Specific cleavage of bovine parathyroid hormone catalyzed by an endopeptidase from bovine kidney.

Cleavage of parathyroid hormone (PTH) is catalyzed by an endopeptidase associated with a partially purified membrane preparation from bovine kidney cortex. This enzyme was found to have an acid pH optimum and to be easily extracted from the membranes by a single freeze-thaw cycle. The cleavage is remarkable in that it appears to be restricted to a small region of the PTH peptide chain, generating fragments which are not further degraded. The dominant products are a large fragment, COOH-terminal in origin, and a small fragment from the NH2 terminus. The small fragment is biologically active and its activity establishes that it contains at least the first 29 amino acids in PTH. The large fragment has no biological activity. The cleavage of PTH was demonstrated both with iodinated PTH and with unlabeled hormone by immunoassay and by labeling the large fragment after its production. Microsequencing of the large fragment showed that, in fact, two products are produced: one with its NH2 terminus at position 38 of PTH and one with its NH2 terminus at position 35. These fragments are remarkably similar to those generated in both the liver and the kidney in vivo.