Leaching from affinity media is a well-known phenomenon, especially for media with large ligands such as proteins and lectins. The amount of leaching varies with the type of media and operating conditions, but there are no completely leach-free media.
A common used method to decrease the leaching of Con A is to rinse out the Con A ligands which are non-covalently bound to the media with a buffer before the medium is used.
Various tricks for decreasing Con A leaching are discussed in the following paper:
Marikar, Y. et al. Leaching of concanavalin A during affinity chromatographic isolation of cell surface glycoproteins from human fetal neurons and glial cells. Anal. Biochem. 201, 306–310 (1992).
As compared to many other types of affinity media, the leaching from Con A Sepharose is more extensive because Con A consists of three subunits, held together by non-covalent bonding. The stability of the ligand depends on whether any individual Concanavalin A molecule has been attached to the matrix by one, two or three of its subunits. Leaching will also vary with pH. As for any type of ligand attached by the CNBr coupling technique, higher leaching is expected at higher pH values, especially at pH values above 8. Possibly a decrease in operating pH can give decreased ligand leaching in this case since operating pH is fairly close to 8. Another typical observation about leaching from these types of affinity media is that leaching will decrease over time as the most loosely bound ligands are removed.
For critical applications where downstream removal of the contaminating Con A in purified material is not an option, there is a possibility to cross-link immobilized Con A before using the medium. Guidance can be found in the following papers:
Miernyk, K. M., and Johnston, M. L. Chemical cross-linking immobilized concanavalin A for use in proteomic analyses. Prep. Biochem. Biotechnol. 36, 203–214 (2006).
Kowal, R. and Parsons, R. G. Stabilization of proteins immobilized on Sepharose from leakage by glutaraldehyde crosslinking. Anal. Biochem. 102, 72–76 (1980).