In my experience with lectins, one needs to use a ton of competing sugar(s)
to block lectin binding.  Most lectins bind more than one sugar.  ConA
binds both mannose and glucose but I don't know about suc-ConA.  I do know
that when I stained intestinal mucin with ConA, I got good staining in a
subpopulation of goblet cells.  This is despite the fact that mucin is
believed by some to only contain 0-linked sugars (i.e., no glu or man).
Either mucin has some N-glycosylation, ConA binds other sugar groups, or
there were additional glycoproteins in the mucous secretory granules of
some goblet cells.  Lectins also greatly prefer sugars that are attached to
proteins.  If all you are trying to do is demonstrate specific binding
rather than specific binding to mannose, you could try competing off the
lectin with some glycoprotein.  I have never been impressed by reports that
mannose can compete off the binding of ConA and therefore the binding site
on the tissue is mannose.  All that this result implies is that mannose
either binds with higher affinity or you have so much that by the mass
action law, you compete off a higher affinity binding site.  good luck
 
 
 
Thomas E. Phillips, Ph.D.
Associate Professor of Biological Sciences
Director, Molecular Cytology Core Facility
3 Tucker Hall
University of Missouri
Columbia, MO 65211
(314)-882-4712 (voice)
(314)-882-0123 (fax)