Possible role and mechanism of action of dissolved calcium in the degradation of bone collagen by lysosomal cathepsins and collagenase.

Equilibrium experiments with bone powder, at pH values ranging from 6.3 to 3.5, show a linear relation between log([Ca2+]/[Ca2+]0) (where [Ca2+]0 = 1 M-Ca2+) and pH, indicating that [Ca2+] could reach levels of 25 mM at pH 5 and 90 mM at pH 4. These elevated Ca2+ concentrations stimulated the lysis of insoluble bone collagen in vitro by purified lysosomes and by mouse bone collagenase, whose activities were additive at acid pH. At neutral pH, the addition of 10-100 mM-CaCl2 did not influence the susceptibility of acid-soluble skin collagen in solution towards bone collagenase, but increased it markedly towards collagen in the fibrillar form. Increasing the [Ca2+] did not influence the susceptibility of collagen to trypsin. Elevated [Ca2+] and a co-operation between lysosomal cysteine proteinases and matrix collagenase could thus participate in the osteoclastic breakdown of bone collagen.