44 MICROSOMAL PARTICLES 



DIVERSITY OF CYTOPLASMIC RIBONUCLEOPROTEINS 



The view that the small, dense particles represent the structural substrate of 

 cytoplasmic basophilia was clearly supported by the isolation of RNP particles 

 from DOC-treated microsomes (fig. 6) and further strengthened by the finding 

 that the pancreatic postmicrosomal fractions (fig. 7), assumed to represent the 

 free particles of the cytoplasmic matrix, are also comprised of ribonucleopro- 

 teins. It should be pointed out, however, that the RNP particles, free and at- 

 tached, do not account for all the RNA of the cytoplasm. Leaving aside the 

 controverted question of the RNA content of mitochondria, we found that the 

 microsomes contain a certain amount of RNA, small in the liver but relatively 

 large in the pancreas, which is solubilized by DOC treatment, under the condi- 

 tions of our experiments, and whose structural connections are unknown. Fi- 

 nally there is a relatively small amount of RNA (~ 10 per cent of the RNA of 

 the cytoplasm) that remains in the supernatant of the last postmicrosomal frac- 

 tion. Evidently we do not know to what extent our findings reflect the situa- 

 tion inside the living cell, or to what extent they are affected by preparation 

 artifacts. It can be argued, for instance, that attached particles can be detached 

 during tissue grinding and fractionation, or that the sedimentation of RNA 

 particles is still incomplete by the end of our last centrifugation. 



With such possibilities in mind, Dr. Siekevitz and I tried to find out in more 

 recent work [54] whether there are functional differences among the various 

 RNP preparations described. In agreement with reports on other tissues [55, 

 56], we noted that the attached particles (DOC-insoluble microsomal material) 

 are more active than the parental microsomes in the incorporation of labeled 

 amino acids into proteins (presumably protein synthesis 10 ). In addition, we 

 found that the activity mentioned is considerably higher in the attached particles 

 than in the free RNP particles of the postmicrosomal fractions. The second 

 postmicrosomal fraction, however, proved to be more active than any other 

 particulate fraction in the incorporation of labeled adenine into RNA. In turn, 

 its activity was greatly exceeded by that of the "soluble" RNA of the final 

 supernatant. 



Such metabolic differences, though not entirely excluding preparation arti- 

 facts, render them more unlikely, and suggest that a variety of RNP's with dif- 

 ferent structural connections and different functions exist within the animal 

 cell. I should also add that by comparing microsomes obtained from starved 

 and fed animals we observed a relatively large increase in microsomal pro- 

 teolytic 11 and ribonuclease activity [57]. What seems to be of considerable 

 interest is the finding that a sizable part of the enzymatic activities of the micro- 



10 In our experiments the incorporation was carried out in vivo; in those reported in 

 references 55 and 56 the labeled amino acids were incorporated either in vivo or in vitro 

 by whole microsomes. 



11 Due mainly to trypsinogen and chymotrypsinogen. These two proteases and the 

 ribonuclease are enzymes synthesized on a large scale by the exocrine cells of the pancreas 

 to be released in the intestine for the digestion of food. 



