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

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).

Characterization of solubilized isoquinoline binding sites from rat intestine using 6,7-dimethoxy 4-(4'-amino, 3'(125I)iodobenzyl)isoquinoline

In rat tissues, the specific binding of 6,7-dimethoxy 4-(4'-amino, 3'(125I)iodobenzyl)-isoquinoline is distributed as follows: aorta greater than pancreas greater than liver greater than intestine greater than stomach greater than lung greater than skeletal muscle greater than heart greater than brain. In aorta and intestine 125I-DMABI is specifically covalently incorporated after direct u.v. photolabeling, in a major polypeptide of Mr 36,000 daltons, and a minor polypeptide of Mr 52,000 daltons. In intestine another smaller minor polypeptide of Mr 26,000 is observed. In intestine a variety of isoquinolines are tested for their ability to inhibit the covalent photo-incorporation of 125I-DMABI. Inhibitory potency is influenced by 6,7-substitutions, e.g. 6,7-dimethoxy, and by the presence of benzyl ring in C-1 and C-4 positions. Isoquinoline is much more potent than tetrahydroisoquinoline. 125I-DMABI intestinal binding site is solubilized using Triton X-100. Layered on Sephadex G-25 column, a high specific peak of radioactivity is eluted in the void volume of the column. The 125I-DMABI binding protein loaded onto a Sephacryl S-300 column is eluted as a single peak corresponding to a species with a Stokes radius of 43.5 A. The sedimentation coefficient of the 125I-DMABI binding protein is measured by ultracentrifugation of 5.5 S, using 5-20% sucrose gradient. The calculated molecular weight of the intestinal 125I-DMABI binding protein is estimated at 110,000 daltons.