DEAN B. COWIK AND RICHARD H. R()IU:RTS 



25 



used together to supply I lie cystine of the proteins. It is concluded that the 

 intact molecule enters the cell at rates sufficient to supply the requirements for 

 growth. 



The outward passage of intact amino acids has been demonstrated. Slight 

 traces of radioactive amino acids appear in the culture medium when C'^- 



100 



90 



80 



70 

 60 



50 3 

 Q 



408 

 u 

 in 



a. 



UJ 



30 "^ 



z 



3 

 O 



u 



20 



12 3 



Fig. 7. Replacement of racliosulfate uptake by the addition of nonradioactive cystine. 



glucose is used as the sole carbon source. The addition of nonradioactive amino 

 acids to this medium often results in the release of considerable quantities of 

 C^^-labeled amino acids to the medium (6). C^*-homoserine is released from 

 the cell in response to the addition of C'--homoserine to a medium containing 

 C'^-glucose. This amino acid is not released as a result of cell lysis since homo- 

 serine is not present in measurable amount in the protein hydrolysates or in 

 other fractions of the cell. In the case of several competitors (glutamate, 



