320 BIOLOGICAL ACTIVITIES OF YEASTS 



mediate fermentation products and that these are subsequently oxi- 

 dized to carbon dioxide and water by the oxygen of air. One of the 

 points in the dissimilation of sugar suggested as the locus of shunting 

 off the fermentive process has been at the acetaldehyde (or pyru- 

 vate) stage where there is a competition between oxidation on the 

 one hand and reduction with the formation of ethyl alcohol (or 

 lactate) on the other.** It has also been suggested that the point of 

 the Pasteur effect is at the triose phosphate stage. ^ In the alcoholic 

 fermentation (see page 326) for the Embden-Meyerhof-Parnas 

 scheme), hexose is initially phosphorylated. This is followed by the 

 cleavage of the diphosphorylated sugar to two phosphorylated tri- 

 oses. After further phosphorylation, the triose is oxidized at the 

 expense of diphosphopyridine nucleotide, DPN (Coenzyme I) which 

 is reduced to Ho-DPN. Under anaerobic conditions, H2-DPN (re- 

 duced Coenzyme I) passes its hydrogen to acetaldehyde (reducing 

 it to ethyl alcohol) and is itself oxidized to DPN. Under aerobic 

 conditions, however, it is proposed that H2-DPN does not pass its 

 hydrogen to acetaldehyde but rather to an enzyme of the flavo- 

 protein type (yellow enzymes) or to some enzyme, as yet unknown. 

 If the hydrogen is passed to a flavoprotein enzyme, it in turn passes 

 the hydrogen to molecular oxygen via the cytochrome-cytochrome 

 oxidase system. Oxygen is the final hydrogen acceptor. Coenzyme I, 

 shuttling back and forth between its oxidized and reduced states, 

 oxidizes more phosphorylated triose molecules. The oxidation prod- 

 uct of the phosphorylated triose is disposed of by oxidation through 

 the intervention of oxidative enzymes. 



Recently, it has been suggested that intermediates are not formed 

 at all but that the locus of the Pasteur effect is at the hexose mono- 

 phosphate stage. Engel'hardt " among others has proposed that the 

 Neuberg ester is the critical compound. If this hexose monophos- 

 phate is further phosphorylated to the hexose diphosphate, the mole- 

 cule undergoes fermentive breakdown. However, if this hexose 

 monophosphate, instead of being phosphorylated, is oxidized to phos- 

 phohexonic acid, oxidative breakdown occurs. A somewhat related 

 hypothesis has been advanced by Johnson.^- He points out that 

 both aerobic and anaerobic breakdown processes involve phosphory- 

 lative reactions. The aerobic processes carry on phosphorylations 

 utilizing relatively large amounts of phosphates and may lower the 

 phosphate concentration to the point where anaerobic glycolytic 

 phosphorylations cannot occur. Therefore fermentive mechanisms 

 are unable to operate. He does not, however, stipulate the point 

 of the Pasteur effect, as does Engel'hardt. 



