Processing of NH2- and COOH-terminal peptides of rat osteocalcin by cathepsin B and cathepsin L

Rat osteocalcin was subjected to proteolysis by cathepsins B and L at acid pH in vitro. Short fragments of fewer than 8 amino acids were liberated from both the NH2- and COOH-termini of the molecule, but the midportion, composed of antiparallel α-helical domains, was resistant to proteolysis. Intact rat osteocalcin bound 10 Ca2+/mol protein at pH 7.5 and the binding decreased to half that amount at pH 5.0, while the midportion fragment (Ala8-Lys43) bound 4–5 Ca2+/mol protein at both pH 5.0 and 7.5. When COOH-terminal-truncated rat osteocalcin (Tyr1-Lys43) was prepared with lysyl-endopeptidase, it showed nearly the same Ca2+ binding as that of the intact molecule. Our results suggest that proteolytic processing of the terminal sequence of osteocalcin alters its intrinsic Ca2+-binding capacity and that its NH2-terminal sequence is probably involved.