Differences in binding affinities of human PTH(1-84) do not alter biological potency: a comparison between chemically...

The purpose of this study was to evaluate receptor binding affinities and biological properties in vitro and in vivo of various recombinant hPTH(1-84) forms representing the natural hormone and a mutagenized hPTH form, [Gln26]hPTH(1-84) (QPTH), after expression in E. coli and Saccharomyces cerevisiae. In LLC-PK1 cells stably transformed with the rat PTH/PTHrP receptor, chemically synthesized hPTH(1-84) and QPTH showed a reduced binding affinity (apparent Kd 18 and 23 nM, respectively) than the recombinant, hPTH(1-84) (apparent Kd 9.5 nM). All recombinant hPTH forms showed a similar potency to stimulate cellular cAMP production (EC50 1.5 nM) and significantly better than chemically synthesized hPTH (EC50 5.7 nM). All hormone forms showed an about equipotent activity in causing elevation in serum calcium, increased excretion of urine phosphate, and cAMP. Thus, the natural recombinant PTH forms showed higher binding affinities and adenylate cyclase activation potencies in LLC-PK1 cells, but the reduced receptor binding affinity exerted by QPTH did not transcend differences in cAMP generation and in vivo biological activities.