Human Cathepsin O2, a Matrix Protein-degrading Cysteine Protease Expressed in Osteoclasts

Abstract

Cathepsin O2, a human cysteine protease predominantly present in osteoclasts, has been functionally expressed in Spodoptera frugiperda Sf9 cells using the Autographa californica nuclear polyhedrosis virus. Following in vitro activation at pH 4.0 with pepsin, active enzyme with an apparent molecular weight of 29,000 was obtained. N-terminal sequencing revealed the typical processing site for cysteine proteases of the papain family with a proline in the position adjacent to the N-terminal alanine residue. The S2P2 subsite specificity of human cathepsin O2 is similar to cathepsin S but distinguished from cathepsins L and B. Similar to cathepsin S, cathepsin O2 is characterized by a bellshaped pH activity profile and is stable at pH 6.5 for 30 min at 37 degrees C. Cathepsin O2 is further distinguished by its potent collagenolytic activity against Type I collagen between pH 5 and 6, and elastinolytic activity against insoluble elastin at pH 7.9. Its capacity to efficiently degrade Type I collagen and its high expression in osteoclasts suggest that cathepsin O2 may play a major role in human osteoclastic bone resorption.