Isolation of mannose-binding proteins from human and rat liver

A binding assay for the detection of mannose-binding proteins was developed, which uses a ligand of mammalian origin, 125I-labelled bovine pancreatic ribonuclease B. The binding assay was validated by using the recognized mannose-binding protein, concanavalin A. Microgram quantities of concanavalin A or mannose-binding proteins could be assayed. A mannose-binding protein was isolated from rat liver by affinity chromatography on mannose-Sepharose 6B. It has a Mr of approx. 900000 under non-dissociating conditions and contains a subunit of approx. 34000 Mr. When ribonuclease B-Sepharose was used as a ligand for affinity chromatography, the predominant mannose-binding material isolated from rat liver had a native Mr of 205000-225000 and consisted largely of a subunit of Mr 70000, which yielded subunits of Mr 28500 and 34000 on reduction. It is suggested that different mannose-binding proteins are isolated by the two affinity-chromatography ligands. A mannose-binding protein was also purified from human liver by affinity chromatography on mannose-Sepharose 6B. It has a native Mr of over 1000000 and consists of subunits with Mr 28000 and 30500. Its isolation suggests that mannose-mediated endocytosis or intracellular transport of glycoproteins occurs in human liver.