Heparin-binding lectin from human placenta: purification and partial molecular characterization and its relationship to basic fibroblast growth factors

Abstract

The heparin-binding lectin from human placenta is isolated on the basis of its tendency to form large aggregates by gel filtration and on the basis of its affinity for heparin by affinity chromatography. The purified lectin dissociates into up to four distinct polypeptides with molecular weight values of 14,400, 15,000, 16,200, and 16,700 and a single isoelectric point of 9.0. Molecular heterogeneity is not due to different degrees of glycosylation, as evidenced by gel electrophoretic analysis after extensive treatment with various endoglycosidases. Despite its similarities of affinity to heparin, molecular size, and isoelectric point to the basic fibroblast growth factor (bFGF), the comparatively high yield of the lectin (approximately 1.5 mg/100 g of placenta), the occurrence of proteolytic fragmentation in the presence of heparin, and the lack of homology to the amino-terminal sequence of the lectin argue against any notable relationship to bFGF. Most importantly, the lack of mitogenic activity in a commonly used bioassay with quiescent 3T3 fibroblasts rules out any FGF-like activity on cell proliferation. The heparin-binding lectin is thus clearly distinguishable from heparin-binding growth factors. By employing biotinylated heparin as labeled ligand to visualize and quantify heparin binding, hapten inhibition in a solid-phase assay reveals that except for heparin no other vertebrate glycosaminoglycan but the sulfated fucan fucoidan can effectively reduce the Ca2+-independent ligand binding. Proteolytic fragmentation by chymotrypsin in two independent assays demonstrates that a fragment of Mr 7800 still retains ability to bind heparin. The interaction of this lectin with naturally occurring heparin-like molecules may physiologically be involved in modulatory regulation of heparin-mediated processes.