106 



MICROSOMAL PARTICLES 



are observed in the presence of p-FPhe (Halvorson and Spiegelman [1]), a 

 direct test of its effect on the rate of protein degradation was undertaken. 



Cells were grown overnight in synthetic medium in the presence of radio- 

 active valine or Phe. They were then centrifuged, washed, resuspended in fresh 

 synthetic medium containing glucose, and shaken for 210 minutes in order to 

 diminish any pool radioactivity that might have been present. The cells were 

 again centrifuged, washed twice, and resuspended to a density of 3 mg dry 

 wt./ml in two flasks containing buffer with or without p-FPhe 10~ 2 M. Protein 

 degradation was followed by the appearance of radioactivity in the soluble pool 

 (cold-TCA-soluble radioactivity). The results in table 1 show elevated pool 

 levels in the presence of p-FPhe. Since the previous experiments show that 

 p-FPhe does not inhibit the amino acid incorporation observed in the presence 

 of an exogenous source of energy, these results indicate that the antagonist ac- 

 celerates the rate of protein degradation. 



TABLE 1. Effect of p-Fluorophenylalanine on Protein Breakdown 



Radioactivity Released f 



Control 







40 

 145 

 270 



54 

 171 

 378 

 613 



p-FPhe 



90 

 210 

 380 

 870 

 463 

 870 

 1460 

 2310 



* Incubated aerobically in phosphate buffer, pH 4.5, at 30° C with or without 0.01 M 

 p-FPhe. 

 t Increase in cpm of cold-TCA-soluble fraction/ml incubation mixture. 

 t 19,860 cpm protein/ml incubation mixture. 

 § 81,320 cpm protein/ml incubation mixture. 



DISCUSSION 



A reanalysis of the effects of /7-FPhe on the growth of yeast shows a strong 

 parallelism with its effects on E. coli. A linear rather than exponential rate 

 of growth is seen in the absence of cell division and without decreasing the dif- 

 ferential rates of synthesis of protein and hot-TCA-soluble material or total 

 carbon incorporation. Furthermore, in resting yeast cells, the incorporation of 

 endogenous amino acids is not influenced by the presence of p-FPhe. The previ- 

 ously observed inhibition of a-glucosidase synthesis by p-FPhe (Halvorson and 

 Spiegelman [1]) may therefore represent another example of inactive enzyme 

 synthesis (Cohen and Munier [8]). In contrast to the results found with E. coli, 

 however, p-FPhe is capable of completely suppressing a-glucosidase induction 

 in resting yeast cells only when added simultaneously with the inducer (Hal- 

 vorson and Jackson [9]). When p-FPhe was added at various times after the 



