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

gamma-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 gamma-glutamyl-tripeptides including gamma-Glu-Cys-Gly (glutathione) and its analogs S-methylglutathione and S-propylglutathione, or dipeptides including gamma-Glu-Ala and gamma-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 gamma-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 gamma-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 gamma-glutamyl peptides and L-amino acids activate the CaR via a common mechanism.