Identification of peptide fragments generated by digestion of bovine and human osteocalcin with the lysosomal proteinases cathepsin B, D, L, H, and S.

We have determined the primary cleavage sites in the bone Gla protein (BGP; osteocalcin) for several of the proteases that could act on the protein during bone resorption and turnover, cathepsins B, D, L, H, and S. The time course of BGP digestion by each cathepsin was first determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. We then incubated human and bovine BGP with each cathepsin for a sufficient time to reduce the level of intact protein by at least 20-fold, isolated the major cleavage peptides, and identified each by N-terminal sequence analysis and by amino acid analysis. Our results show that BGP has relatively few cathepsin-sensitive sites and that these sites are located at the N and C terminus of the 49-residue protein. Cathepsins B, L, H, and S readily cleave BGP at the G7-A8 bond; cathepsin L also cleaves at R43-R44; cathepsin B also cleaves at R44-F45; and cathepsin D cleaves only at A41-Y42. The immunoreactivity of the major peptides generated by cathepsin cleavage was evaluated using the original radioimmunoassay developed for the detection of BGP in human serum. The BGP 8-49 fragment cross-reacts identically with native BGP, while the 8-43 and the 1-44 fragments require 20- to 40-fold higher concentrations to achieve the same level of displacement as the native protein. The 1-41 and 8-41 fragments are unable to significantly displace the labeled native BGP tracer at any concentration tested. These results demonstrate the utility of peptides generated by cathepsin digestion in the mapping of the antigenic epitopes recognized by a given BGP immunoassay.