Enzymes and Fermentation in Yeast Cells 288 



position of glucose to yield pyruvate takes place so rapidly 

 during the first seconds that the pyruvate decarboxylase 

 cannot keep step with this supply. Moreover, the behaviour 

 of the pyruvate concentration under aerobic conditions (Fig. 4) 

 shows that under aerobic conditions, as well as on the addition 

 of acetaldehyde under anaerobic conditions, pyruvate accumu- 

 lates rapidly. It seems that in the presence of oxygen there is 

 no inhibition of triose phosphate dehydrogenation due to a 

 lack of oxidized DPN, since DPNH now can transfer its 

 hydrogen to oxygen. 



In summary, it may be said that all measurements shown 

 in Figs. 1-4 agree with the assumption that a lack of oxidized 

 DPN limits the rate of fermentation during the first seconds 

 after the addition of glucose to starved yeast cells. 



Increase of aerobic fermentation by addition of 

 NH^ ions to glucose -oxidizing yeast cells 



If NH^ ions are added to glucose-oxidizing yeast cells, soon 

 a strong increase of aerobic fermentation, and after some time 

 (in the presence of the necessary growth factors) the occur- 

 rence of growth and cell multiplication, are observed. In 

 connexion with studies on carbohydrate metabolism in grow- 

 ing cells and tissues we have been interested in the mechanics 

 of how the rise in aerobic fermentation following the addition 

 of NH^ ions comes about. Fig. 5 shows the changes in the 

 concentration of orthophosphate, pyruvate, acetaldehyde 

 and a-ketoglutarate after the addition of NH^ ions to glucose- 

 oxidizing yeast cells (Holzer, 1959, 1958; Holzer and Witt, 

 1958). From these measurements we conclude that the 

 following sequence of events occurs: at first a-ketoglutarate 

 is aminated reductively to glutamate by means of DPNH and 

 reduced triphosphopyridine nucleotide (TPNH), respectively 

 — the two glutamate dehydrogenases can be concentrated and 

 separated from each other (Holzer and Schneider, 1957); the 

 lack of a-ketoglutarate causes an inhibition of respiration as 

 well as oxidative phosphorylation, by which the observed 



