Molecular properties of the papaverine binding sites identified in human colonic membranes with 6,7-dimethoxy-4-(4'-amino,3'-[125I]iodobenzyl) isoquinoline as probe

• Published in: European journal of pharmacology Vol. 149; no. 3; p. 371
• Main Authors: Servin, A L, Christinaki, H, Viel, C
• Format: Journal Article
• Language: English
• Published: Netherlands 10.05.1988
• Subjects: Affinity Labels; Animals; Binding, Competitive; Colon - metabolism; Humans; In Vitro Techniques; Male; Muscle Relaxation - drug effects; Muscle, Smooth - drug effects; Muscle, Smooth - metabolism; Papaverine - analogs & derivatives; Papaverine - metabolism; Rats; Rats, Inbred Strains
• ISSN: 0014-2999
• DOI: 10.1016/0014-2999(88)90671-1

The binding sites for papaverine in human intestine were studied with the radioiodinated papaverine derivative, 6,7-dimethoxy-4-(4'-amino,3'-[125I]iodobenzyl)isoquinoline [( 125I]DMABI) as a probe. This drug was shown to bind to two specific binding sites in human colon membranes; one with a low capacity (Kd = 0.02 +/- 0.01 microM, Bmax = 0.77 +/- 0.15 fmol/mg) and another with a high capacity (Kd = 12 +/- 1.5 microM, Bmax = 167 +/- 20 fmol/mg). The ability of various 4-benzyl isoquinolines derivatives, to inhibit radioligand binding was not influenced by 4'-substitutions, but was influenced by 6,7-substitutions, e.g. 6-hydroxy greater than 7-hydroxy greater than 6,7-dihydroxy; and by other substitutions, e.g. 1-CH3 greater than N-oxyde. The papaverine probe was further used to examine structural aspects of human colon papaverine binding sites. For this purpose, [125I]DMABI-labeled membranes were irradiated with U.V. for 15 min at 4 degrees C. Subsequent sodium dodecyl sulfate-polyacrylamide gel electrophoresis of membranes revealed a major, specifically labeled protein of Mr = 36,000 Da. This photoaffinity labeling was protected by unlabeled DMABI in a dose-dependent manner (IC50: 0.5 microM).