Type I collagen fibers account for 90% of the organic matrix of bone. The degradation of this collagen is a major event during bone resorption, but its mechanism is unknown. A series of data obtained in biological models strongly suggests that the recently discovered cysteine proteinase cathepsin K plays a key role in bone resorption. Little is known, however, about the actual action of cathepsin K on type I collagen. Here, we show that the activity of cathepsin K alone is sufficient to dissolve completely insoluble collagen of adult human cortical bone. We found that the collagenolytic activity of cathepsin K is directed both outside the helical region of the molecule, i.e. the typical activity of cysteine proteinases, and at various sites inside the helical region, hitherto believed to resist all mammalian proteinases but the collagenases of the matrix metalloproteinase family and the neutrophil elastase. This property of cathepsin K is unique among mammalian proteinases and is reminiscent of bacterial collagenases. It is likely to be responsible for the key role of cathepsin K in bone resorption.The only mammalian proteinases that have been shown to attack the native triple helical region of type I collagen are the collagenases of the MMP 1 family (1-3) and the neutrophil serine elastase (4) They cleave the type I collagen triple helix across all three chains (i.e. two ␣1 chains and one ␣2 chain) only at a specific point three-quarters of the way to the Nterminal end of the collagen molecule. Proteinases with broad specificity, such as cysteine proteinases, attack only the extrahelical regions that are located at either end of native collagen (telopeptides) and that represent only 4% of the molecule (5). Because the telopeptides are involved in intra-and intermolecular links, this attack may separate individual molecules. The latter proteinases may also attack destabilized triple helices, acting thereby as gelatinases. At 37°C, such a destabilization may transiently affect a small proportion of collagen molecules, because the melting temperature of soluble collagen is only a few degrees higher. It has also been emphasized that when collagen molecules are cross-linked and arranged in insoluble fibers they become more resistant to proteolysis (6). However, the co-operation of proteinases with distinct specificities toward the chemical bonds of collagen fibers has been shown to favor the efficiency of collagenolysis (7).Insoluble type I collagen fibers constitute 90% of the organic matrix of bone, and their degradation is necessary for bone resorption (8). The test tube experiments that have been performed so far showed that it is difficult to achieve complete degradation of adult lamellar bone with a single bone proteinase (9, 10). On the other hand, various biological approaches have shown that both MMPs and cysteine proteinases participate in the bone resorption processes (8,(11)(12)(13). Representatives of these two types of proteinases were identified in osteoclasts, the cells responsible for bone resorptio...

These in vitro and ex vivo experiments of human cartilage suggest that Helix-II and CTX-II could be released in part by different enzymatic pathways. Helix-II and CTX-II alone reflect only partially overall cartilage collagen degradation. These findings may explain why these two biological markers could provide complementary information on disease progression in OA.

Helix-II and CTX-II show to some degree differential selectivity for specific features in cartilage tissue. CTX-II detection close to bone may be relevant to the possible role of subchondral bone in OA. The restricted co-localization of breakdown markers and PIIANP suggests that collagen fragments can result only partially from newly synthesized collagen. Our study strengthens the interest for the question whether combining several markers reflecting different regional cartilage contributions or metabolic processes should allow a broader detection of OA activity.