Evaluation of novel parathyroid hormone analogs using a bovine renal membrane receptor binding assay

A PTH membrane receptor binding assay based on a stable hormone analog radioligand was refined and used with bovine renal cortical membranes to evaluate PTH reference peptides and novel analogs of the hormone. Systematic studies were performed to optimize several aspects of the receptor binding assay. The sulfur-free agonist analog [Nle8,18,Tyr34]bovine PTH-(1-34)NH2 was iodinated using Iodogen. The monoiodinated derivative was purified and isolated by reverse phase HPLC. Immediate dilution of the purified radioligand in albumin-containing buffer and cold storage of aliquots yielded a tracer that was stable for at least 2 months. When used in the binding assay, the radioligand displayed specific, high affinity, and saturable binding to both bovine and canine renal cortical membranes. The Kd values were 0.47 +/- 0.07 and 0.63 +/- 0.08 nM for bovine and canine membranes, respectively. Maximum binding values were 475 +/- 46 and 395 +/- 48 fmol/mg protein for bovine and canine membranes, respectively. Furthermore, when used on a routine basis, this assay system proved reliable and reproducible. A series of previously characterized PTH agonists and antagonists was tested as reference peptides to validate this assay. Inhibition of binding by agents was dose dependent, parallel concentration-dependent binding curves were observed, and a close correlation between binding affinity and adenylate cyclase activity was obtained. In addition, a series of novel analogs designed to determine the consequences of sequence elongation at the N- and/or C-terminus or incorporation of amino acid substitutions to enhance resistance to enzymatic degradation were examined. With the exception of C-terminal extension from position 34 to position 38, which diminished receptor affinities, all modifications retained full biological activity. This refined receptor binding assay should facilitate future studies of newly designed PTH analogs.