GARFINKEL 



25 



TABLE 1 



Sedimentable Protein, mg/ml 



Before digestion 

 After digestion 



Light Fraction 



17 



8 



Bulk Fraction 



35 

 20 



for 10 days to 2 weeks is sufficient to solubilize nearly all the cytochrome; 

 this is in fact the method of preparing cytochrome b 5 [6, 7]. The nucleopro- 

 tein granules have practically disappeared in the process. Examination of the 

 sedimentable protein content of these two suspensions shows that much more 

 protein is left in the bulk fraction. Nearly all the nucleoprotein has been re- 

 moved, and since the cytochrome contents were equal there should be the same 

 amount of residue from the smooth-surfaced microsomes in both fractions. 

 Apparently there is something present in the bulk fraction (of which there is 

 much less in the light fraction) to account for the difference in protein con- 

 tent. Since the other two forms were eliminated, this must be the rough-sur- 

 faced microsomes. That there is a qualitative difference in the residues can be 

 seen by looking at the centrifuged pellets. Both contain a transparent amber 

 layer, but this is all that is left of the light microsomes, whereas the bulk-frac- 

 tion pellet has below this transparent layer an opaque tan one, showing some 

 signs of further layering. This bulk-fraction pellet still contains the noncyto- 

 chrome heme originally present. The rough-surfaced microsomes, which have 

 not previously been characterized biochemically, are therefore found to contain 

 a tan pigment and the noncytochrome heme, and to be distinct from the smooth- 

 surfaced ones. This finding is summarized in table 2. It should be kept in mind 

 that the expression "rough-surfaced microsomes" means the vesicles themselves 



