DEAN n. COWIK AND RICHARD B. ROBKRTS 



23 



Other Methods of Demonstrating Cellular Permeability. The isotopic com- 

 petition method has been employed in this laboratory to investigate the 

 metabolic pathways involved in the utilization of inorganic ions and amino 

 acids for protein and nucleic acid synthesis (1-4, 9, 10, 17, 24, 26). This method 

 reveals that exogenous supplements of a nonradioactive amino acid may 



FRONT 



GLUTAMIC 

 ACID 



c'4 



RADIOACTIVITY 



n (no radioactivity) 



->- BUTANOL-FORMIC ACID 



Fig. 5. A line drawing of a chromatogram of the wash solution obtained from cells immersed 

 in C'^-glutamic acid. 



eliminate, or markedly suppress, the formation of that particular amino acid 

 from uniformly C'^-labeled glucose. Since the total quantity of amino acids 

 of the proteins remains constant, it appears that the intact carbon chain of 

 the exogenous amino acid is being utilized and that penetration of the cellular 

 membrane occurs. In reciprocal experiments, in which nonradioactive glucose 

 is used and the amino acid is uniformly labeled with C^*, additional direct 



