Fibromodulin from bovine articular cartilage has been subjected to lectin affinity chromatography by Sambucus nigra lectin which binds 2-6)- linked N-acetylneuraminic acid, and the structure of the keratan sulphate in the binding and non-binding fractions examined by keratanase II digestion and subsequent high pH anion exchange chromatography. It has been confirmed that the keratan sulphate chains attached to fibromodulin isolated from bovine articular cartilage may have the chain terminating N-acetylneuraminic acid residue 2-3)- or 2-6)-linked to the adjacent galactose residue.
Although the abundance of 2-6) linked N-acetylneuraminic acid (ca. 22%) is such that this could cap one of the four chains in almost all fibromodulin molecules, it was found that ca. 34% of the fibromodulin proteoglycan molecules from bovine articular cartilage were capped exclusively with 2-3)-linked N-acetylneuraminic acid. The remainder of the fibromodulin proteoglycans which bound to the lectin had a mixture of 2-3)- and 2-6)-linked N-acetylneuraminic acid capping structures.
The keratan sulphates attached to fibromodulin molecules capped exclusively with 2-3)- linked N-acetylneuraminic acid were found to have a higher level of galactose sulphation than those from fibromodulin with both 2-3) and 2-6)-linked N-acetylneuraminic acid caps which bound to the Sambucus nigra lectin. In addition, both pools contained chains of similar length (ca. 8 - 9 disaccharides). Both also contained 1-3)-linked fucose, showing that this feature does not co-distribute with 2-6)-linked N-acetylneuraminic acid, although these two features are present only in mature articular cartilage.
These data show that there are discrete populations of fibromodulin within articular cartilage which may have differing impacts upon tissue processes.