Parathyroid hormone (PTH) regulates
calcium and
phosphate metabolism through a
G-protein-coupled receptor which is shared with PTH-related
protein (
PTHrP). Therefore, structure-activity studies of PTH and
PTHrP with their common receptor provide an unusual opportunity to examine the structural elements in the two
hormones and their common receptor which are involved in the expression of
biological activity. Our approach to studying the nature of the bimolecular interface between
hormone and receptor is to use a series of specially designed photoreactive
benzophenone- (BP-) containing PTH analogs in "photoaffinity scanning" of the
PTH/PTHrP receptor. In this report we describe a series of BP-containing agonists and antagonists which have been synthesized by solid-phase methodology and characterized physiocochemically and biologically. Each of the 12 analogs contains a single BP moiety at a different defined position. Examples of BP-containing agonists prepared and studied in human
osteogenic sarcoma Saos-2/B-10 cells are [Nle8,18,Lys13(epsilon-pBZ2),L-2-Nal23,Tyr34]
bPTH(1-34 )NH2(K13)(Kb = 13 nM; Km = 2.7 nM) and [Nle8,18,L-Bpa23,Tyr34[
bPTH(1-34)NH2(L-Bpa23) (Kb = 42 nM; Km = 8.5 nM). Another BP-containing analog, [Nle8,18,D-2-Nal12,Lys13(epsilon-pBZ2),L-2-Nal23 ,Tyr34]
bPTH(7-34)NH2, was a potent antagonist (Kb = 95 nM; Ki = 72 nM). The
amino acids substituted by residues carrying the BP moiety span the biologically active domain of the
hormone (Phe7, Gly12, Lys13, Trp23, and Lys26). Analysis of photo-cross-linked conjugates of
PTH/PTHrP receptor with BP-containing PTH analogs should help to identify the "contact points" between
ligand and receptor.