Different affinity of galectins for human serum glycoproteins: galectin-3 binds many protease inhibitors and acute phase proteins

Here we report the first survey of galectins binding to glycoproteinsof human serum. Serum was subjected to affinity chromatographyusing immobilized galectins, and the bound glycoproteins wereanalyzed by electrophoresis, Western blotting, and mass spectrometry.Galectins-3, -8, and -9 bound a much broader range of ligandsin serum than previously known, galectin-1 bound less, and galectins-2,-4, and -7 bound only traces or no serum ligands. Galectin-3bound most major glycoproteins, including alpha-2-macroglobulinand acute phase proteins such as haptoglobin. It bound onlya selected minor fraction of transferrin, and bound none orlittle of IgG. Galectins-8 and -9 bound a similar range of glycoproteinsas galectin-3, but in lower amounts, and galectin-8 had a relativepreference for IgA. Galectin-1 bound mainly a fraction of alpha-2-macroglobulinand only traces of other glycoproteins. The binding of galectin-3to serum glycoproteins requires affinity for LacNAc, since amutant (R186S), which has lost this affinity, did not bind anyserum glycoproteins. The average affinity of galectin-3 forserum glycoproteins was estimated to correspond to K_d 1–5µM by modeling of the affinity chromatography and a fluorescenceanisotropy assay. Since galectins are expressed on endothelialcells and other cells exposed to serum components, this reportgives new insight into function of galectins and the role oftheir different fine specificity giving differential bindingto the serum glycoproteins.