Determination of amino acid sequence responsible for suppression of bone resorption by serum calcium-decreasing factor (caldecrin).

We previously reported on the serum calcium-decreasing activity of recombinant protein factor referred to as caldecrin [Tomomura et al. (1995) J. Biol. Chem. 270, 30315-30321]. To address the mechanism of this serum calcium-decreasing activity, we investigated the effect of rat caldecrin on osteoclastic bone-resorbing activity. Wild-type caldecrin suppressed resorption pit formation by osteoclast on a dentine slice in a dose-dependent manner. The suppressive effect on the bone resorption was not affected by treatment of caldecrin with phenylmethyl sulfonyl fluoride or by use of protease-deficient mutant caldecrins. Recombinant procaldecrin (-13-239), and its fragments (-13-125), (1-111), (1-46), (47-111), and (126-239) were expressed as His-tagged thioredoxin fusion proteins and investigated for their ability to suppress bone resorption. The proform (-13-239) and fragment (-13-125) did not affect the suppressive activity, whereas fragments (1-111) and (126-239) did suppress the bone resorption. The bone-resorbing activity was also suppressed by fragment (47-111), not by fragment (1-46). Overlapping fragments (47-62), (47-79), (47-98), (56-111), (71-111), and (85-111) were compared for their suppressive activity. The fragments (47-62) and (85-111) did not affect the activity, but the other fragments suppressed the bone resorption. A synthetic peptide having the (71-79) sequence suppressed the bone resorption. These results suggest that amino acid sequence corresponding to rat caldecrin (aa 71-79) is responsible for the suppression of bone resorption by caldecrin.