ETHYL ALCOHOL FORMATION 141 



CH2OPO3H2 /'CHj.CHzOH 



' ) + 



CHOH — -^ < H.COOH 



t I + 



CH2OH u^ V Phosphate 



H2O3 POHi C /^ \CH2OPO3H2 



OH 

 HO H CH2OPO3H 



CH3.CO.COOH 



In the absence of fluoride, phosphoglyceric acid is fermented 

 to pyruvic acid, as described, and a-glycerophospbate is 

 fermented with the formation of ethyl alcohol and formic 

 acid. Dihydroxyacetone-phosphate and glyceraldehyde- 

 phosphate form an equilibrium mixture; consequently the 

 proportions of ethyl alcohol to pyruvic acid formed will 

 depend upon the reaction velocities of the various inter- 

 mediate reactions involved. 



It will be noted that the oxidation of glyceraldehyde- 

 phosphate can be coupled either with a reduction of dihydroxy- 

 acetone-phosphate or with the reduction of pyruvic acid to 

 lactic acid. Since pyruvic acid will not accumulate until 

 the glycolysis cycle has proceeded through all the intermediate 

 steps, it follows that the reduction of dihydroxyacetone- 

 phosphate will occur predominantly during the starting-up 

 of the cycle and so can be looked upon as a " starter reaction " 

 which enables the cycle to get under way and consequently 

 to produce the main H-acceptor, pyruvic acid. 



Investigations using isotopic-carbon have indicated that, 

 during the fermentation of glucose, ethyl alcohol can also be 

 formed by the reduction of acetic acid. Ethyl alcohol is not 

 formed during the fermentation of pyruvic acid but it is 

 possible that the reduction of acetic acid requires reduced 

 coenzyme I as H-donor, and this, in turn, requires the triose- 

 phosphate dehydrogenase system, as in the case of lactic acid 

 formation. This second method of alcohol formation has 

 not yet been confirmed by other techniques. 



