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Adenylate cyclase stimulating activity immunologically similar to parathyroid hormone‐related peptide can be extracted from fetal rat long bones

We observed that culture medium conditioned with fetal rat long bones contained peptides immunologically related to the parathyroid hormone‐related peptide of malignancy (PTHrP) and stimulated cyclic AMP production in canine renal cortical membranes. Because the adenylate cyclase stimulating activity (CSA) of the medium increased when bone resorption was stimulated, it was suspected that these peptides were stored in the matrix and released during the resorption process. In this work, we extracted the noncollagenous proteins of fetal rat long bones and found that the extract contained significant amounts of CSA. The biologic activity of the extract was abolished after trypsin digestion and eluted at 24 and 37 kD on filtration HPLC. The CSA of bone extract and of both HPLC peaks could be inhibited by 3–34 and 7–34 parathyroid hormone analogs. It was not blocked by an antiserum directed against the N‐terminal region of parathyroid hormone, but it was significantly inhibited after an overnight preincubation with an antiserum directed against the 1–11 fragment of PTHrP. One band migrating at 18 kD could be visualized after SDS‐PAGE electrophoresis of bone extract and immunoblotting with the anti‐PTHrP antiserum. We conclude that an adenylate cyclase stimulator immunologically similar to PTHrP is present in the matrix of fetal rat long bones. Adenylate cyclase stimulating peptides of lower molecular weight found in bone‐conditioned medium could be active fragments formed by proteolysis during the resorption process.