86 NITROGEN METABOLISM 



the absence of glucose fermentation. It may be concluded 

 that the presence of high concentrations of other amino-acids 

 in the internal environment promotes the retention of lysine 

 inside the cells and that their influence is overcome by 

 glucose fermentation. The converse is also true, since the 

 presence of acidic or basic amino-acids in the external 

 environment retarded the uptake of lysine by amino-acid 

 deficient cells in the absence of glucose. The effects of 

 orthophosphate on the outw^ard migration of lysine have 

 been investigated [i6], but the results are difficult to inter- 

 pret and the reader is referred to the original paper. 



From all these results, it may be deduced that the accumu- 

 lation of lysine by Strep, faecalis involves a diffusion mech- 

 anism which is independent of metabolic energy and may 

 be due to the establishment of a Donnan type of equilibrium. 

 Such a hypothesis is supported by the observation that, 

 when suspended in an amino-acid free medium, lysine 

 migrated out of cells which contained only this amino-acid 

 in large amounts. However, it must not be assumed that the 

 mechanism of lysine absorption is the same in all Gram- 

 positive organisms. Although similar results were obtained 

 with Staph, aureus, the uptake of lysine by Sac. cerevisiae 

 was dependent on metabolic energy [6] and is therefore 

 comparable to the uptake of glutamic acid by Strep, faecalis 

 and Staph, aureus. 



Uptake of glutamic acid [5] 



Irrespective of the pH of the inorganic salt medium there 

 was no absorption of glutamic acid by the amino-acid de- 

 ficient cells unless glucose or a tryptic digest of casein was 

 also added. Hence, unlike lysine, the uptake of glutamic 

 acid appeared to be an endergonic process utilizing meta- 

 bolic energy made available by the fermentation of glucose 

 or by the metabolism of amino-acids. Arginine promoted 

 the absorption of glutamic acid, although not as efficiently 

 as glucose, and the fact that the organisms possess arginine 

 dihydrolase (p. 27) may be of some significance. In the 

 presence of glucose, glutamic acid was not only absorbed 

 but also concentrated in the cells, and the relationships 



