BIOSYNTHESIS OF PROTEIN 635 
ditions as much as 90%, of the incorporated radiocarbon may be found in the TCA- 
soluble fraction. If these specifically labeled cells are washed and transferred to 
non-radioactive medium, there is a rapid loss of pool radioactivity, the major portion 
of the carbon being transferred to the protein of the cell (TCA-precipitable fraction). 
These results indicate that there is a precursor-end-product relationship between 
the pool amino acids and the proteins of the cell. As shown in Fig. 2 very little if 
any of the labeled carbon is lost to the medium. 
The transfer of carbon from individual amino acids of the pool to protein has 
also been investigated. Fig. 3 shows the rate of loss of radiocarbon from several 









=a 






all 
Glutamic acid 
=| 





in counts per second OWO 










Radioactivity 

Fig. 3. Time course of MC concentration in 
amino acids of the cold TCA-soluble fraction. 
Data represents the measured radioactivity 
20 40 60 BO 100. 120 of individual amino acids appearing on chro- 
Time in minutes matograms. 





pool amino acids. The data were obtained from cells briefly labeled as above with 
[44C fructose and reinoculated into non-radioactive medium. Samples were removed 
at the indicated times, the cells chemically fractionated, and two-dimensional 
chromatograms made of aliquots of hydrolyzates of the cold TCA-soluble and -pre- 
cipitable fractions. Chromatographic examination of the TCA-soluble material 
immediately after the labeling period showed that “parental” amino acids were 
highly radioactive while related “member” amino acids were found to be relatively 
non-radioactive. 
The exponential rate of loss of radiocarbon from pool alanine and glutamic acid 
indicate that this loss is a simple process. Each alanine has an equal probability of 
being incorporated into protein, whether radioactive or not. 
At the start of the experiment arginine was found to be only slightly radioactive 
(Fig. 3) but increased in radioactivity for 25 min. This increase was then followed 
by an exponential loss of “C equal to the rate observed for glutamic acid. These 
References p. 645 
