Abstract
A series of phenoxy-substituted methylimidazoline derivatives were synthesized and used to define the ligand recognition properties of the imidazoline-binding domain (IBD) on monoamine oxidase (MAO)-B and its role in substrate processing. The rank order of potency for selected compounds in competitive binding studies with the imidazoline [3H]idazoxan was different from that in enzyme activity assays, suggesting that the IBD and the site involved in enzyme inhibition are distinct. IC50 values for inhibition of MAO-B activity by imidazoline/guanidinium ligands were one to two orders of magnitude greater than ligand concentrations that probably saturate the IBD, but were equal to theKd values of these ligands in competitive binding assays with the reversible MAO-B inhibitor [3H]Ro 19-6327. In addition, the degree of enzyme inhibition by these ligands was similar in platelet and liver, tissues exhibiting 10-fold differences in the amount of the IBD-accessible enzyme subpopulation. These data suggested that the inhibitory effect of these compounds on MAO-B activity involved a secondary interaction with the enzyme domain recognizing the inhibitor Ro 19-6327 and does not involve interaction with the IBD. Subsequent radioligand-binding studies indicated that human liver MAO-B actually existed as two distinct populations that differed in the accessibility of their IBD. The relatively small amounts of MAO-B possessing an accessible IBD (∼5% in human liver) precludes determination of the functional consequences of ligand binding to the IBD. This subpopulation of MAO-B may be selectively regulated or generated in different individuals or tissues and targeted by pharmacologically active compounds in a cell type-specific manner.
Footnotes
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Send reprint requests to: Stephen M. Lanier, Ph.D., Department of Pharmacology, Medical University of South Carolina, 171 Ashley Ave., Charleston, SC 29425-2251. E-mail:laniersm{at}musc.edu
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↵1 This work was supported by National Institutes of Health Grants NS35875-01 (to S.M.L.), GMR43 GM54457 (to V.B.), T32-HL-7260 (to R.R.), and F32 NS10332 (to R.R.). The Brain Tissue Resource Center is supported in part by Mental Health/Neuroscience 31862. The National Neurological Research Specimen Bank is supported by National Institute of Neurological Disorders and Stroke/National Institute of Mental Health, National Multiple Sclerosis Society, Hereditary Disease Foundation, Comprehensive Epilepsy Program, Tourette Syndrome Association, Dystonia Medical Research Foundation, and Veteran's Health Services and Research Administration.
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↵2 The imidazoline BU224 and the guanidinium guanabenz exhibit Ki values of 1 to 10 nM for the IBD on MAO-B as determined in radioligand-binding studies with [3H]idazoxan or photoaffinity labeling with [125I]AZIPI.
- Abbreviations:
- MAO
- monoamine oxidase
- IBD
- imidazoline-binding domain
- [125I]AZIPI
- 2-[3-azido-4[125I]iodophenoxy] methyl imidazoline
- 2BFI
- 2-(2-benzofuranyl)-2-imidazoline
- Ro 19-6327
- N-(2-aminoethyl)-5-chloro-2-pyridine carboxamide
- Ro 16-6491
- N-(2-aminoethyl)-p-chlorobenzamide
- BU224
- 2-(4,5-dihydroimidaz-2-yl)-quinoline
- BOC
- N-t-butoxycarbonyl
- MS
- mass spectrometry
- HEK
- human embryonic kidney
- α2-AR
- α2-adrenoceptor
- Received June 25, 1999.
- Accepted November 13, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
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