COENZYMES DERIVED FROM B VITAMINS 



161 



This reaction is a necessary step in the production of ethanol from sugar 

 and constitutes the primary method of pyruvate metabolism in yeast 

 when cultured anaerobically. Carboxylase, the enzyme catalyzing this 

 reaction, has been shown to occur in yeast, bacteria, fungi, and higher 

 plants, but it never has been found to constitute a part of the enzymatic 

 systems by which carbohydrates are utilized in animal metabolism. Free 

 phosphoric acid is not required for this particular reaction since no 

 utilizable energy units are produced. 



(2) Acetoin Formation. When acetaldehyde is added to preparations 

 from animal tissues capable of metabolizing pyruvic acid, it is found 

 that acetoin is formed. 138 The reaction is presumed to involve a con- 

 densation of a reactive ketenyl radical (arising from pyruvic acid) and 

 a molecule of acetaldehyde forming diacetyl which then acts as the 

 acceptor for the two hydrogen atoms. 



+ co 2 



The requirement for inorganic phosphate ion in connection with this 

 reaction has not been settled. If an energy-containing phosphate inter- 

 mediate were formed, it would be decomposed and its energy utilized in 

 the condensation creating the carbon-to-carbon bond (p. 189) . When 

 propionaldehyde was used as a substrate with pyruvate instead of 

 acetaldehyde, the homologue of acetoin, acetylethylcarbinol, was the 

 product of the reaction. 



In the absence of acetaldehyde muscle tissues still produce acetoin 

 from pyruvate but only at one-fourth the rate, and the yield from a given 

 amount of pyruvate is only half that which would be obtained in the 

 presence of acetaldehyde. Although the investigators could not detect 

 free acetaldehyde as an intermediate under these conditions, it is pre- 

 sumed that the reaction is the result of a two-step process in which 

 acetaldehyde is a transitory intermediate. The requirement of phosphate 

 for this reaction has not been determined. 



In certain bacteria acetoin is the primary product from the anaerobic 

 decarboxylation of pyruvic acid. 139 The mechanism of acetoin formation 

 in Aerobacter aerogenes apparently is somewhat different from that in 



