I had posted about use of the term isozyme. Since I have gotten a few
questions about how "subtle" the structural differences are between forms, let
me just give some specifics. I had hesitated on this to avoid possibly
"blowing my cover" as a reviewer. The enzymes are the superoxide
dismutases (SODs). Two of them have active sites which contain copper and
zinc, the other has an active site with manganese. The MWs are quite
different for all 3, and antibodies to one generally don't react with the
other two. Cells contain one of the Cu-Zn enzymes (mostly in the cytosol),
and the one Mn enzyme (in the mitochondria). The other SOD is secreted to the
outside of cells and is called extracellular SOD. Thus, these are three
structurally distinct enzymes which have a similar enzyme activity.