Inhibition of parathyroid hormone bioactivity by human parathyroid hormone (PTH)-(3-84) and PTH-(8-84) synthesized in Escherichia coli.

Human PTH (hPTH)-(3-84) and hPTH-(8-84) were synthesized in Escherichia (E.) coli when the cells were transformed with a multicopy plasmid in which the transcription of human preproPTH cDNA is directed by the E. coli lac promoter. PTH fragments were extracted from cells and purified by reverse phase HPLC. PTH bioactivity and PTH antagonist activity were estimated in a renal cytochemical bioassay. hPTH-(3-84) and hPTH-(8-84) exhibited less than 1% and less than 0.1%, respectively, of the biological activity of synthetic hPTH-(1-84). hPTH-(8-84) had 1% of the PTH inhibitory activity of synthetic [Nle8,18,Tyr34]bovine PTH-(3-34)amide, whereas hPTH-(3-84) was 100 times more active as a PTH inhibitor than the synthetic bovine PTH-(3-34) analog. The latter has so far been recognized as the most potent PTH antagonist in vitro. A 5-fold molar excess of hPTH-(3-84) over hPTH-(1-84) completely blocked the biological action of intact hPTH-(1-84) in the renal cytochemical bioassay. These findings suggest that the carboxyl-terminal portion of the intact hPTH-(1-84) molecule contributes importantly to inhibitor potency.