4 



Biochemical Characterization and 



Electron-Microscopic Appearance of 



Microsome Fractions 



DAVID GARFINKEL 1 



Eldridge Reeves Johnson Foundation for Medical Physics 

 University of Pennsylvania 



The electron-microscope studies on microsomes by Palade and Siekevitz [1] 

 have resulted in the definition of three kinds of microsomes: granules of 150 A 

 diameter; smooth-surfaced vesicles; and rough-surfaced vesicles which differ in 

 appearance from the smooth-surfaced ones primarily by having the granules 

 attached to them. Biochemical studies of microsomes have resulted in the iso- 

 lation of two varieties of microsomes — the 150 A granules just mentioned, 

 which are the principal subject of interest in this symposium, and which are 

 rich in RNA but poor in lipid and cytochrome b 5 , and a particle isolated by 

 Perm and Mackler [2] which is rich in cytochrome b 5 and lipid and poor in 

 RNA. It will be shown here that there are at least three biochemically dis- 

 tinct varieties of microsomes, correlated with those observed in the electron 

 microscope. 



It is possible to fractionate mammalian liver microsomes (the work here 

 described is with pig liver) so as to obtain, in addition to the microsomes as 

 they are usually prepared, a small light fraction of microsomes which is usually 

 found to contain about twice as much cytochrome b 5 and less than half as 

 many ribonucleoprotein granules per unit biuret protein as the bulk of the 

 microsomes, hereafter referred to as the bul\ fraction. This fractionation may 

 be made, for instance, by centrifuging a concentrated (1 part liver to 2 parts 

 0.25 M sucrose) homogenate, after the mitochondria have been removed, at 



1 Public Health Service Research Fellow of the National Cancer Institute, 1955-1957. 



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