GOLGI MEMBRANES I27 



Dr. Green: Non-localization of components or activities bespeaks incomplete separa- 

 tion. Paucity of assays may make assignment of enzymatic specificity more difficult. 



Dr. Kuff: Of course, it is most dinicult to decide whether the presence in a frac- 

 tion of some biochemical component at low concentration represents a true as.sociation of 

 thai (.'oinponcnt with the particle in ciucstion or merely reflects an incomplete separa- 

 tion of biochemically distinct entities. It woulil a{ipear that complete separations of 

 cytoplasmic particulates are rather rare. With regard to the number of enzymes assayed, 

 admittedly it is small, since the work is in a fairly early stage. 



Dr. Rumney: Could you define the post-microsomal fraction in terms of method of 

 preparation.' Is this fraction deri\ctl from the microsomes.'' 



Dr. Kuff: In the fractionation of epididymal homogenates in o.ScS m sucrose, a main 

 microsomal fraction was obtained by centrifugation of the mitochondrial supernatant 

 fluid for 30 minutes at 40,000 rpm in the no. 40 rotor of the Spinco Model L ultra- 

 centrifuge. The supernatant fluid from this run was diluted with water to a sucrose 

 concentration of 0.25 m and centrifuged for 60 minutes at the same speed. The sedi- 

 ments thus obtained constituted the so-called 'post-microsomal' fraction. Electron micros- 

 copy showed that the main microsome fraction consisted of vesicular elements, many of 

 which carried ribonucleoprotein particles on their surfaces. The post-microsomal 

 fraction contained a great preponderance of smooth-surfaced vesicles. Certain biochem- 

 ical differences were also observed (fig. 10). However, from the operational point of 

 view, both fractions would be thought of as microsomal in nature, in that they con- 

 sist of submicroscopic particulate material. 



