122 



S. S. COHEN 



of excliange of jjrotem-bound sulfur, for instance, was only one-thousandth 

 as rapid as the synthesis of protein during the logarithmic phase of bacterial 

 growth. 



Radioactive S \ 

 inducer 



Non radioactive S 

 No Inducer 



III 



Non radioactive S inducer 



M: 



Sulfur 

 starvation 



Factor increase . 

 enzyme 



Factor decrease 

 specific radioactivity 



1-47 X 

 1-53 X 



4-85X 

 4-46x 



7-90 X 

 7-40 X 



Fig. 23. The stability of |8-galactosidase (Cohn, 1954). 



Diagrammatic representation of experiment in which Escherichia coli was groTVTi with 

 radioactive S plus inducer (crosshatched areas) I. The inducer was then removed, and 

 nonradioactive S substituted. The bacterial cells increased 11 -fold, but the enzyme 

 remained constant (II). The III mchicer was added to the nonradioactive S and 

 growth continued as shown in (a), (b) and (c). The degree of radioactivity in the cells 

 is shown by the closeness of the Crosshatch ing, and the amount of enzyme by the inner 

 clear rectangles. 



It was then asked whether the apparent turnover of any proteins in the 

 mammal did not reflect the death and resynthesis of cells, rather than of 

 components of intact cells. As of this writing, definitive experiments with 

 animal cells have not yet been performed to prove or disprove this hypothesis. 

 Vehck (1956) studied three well-defined crystallizable enzymes, aldolase, 

 glyceraldehyde-3-phosphate dehydrogenase, and phosphorylase in muscle, 

 which has a long life span and a slow protein metabohsm. He found that the 

 three enzymes did not become labeled at the same rate, a result which at 

 least suggests the independent metabohsm of individual proteins in intact 

 and functionmg cells. He estimated half- life times of 20 to 100 days for 

 different glycolytic enzymes and contractile protems of muscle. 



Related to this question have been the observations on the changes in 

 enzyme content of various tissues as a consequence of depletion of some 

 essential metabolite. As a tyj^ical report may be noted the study of Van 

 Pilsum et al. (1957), in which it was shown that. a deficiency for tryptophan 

 or other essential amino acids did not reduce the activity in rat fiver of 

 arginase, aconitase, and D-amino acid oxidase, but did decrease the content 

 of catalase and xanthine oxidase. Although the authors thought this was best 



