What's a MICROSOME?

[Michael S. Straka]

Calvino Ka-wing Cheng <ckwcheng at acs4.acs.ucalgary.ca> wrote:

>OOPS!  I made a mistake in my posting.   I've been looking all
>over for the definition of a MICROSOME, not microbody.  Are they
>the same?
>
>I need some help.
>
>Cal


Hi Cal.

Subcellular fractionation is generally carried out by sequential 
centrifugation, during which the denser fractions are pelleted at lower g 
forces, less dense fractions at higher g, etc.  There can be considerable 
overlap, depending upon the care with which the prep is done, and thus 
purity of fractions needs to be determined, particularly when you're just 
starting out.  Immuno-isolation is also done, but that obviously requires 
an antiserum to proteins of the specific fraction desired (and a whole 
new set of criteria to determine purity and specificity).  

The classical definition of microsomes among cell biologist types is 
(generally) those roughly spherical vesicles which are pelleted by a spin 
of about 100,000 x g.  They are considered to be fragments of sheet ER 
which have been disrupted and re-formed during the process of 
homogenization.  The purity of the prep can be ascertained by measuring 
the enrichment of ER enzymes (eg NADPH-cytochrome c reductase or other 
cytochrome P450 enzymes) vs enrichment of enzymes of other fractions, 
such as alkaline phosphatase (plasma mb), galactosyl transferase 
(trans-Golgi), citrate synthase (mitochondria) and 
N-acetylglucosaminidase (lysosomes).

I was in this field for a number of y, however, not for the past 4, so 
unfortunately I don't have specific refs handy.  If you need extensive 
amts of info, I would recommend starting with J.Cell Biol, Annu.Rev.Cell 
Biol., Cell, etc, concentrating on keywords such as trafficking, marker 
enzymes, endocytosis, membranes.  I imagine that most of the info re 
markers will be found prior to 1990.

Good luck.

-Mike Straka