618 



PANTOTHENIC ACID 



DONOR SYSTEMS 

 TRANSACETYLASES: 



Ac~Ph 



ACCEPTOR SYSTEMS 



ACETOKINASES: 



Acetyl SAM, choline 



Histamine 

 Glucosamine 

 Amino acids 

 Hydroxylamine 



Acetylformate 

 (pyruvate) ■* 



Acetoacetate 



>- Citrate 



Acetate +CoA+ ATP 



Acetoacetate 

 Fig. 8. The acetyl transfer system. 



and acceptor territories, which are bridged by the acetyl-CoA carrier 

 system. This is illustrated by Fig. 8." 



As mentioned, a final confirmation and amplification was obtained by 

 Lynen and Reichert*^' ^' when they isolated acetyl-CoA from yeast. Of 

 great importance is their finding that the SH- group of CoA is the functional 

 group, that is to say, when functioning, CoA shuttles back and forth be- 

 tween sulfhydryl- and thioacetyl-CoA. This explains the long-known need 

 of large amounts of SH-compounds like cysteine or glutathione to activate 

 all acetylation systems.^"- ''^ This activation occurs by conversion of in- 

 active CoA disulfide into the active sulfhydryl-CoA. 



It is quite remarkable that in CoA-linked acetyl the methyl end — we 

 call this in our laboratory the "tail" end — as well as the carboxyl end or 

 "head" is activated. Accordingly, we speak of "tail" or "head" condensa- 

 tions. ^^ Thus citric acid synthesis is a tail condensation. Straightforward 

 acetylation reactions are, of course, head condensations. One of the most 

 interesting reactions in this respect is acetoacetate synthesis. Stadtman 

 et at. showed that acetoacetate is synthesized by interaction of two active 

 acetyls, i.e., two acetyl-CoA's, the head of one reacting mth the tail of 

 another.''^ 



" D. Nachmansohn and A. L. Machado, Neurophysiol. 6, 397 (1943). 



" F. Lipmann, Cold Spring Harbor Symposia Quant. Biol. 13, 127 (1948). 



" E. R. Stadtman, M. Doudoroff, and F. Lipmann, /. Biol. Cheni. 191, 377 (1951). 



