Effects of rat fetuin on stimulation of bone resorption in the presence ofparathyroid hormone

Rat fetuin, which is the rat counterpart of human alpha(2)-HS glycoprotein and bovine fetuin, is only detectable in calcified tissues such as bone mat rices and dentin, and bone cells such as osteoblasts and osteocytes immunoh istochemically, The effect of this protein on bone resorption was examined to study its physiological role in bone metabolism. Rat fetuin increased bo ne resorption in the presence of low concentrations of parathyroid hormone (PTH), but it had no activity on bone resorption without PTH, The increase in bone resorption by PTH and PTH plus rat fetuin was inhibited by the addi tion of chymostatin, an inhibitor for cathepsin L, Moreover, we found that when type I collagen from rat was preincubated with rat fetuin, the digesti on of rat type I collagen by cathepsin L was increased. These findings sugg est that rat fetuin present in bone matrix is important in bone resorption.