26 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



glycine, alanine, and valine), in which the C'^ compound has been supplied in 

 addition to C'^-glucose, an amount of the C^^ amino acid roughly equal to 

 that normally synthesized by the cells from glucose has been found in the 

 external medium. 



In these cases the exogenous amino acid supplies most of the material used 

 for protein synthesis. The amino acid in the protein consists of 90 to 95% 

 C'^-amino acid (exogenously supplied) and 5 to 10% C"* amino acid (internally 

 synthesized). 



The rate of synthesis of the amino acid by the cell is not strongly reduced, 

 but the external amino acid dilutes that internally synthesized by about 

 a factor of 10, as far as utilization by the cell is concerned. Therefore, there 

 must be a flow of amino acid into and out of the cell at a rate at least 10 times 

 as great as the rate of utilization by the cell for protein synthesis. 



This demonstration of the permeability of amino acids is not restricted to 

 the examples given above. All the amino acids found in the bacterial proteins 

 as well as the nucleic acid bases have been shown to be effective competitors. 

 Sometimes the exogenous compound eliminates or reduces the biosynthesis of 

 only one particular substance in the cell. Often whole groups of related com- 

 pounds are no longer derived from glucose when an exogenous compound is 

 used to supplement the medium. Accordingly these exogenous compounds 

 must penetrate rapidly to the sites of amino acid synthesis. 



The isotopic competition method of analysis has also revealed similar char- 

 acteristics in the yeast and the mold (Torulopsis utilis and N eurospora crassa) 

 (24). That is, the direct incorporation of the carbon of many amino acids occurs 

 so rapidly that little of the internally synthesized compounds from other car- 

 bon sources (glucose, CO2, and acetate) appears in the cell. 



Permeability of Escherichia Coli to Peptides. Glutathione is one of the end 

 products of the metabolism of sulfate by E. coli (25). Twenty-five per cent of 

 the sulfur of the cell is found in this product. When glutathionine is used as 

 the sole sulfur source, the optimal growth rate is observed. When both radio- 

 sulfate and glutathione are present, the formation of S'^-glutathione from the 

 radiosulfate is greatly reduced although the quantity of glutathione in the 

 cell is unchanged (24). These facts demonstrate that at least the sulfur of this 

 peptide penetrates the cell membrane. 



Glutathione labeled with S'''^ was prepared by extracting E. coli (grown in 

 the presence of S^'^04=') with cold TCA. At least 95% of the S^^ of this fraction 

 is in glutathione. After the TCA was removed by shaking with ether, the 

 radioactive glutathione was added to synthetic medium containing either 

 reduced or oxidized S^^-glutathione. Chromatograms showed that the excess 

 of the inert material converted the radioactive material to the corresponding 

 oxidation state. These solutions were then used to resuspend two pellets of 

 cells for the usual water-space measurements (table 21). 



