24 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



evidence is obtained for the penetration of the compound. C'--glutamic acid 

 suppresses incorporation of C^"* from C^'*-glucose into the glutamic acid of the 

 protein whereas C^''-glutamic acid, used to supplement the normal nonradio- 

 active medium, is found in the cell proteins with the same specific radioactivity 

 (±io%) as the exogenous compound. 



A mixture of S^*- and 2-C' "'-methionine added to the medium results in the 

 incorporation of both labeled atoms in the cell proteins (8) with the same 

 specific radioactivities as the original mixture. Furthermore, it has been shown 

 (31) by using C'^ in the methyl group that in E. coll the same specific radio- 

 activity is found in the protein methionine as in the exogenous methionine. 



Fig. 6. Identification of glutamic acid obtained from the wash solution of cells immersed 

 in C"-glutamic acid. ^ is a photograph of the glutamic acid region of a paper chromatogram 

 sprayed with ninhydrin. B shows the identical "fingerprint" obtained with a radioautograph 

 of the same region of this chromatogram. 



The time required for the competitive exogenous supplements to replace 

 endogenous synthesis has been studied using S^--cystine to compete with S^*04^. 

 The rate at which the replacement of sulfate occurs is rapid, as shown by the 

 sharp break in the radiosulfate uptake curve of figure 7. In this figure the ex- 

 ponentially growing cells take radiosulfate at a rate proportional to growth. 

 At the time of the addition of the nonradioactive cystine, the radiosulfate 

 uptake ceases, but growth continues at the same optimal rate. The unlabeled 

 cystine sulfur now supplies all sulfur requirements. That the carbon of the 

 amino acid can also penetrate is shown by supplying nonlabeled cystine to 

 medium containing C ^-glucose as the only other carbon source. Under these 

 conditions little of the C'' appears in the cystine of the proteins. This effective 

 reduction of the biosynthesis of both the sulfur and the carbon of cystine when 

 the exogenous amino acid is present suggests that the carbon and sulfur are 



