Photoaffinity labeling of parathyroid hormone receptors in clonal rat osteosarcoma cells

A photoreactive derivative of a sulfur-free bovine parathyroid hormone (PTH) analogue, [Nle8,N-epsilon-(4-azido-2-nitrophenyl)Lys13,Nle18,Tyr34]bovine PTH-(1-34)-NH2 (NAP-NlePTH), was purified from the products of the reaction of [Nle8,Nle18,Tyr34]bovine PTH-(1-34)-NH2 (NlePTH) with 4-fluoro-3-nitro-phenylazide and was used to identify binding components of the PTH receptor in clonal rat osteosarcoma cells (ROS 17/2.8). The purified analogue, NAP-NlePTH, is a fully active agonist in three different ROS 17/2.8 cell bioassays: 1) specific binding to saturable PTH receptors; 2) stimulation of cyclic AMP accumulation; and 3) inhibition of cellular alkaline phosphatase activity; this analogue gave dose response curves parallel to and 25-33% as potent as its parent molecule, NlePTH. Radioiodinated NAP-NlePTH (125I-labeled NAP-NlePTH) retained maximal receptor-binding potency. Radioligand saturation studies in intact cells showed that the Kd of PTH receptors for the photoligand was slightly less than that for 125I-labeled NlePTH (2.8 and 0.8 nM, respectively), but that the Bmax was essentially identical for both radioligands (8 fmol/10(5) cells). Photoaffinity labeling of ROS 17/2.8 cells revealed several 125I-labeled macromolecular components by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. One predominant 125I-labeled band, having an apparent Mr of 80,000 daltons (including Mr = 4,347 ligand; hereafter referred to as the Mr = 80,000 protein), was consistently demonstrated in both reducing and nonreducing conditions. Its labeling was completely inhibited by coincubation with NlePTH (10 nM) at 26-fold molar excess to the photoligand, but not by biologically inactive PTH fragments or unrelated hormone. Labeling of several other macromolecular components persisted in the presence of NlePTH (1 microM). Only the labeling of the Mr = 80,000 protein showed saturation kinetics for photoaffinity labeling; the dose of 125I-labeled NAP-NlePTH (0.8 nM) to half-saturate labeling of the Mr = 80,000 protein was close to the Kd (2.8 nM) of specific binding of the photoligand to receptors in intact ROS 17/2.8 cells. Pretreatment of the cells with NlePTH and dexamethasone led to the predicted proportional decrease or increase, respectively, in labeling of the Mr = 80,000 protein. Our data, using a highly purified photoactive derivative of PTH, having carefully defined chemical and biological properties, show a plasma membrane component of Mr = 80,000 in ROS 17/2.8 cells that possesses the affinity, binding capacity, and physiological characteristics of the PTH receptor.