Formation of parathormone 8-34 by cathepsin-D digestion of parathormone and its efficacy as a hormone antagonist.

It previously has been shown that digestion of bovine parathormone (bPTH) with cathepsin-D results in rapid cleavage of the hormone between Phe34 and Val35 yielding PTH(1-34) and PTH(35-84). Since bPTH also contains a Phe at residue 7 we have conducted additional studies to determine whether cleavage at this position could occur. We have found that following longer incubation periods of hormone and enzyme, 2 additional peptides are generated; PTH(8-34) and PTH(1-7). Time course studies demonstrated that these 2 fragments are formed from the (1-34) peptide generated through the initial cleavage at Phe34-Val35 of PTH. The identification of the bPTH(8-34) was accomplished through amino acid analysis and N-terminal sequencing. bPTH(8-34) behaved as a PTH antagonist in an in vitro mouse calvarial bone resorption assay. Although bPTH(8-34) did not affect the PTH-stimulated cAMP response when added simultaneously with PTH, preincubation of bone cells with this peptide caused desensitization of the PTH-stimulated cAMP response.