286 Helmut Holzer 



already shown in Fig. 5, a rapid decrease of the glucose-6-phos- 

 phate concentration and a rapid increase of the concentration 

 of oxidized TPN have been observed. We ascribe this to the 

 fact that in yeast cells as well as in animal tissues the dehydro- 

 genation of glucose-6-phosphate caused by the "Zwischen- 

 ferment" is limited by a lack of oxidized TPN, as most of 

 the TPN is present in its reduced form. On the addition of 

 NH^ ions, TPNH is used up for the reductive amination of 

 a-ketoglutarate, thus yielding oxidized TPN, which now 

 makes possible the dehydrogenation of glucose-6-phosphate. 

 This starts the oxidative pentose phosphate cycle. Since the 

 dehydrogenation of glucose-6-phosphate leads to an accumu- 

 lation of pentoses, here is a reasonable mechanism by means 

 of which the NH/ ions, which are causing growth, promote the 

 formation of pentoses and therefore of ribonucleic acids, the 

 latter being necessary for growth. In experiments with 

 extracts from yeast and liver we could demonstrate that an 

 activation of the pentose phosphate cycle by NH^ ions as 

 mentioned above actually takes place, leading to an accumu- 

 lation of pentoses (Witt and Busch, 1958, unpublished). At 

 present we are investigating the in vivo operation of this 

 mechanism, with the help of ^*C-labelled glucose. 



Summary 



(1) Some problems of the enzymic regulation of carbo- 

 hydrate metabolism in yeast cells have been investigated by 

 measuring changes in metabolite concentrations. 



(2) During the first minute after the addition of glucose to 

 starved yeast cells the degradation of glucose is limited by lack 

 of oxidized DPN, i.e. by a too slow reoxidation of DPNH. 



(3) The origin of the aerobic fermentation, appearing after 

 the addition of NH^ ions to glucose-oxidizing yeast cells, may 

 be explained by a regulation via the adenylic acid system. 



(4) It has been shown that in glucose-oxidizing yeast cells 

 the direct oxidation via the pentose phosphate cycle is 

 limited by a lack of oxidized TPN. Addition of NH^ ions 



