Allosteric Modulation of the Calcium-sensing Receptor by γ-Glutamyl Peptides INHIBITION OF PTH SECRETION, SUPPRESSION OF INTRACELLULAR cAMP LEVELS, AND A COMMON MECHANISM OF ACTION WITH l-AMINO ACIDS

γ-Glutamyl peptides were identified previously as novel positive allosteric modulators of Ca2+o-dependent intracellular Ca2+ mobilization in HEK-293 cells that bind in the calcium-sensing receptor VFT domain. In the current study, we investigated whether γ-glutamyl-tripeptides including γ-Glu-Cys-Gly (glutathione) and its analogs S-methylglutathione and S-propylglutathione, or dipeptides including γ-Glu-Ala and γ-Glu-Cys are positive allosteric modulators of Ca2+o-dependent Ca2+i mobilization and PTH secretion from normal human parathyroid cells as well as Ca2+o-dependent suppression of intracellular cAMP levels in calcium-sensing receptor (CaR)-expressing HEK-293 cells. In addition, we compared the effects of the potent γ-glutamyl peptide S-methylglutathione, and the amino acid l-Phe on HEK-293 cells that stably expressed either the wild-type CaR or the double mutant T145A/S170T, which exhibits selectively impaired responses to l-amino acids. We find that γ-glutamyl peptides are potent positive allosteric modulators of the CaR that promote Ca2+o-dependent Ca2+i mobilization, suppress intracellular cAMP levels and inhibit PTH secretion from normal human parathyroid cells. Furthermore, we find that the double mutant T145A/S170T exhibits markedly impaired Ca2+i mobilization and cAMP suppression responses to S-methylglutathione as well as l-Phe indicating that γ-glutamyl peptides and l-amino acids activate the CaR via a common mechanism.