorylated derivative) 

 equal to 1 per cent of its body weight. 243 In the meantime, other investi- 

 gators found that acetic acid was a substrate for (1) the reaction in 

 which acetylcholine is formed, and (2) the enzymatic acetylation of 

 aromatic amines. Previously it had been recognized that the acetate ion 

 which is used for buffering media in which bacteria are grown has func- 

 tions independent of its buffering capacity, 244 and later this metabolic 

 utilization of acetic acid by bacteria was shown to be related to lipide 

 synthesis. 245, 246 



To discuss the reactions in which acetic acid can participate it is nec- 

 essary to consider the chemical nature of the activated acetyl molecule 

 which is first formed and with which pantothenic acid is associated. This 

 reactive compound, which can be formed enzymatically from acetic acid, 

 is believed actually to be the substance participating in most, if not all 

 of the acetate reactions. It is a molecule which can act both as a phos- 

 phorylating and as an acetylating agent 247 but it is not identical with 

 acetyl phosphate. 248, 249> 250 Although there were a number of reasons for 

 assuming the active intermediate to be acetyl phosphate, this compound, 

 when prepared synthetically, was inactive and could not be substituted 

 as the substrate replacing the so-called "active acetate" arising from the 

 oxidative decarboxylation of pyruvate. 249 Moreover, the "active acetate" 

 is not decomposed readily by the specific enzyme, acetyl phosphatase, 

 whereas the synthetic acetyl phosphate is. 248 Partial purification of the 



