Molecular Pharmacology, Vol 17, 328-338, Copyright © 1980 by the American Society for Pharmacology and Experimental Therapeutics

Iodohydroxybenzylpindolol: Preparation, Purification, Localization of Its Iodine to the Indole Ring, and Characterization as a Partial Agonist

¹ Department of Cell Biology, Baylor College of Medicine, Houston, Texas 7703
² Department of Medicine, Baylor College of Medicine, Houston, Texas 77030
³ Department of Cell Biology, Baylor College of Medicine, Houston, Texas 7703, and Physiologisches Institut der Universität Dusseldorf, Lehrstuhl fur Klimische Physiologie, Dusseldorf, West Germany

Hydroxybenzylpindolol (HYP) has been subjected to iodination using a chloramine-T method. We have studied the products of iodination and determined both the chemical and pharmacological properties of one of the iodinated species of HYP. Routine iodination consistently resulted in two labeled products separable by QAE-Sephadex, paper and high-pressure liquid chromatography. The pattern of elution on high-pressure liquid chromatograms varied with the source and age of the ¹²⁵I used. Optimized conditions led to a major peak of iodohydroxybenzylpindolol (I-HYP). This peak was relatively stable to incubation at 37°C (66% remaining after 2 h). It chromatographed on paper with an R_f similar to that reported for I-HYP by M. E. Maguire, R. A. Wiklund, H. J. Anderson and A. G. Gilman (J. Biol. Chem. 251: 1221-1231, 1976). Ultraviolet titration curves and NMR spectra indicated that iodination occurs neither on the hydroxybenzyl moiety of HYP nor on the benzylic portion of the indole moiety of HYP. Spectra are consistent with iodination occurring at position 3 of the indole moiety of HYP. Pharmacological properties were studied both on kitten ventricular membranes and on isolated kitten atria and papillary muscles. I-HYP competitively blocked isoproterenol stimulation of kitten ventricular membrane adenylyl cyclase with pK_B’s (-log K_B) that ranged from 9.2 to 9.4, which were indistinguishable from pK_B’S for HYP determined simultaneously. I-HYP acted as a partial agonist in the isolated organ assays. Calculations performed as described by M. Marano, and A. J. Kaumann (J. Pharmacol. Exp. Ther. 198: 518-525, 1976) yielded pK_p’s for I-HYP for competition against isoproterenol’s interaction with chronotropic and inotropic -adrenergic receptors in right atria, left atria, and papillary muscles of 9.2, 9.4, and 8.7, respectively. EC₅₀’s (effective concentrations giving 50% of maximum response for partial stimulation of chronotropic and inotropic receptors) in the same tissues, on the other hand, were (-log molar) 7.3, 7.3, and 7.1, respectively. Thus, I-HYP, as HYP (Kaumann, A. J., L. Birnbaumer and R. Wittman, in Receptors and Hormone Action, L. Birnbaumer and B. W. O’Malley, Eds., Vol. 3, 134-177, Academic Press, New York, 1978) behaves in kitten cardiac tissues as a partial agonist that initiates stimulation of response only at concentrations at which it causes better than 97.5% occupancy of -receptors.

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ACKNOWLEDGMENTS We thank Dr. Wayne Wray for the use of his Heath spectrophotometer, Mrs. Barbara Adrian for her excellent technical assistance, and Ms. Nancy Gammage for her efficient preparation of the manuscript.