84 NITROGEN METABOLISM 



lysine most readily enters the cells as the zwitterion. When 

 the cells fermented glucose, the uptake of lysine was de- 

 pressed but could be restored, although not completely, if 

 glutamic acid was also added to the system. The factors 

 influencing the outward migration of lysine were also of 

 importance, if only to show that washing the cells prior to 



1 1 1 



O 5 10 15 



EXTERNAL CONG. OF AMINO- ACID (a' mole/mi.) 



FIG. 6.1. — Effect of external concentration of L-Iysine (#) and 

 L-glutamic acid (O) on the rate of entry of the amino-acid 

 into Strep, faecalis 



assay did not remove any of the internal amino-acids. Cells 

 containing large amounts of lysine, glutamic acid and 

 probably several other amino-acids were obtained from cul- 

 tures grown in the presence of a tryptic digest of casein. 

 When these 'amino-acid rich cells' were incubated in an 

 amino-acid free salt medium, there was no outward migra- 

 tion of either lysine or glutamic acid unless the cells were 

 fermenting glucose. 



