Biological Properties of Synthetic Human Parathyroid Hormone: Effect of Deamidation at Position 76 on Agonist and Antagonist Activity*

Abstract

Recent studies have suggested a role for the carboxyl-terminus of PTH in the binding of the molecule to renal and skeletal receptors, but the functional significance of this binding remains uncertain. We have investigated the possible role of this region by examining the effect of substituting the asparagine residue at position 76 of the native human molecule [Asn76]hPTH-(1-84) with an aspartate residue, [Asp76] hPTH-(1-84) on activity in both renal and skeletal cytochemical (CBA) and adenylate cyclase (AC) bioassays. In the renal CBA, [Asp76]hPTH-(1-84) was considerably less potent than [Asn76]hPTH-(1-84) and produced dose-dependent inhibition of the bioactivity of intact bovine (b) PTH-(1-84), bPTH-(1-34), and [Asn76]hPTH-(1-84). [Asp76]hPTH-(39-84) inhibited the response to intact PTH to a lesser extent, whereas [Asp76]hPTH-(53-84) had no antagonistic activity. In the metatarsal CBA, [Asp76]hPTH-(1-84) inhibited the response to intact PTH, but was less potent than in the renal CBA. In both renal (OK) and skeletal (UMR) cell AC assays [Asp76]hPTH-(1-84) and [Asn76]hPTH-(1-84) were equipotent agonists. Therefore, the CBAs are much more sensitive to modification of the carboxyl end of the molecule than AC assays. The antagonist properties of [Asp76]hPTH-(1-84) appeared to be mediated by phosphodiesterase activation as theophylline abolished the antagonism of this analog. These studies indicate that generation of PTH analogs, modified at the carboxyl-terminal region as well as at the amino-terminus, may be useful for developing potent PTH antagonists.