45G 



F. B. STRAUB 



of the pancreatic secretion respectively. However, a time curve of incorporation, 

 showing a lag period, can only be accepted if transport processes and bound 

 forms of the protein can be excluded. On this ground, all the above experiments 

 were rightly criticized. We have therefore studied the time course of the incor- 

 poration of radioactive amino acids into the amylase of pancreas slices in vitro, 

 taking care to isolate not only the free, but also all of the bound enzyme. 



15000 



10000 



5000 



600 



400 



200 



Fig. I. Time course of incorporation of ['^COOH]glycine 

 into pancreas components. 



Fig. I shows the result of such an experiment. It is clear that the labelling 

 shows a definite lag period of about lo minutes. In further experiments we have 

 studied the rate of labelUng of the amylase bound to several tissue components. 

 After incubating the shces of pancreas, we have homogenized them and isolated 

 the nuclear fraction, the fraction of heavier and Ughter granules, moreover the 

 fraction which is rich in zymogen granules, and finally the soluble fraction. From 

 all these fractions the amylase was isolated and separately examined for the label 

 it contains. Without going into details I want to mention only that the labelling 

 of the amylase found in the heavier granules showed the same lag period. The 

 labelling of the amylase in all other fractions was nearly linear. In short-time 

 experiments the labelling of the amylase in the fighter granules (microsomes) 

 was the highest, yet its time course was also linear (Table i). 



It seems therefore that the synthesis of amylase protein begins in the micro- 

 somes, where a precursor is formed from amino acids. This is transferred into 

 the heavier granules, where it is syntheticafiy transformed into the active enzyme. 



I do not want to go into details of our work on the second phase of these 

 events. The soluble system affords good opportunities for the study of the 

 factors involved. We have succeeded in showing the presence of several compo- 

 nents, all of them necessary for the transformation of the precursor protein into 



