Surfactant protein D is a divalent cation-dependent carbohydrate-binding protein

Surfactant protein D (SP-D, CP4) is a collagenous surfactant-associated glycoprotein synthesized by lung type II epithelial cells. SP-D can be selectively and efficiently eluted from isolated rat surfactant with glucose, maltose, and certain other saccharides. We therefore examined the ability of the purified protein to interact with carbohydrates in vitro. Saccharide-substituted bovine serum albumins (BSA neoglycoproteins) were adsorbed to plastic wells, and binding of purified SP-D was quantified with monospecific antibodies to SP-D using an indirect immunoassay. SP-D showed specific calcium-dependent binding to α-D-glucosidophenyl isothiocyanate-BSA and maltosyl-BSA, but negligible binding to β-D-glucosidophenyl isothiocyanate-BSA or unconjugated BSA. The most efficient inhibitors of SP-D binding were α-glucosyl-containing saccharides (e.g. isomaltose, maltose, malotriose). SP-D showed quantitative binding to maltosyl-agarose and was specifically eluted with maltose or EDTA. High affinity binding to maltosyl-BSA was also demonstrated using a solution-phase polyethylene glycol precipitation assay. These studies demonstrate that SP-D is a calcium dependent lectin-like protein and that the association of SP-D with surfactant is mediated by carbohydrate-dependent interactions with specificity for α-glucosyl residues.