A mouse monoclonal antibody (AN9P1) to keratan sulphate is described. In a competitive-inhibition solution-phase radioimmunoassay employing 125I-labelled intact proteoglycan, it reacts preferentially with keratan sulphate bound to the core protein of adult human articular-cartilage proteoglycan and to a much lesser degree with keratan sulphate purified from this proteoglycan. Proteolytic cleavage of the proteoglycan by pepsin and trypsin has little effect on antibody binding, but treatment with papain decreases binding considerably and more than does treatment with keratanase. An even greater decrease in binding is observed after treatment with alkaline borohydride. A comparison of binding of antibody AN9P1 with that of another previously described monoclonal antibody, 1/20/5-D-4, to keratan sulphate [Caterson, Christner & Baker (1983) J. Biol. Chem. 258, 8848-8854] revealed similar binding characteristics, both showing much diminished binding after papain digestion of proteoglycan and even less with purified skeletal keratan sulphate. Removal of the Fc piece of antibody AN9P1 had no significant effect on the differential binding of divalent F(ab′)2 fragment to proteoglycan, to papain-digested proteoglycan and to keratan sulphate, although there was a small decrease in binding to papain-digested proteoglycan. Conversion of the antibody into univalent Fab fragment with removal of the Fc piece resulted in diminished binding to proteoglycan, compared with that observed with IgG, and in enhanced binding to free keratan sulphate and to papain-digested proteoglycan. These results suggest that close proximity of keratan sulphate chains on the core protein of proteoglycans favours preferential reactivity of bivalent antibody with these species through cross-bridging of chains by antibody. Conversely, much decreased binding to keratan sulphate on proteoglycan core-protein fragments and to free keratan sulphate results from a lack of close proximity of keratan sulphate. By using univalent Fab fragment in these assays these differences in binding are minimized by preventing cross-bridging and thereby enhancing detection of smaller fragments without sacrificing too much sensitivity of detection of larger proteoglycan species. The persistent preferential binding of Fab fragment to proteoglycan is probably in part the result of the increased epitope density in the intact molecule compared with keratan sulphate in a more disperse form.

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