AMINO ACID INCORPORATION BY LIVER MICROSOMES 



233 



the soluble fraction. It seemed possible, therefore, that the increased in- 

 corporation rates described here might be due to a facilitated release of 

 already labelled proteins from the anabolic sites on the RNP-particles. 

 The distribution of incorporated ^^C-amino acids in different subfractions 

 of the incubation mixture was therefore studied in experiments with 

 mitochondria-free homogenates from normal and aminofluorene-treated 

 animals. An experiment of this kind is shown in Table IX. When the 



TABLE IX 



Incorporation of ["C]-l-Leucine into Protein by Mitochondria-Free Rat 



Liver Homogenates from Normal and Aminofluorene-Treated Animals. 



Distribution of Isotope between the Proteins of Different Subfractions 



OF THE Incubation System. 



Experimental conditions as in Table V, 2-aminofluorene (AF, 25 mmoles/kg. 

 body weight) being administered 21 hr. prior to decapitation. 



incorporation system from the aminofluorene-treated animals is compared 

 with the control system, it appears in fact that an especially high increase 

 in specific activity was obtained in the soluble fraction. A significantly 

 smaller increase was observed in the proteins of the microsomes. 



Discussion 



The experiments described above indicate that under certain con- 

 ditions the activity of RNP-particles in cell-free incorporation systems 

 may be determined by factors, established in the intact cells and still 

 retaining their influence in the disintegrated preparations. According to 

 the experimental evidence, positive effects of this kind may be observed as 

 well as negative. The examples which were given of negative after-effects 

 in cell-free systems are most easily explained on the basis of a decreased 

 functional stability of the microsomal membranes. In the experiments 

 mentioned, this effect was induced under the action of a characteristic 

 group of liver poisons which also includes some potent carcinogens. 

 The experiments clearly illustrate the general importance of the metabolic 



