58 NITROGEN METABOLISM 



the cells unless glucose was also added to the experimen- 

 tal system. On the other hand, a,}/-diethylglutamate and 

 N-phosphorylglutamic acid were absorbed in the absence 

 of glucose and gave rise to internal glutamic acid [8]. Hence 

 it is possible that glutamic acid passes through the cell wall 

 in a phosphorylated form, and irrespective of whether the 

 phosphate group is on the amino group or on one or both 

 of the carboxyl groups, the synthesis of such a compound 

 would be endergonic. But, the evidence in favour of this 

 hypothesis is far from conclusive. Many cells are known to 

 be impermeable to phosphorylated compounds and the Q^q 

 for the uptake of N-phosphorylglutamic acid was inter- 

 mediate between that for free diffusion and for an enzymic 

 process. 



When amino-acid deficient cells of Staph, aureus were 

 incubated with glutamic acid and glucose together with 

 mixtures of other amino-acids, the accumulation of free 

 glutamic acid was reduced and sometimes ceased. This was 

 the outcome of a marked reduction in the amount absorbed 

 and also an increase in the cellular combined glutamic acid. 

 If the concentration of glutamic acid in the external medium 

 was very much greater than that of the other amino-acids, 

 assimilation into cell substance was suppressed and some 

 accumulation of the free acid did take place. Evidently 

 whether glutamic acid entered the cells and accumulated as 

 the free acid or whether it was converted into a combined 

 form, depended on the ratio of glutamic acid to other amino- 

 acids in the external medium. The presence of single acids 

 such as aspartic, cysteine, glycine, serine and alanine pro- 

 duced a marked decrease in the rate of glutamic acid accu- 

 mulation. Valine, leucine and isoleucine caused a small 

 increase in the latter, whilst all the other amino-acids 

 examined had no effect. Aspartic acid acted as a competitive 

 inhibitor, but this explanation did not apply to cysteine and 

 alanine, the addition of which led to the synthesis of extra- 

 cellular peptides containing glutamic acid [lo, ii]. 



There was a small leakage of glutamic acid when cells of 

 Staph, aureus containing large amounts of several amino- 

 acids were incubated in an amino-acid free medium: the 



