Comparison of the in vitro binding characteristics of the β-carbolines harman and norharman in rat brain and liver and in bovine adrenal medulla

Abstract

The in vitro binding of the naturally occurring β-carbolines harman and norharman in their tritium-labelled forms to cell membranes from the rat brain and liver and from bovine adrenal medulla was investigated. Displacement of the specific [³H]harman binding in bovine adrenal medulla and rat liver by several β-carbolines and monoamine oxidase (MAO) inhibitors revealed the pharmacological profile of a single, high-affinity binding site (K _D 4.92±0.43 nmol/l, B_max 8.47±0.17 pmol/mg protein; adrenal medulla) which corresponded to the active site of MAO type A (MAO-A). Similar characteristics have previously been found for brain tissue from rat, marmoset and pig. In order to determine the temperature dependence of the [³H]harman binding, the K _D and B_max values for rat cerebral cortex were calculated from the results of saturation experiments at 5 temperatures (range: 0°C–37°C). Whereas the B_max values under all conditions were − 4 pmol/mg protein, the K _D values, with increasing temperature, ranged from ≈ 3 nmol/l to 30 nmol/l. The calculated linear van't Hoff plot (-In K _D against 1/T) suggested an enthalpy-driven binding of [³H]harman to MAO-A.

At least three different [³H]norharman-binding sites were detected. In the rat forebrain, ≈85% of the specific binding (at about 2 nmol/l of [³H]norharman) can be attributed to a MAO binding site of type B: the binding is displaceable, in nmol/l concentrations by the potent and selective MAO-B inhibitors MDL 72,974A, R(−)-deprenyl and pargyline and, in μmol/l concentrations, by S(+)-deprenyl and the potent and selective MAO-A inhibitors clorgyline, harmine, harman, harmaline, brofaromine 5-F-α-methyltryptamine. After suppression of the MAO binding sites with 1 μmol/l clorgyline and μmol/l R(−)-deprenyl, a second binding site was found. However, the binding at this site was biphasically displaceable by harman and norharman (Hill-slopes about 0.5 and 0.6, curvilinear Rosenthal plots) suggesting the presence of negative co-operativity or of two binding sites (states). A similar clorgyline/R(−)-deprenyl resistent single (Hill-slopes of displacement by norharman, harman and 6-hydroxy-β-carboline about unity; linear Rosenthal plots) high affinity binding site (K _D 7.5±2 nmol/l, B_max 130±30 fmol/mg protein) was found in bovine adrenal medullary cell membranes. A third quite different clorgyline/R(−)-deprenyl resistent high-affinity (K _D≈14 nmol/l) and high-density (B_max 10–30 pmol/mg protein) binding site was detected in the liver. The specific binding at this site was not displaceable by harman or most other substituted β-carbolines or by tetrahydro-β-carbolines, but was displaced by norharman and several newly synthesized 6-substituted aromatic β-carbolines (e.g. F-, CH₃-, CH₃O-, HO-). The [³H]norharman binding site in the liver is certainly not identical with any of the binding sites for MAO-inhibitors, benzodiazepines or sigma receptor ligands and is slightly enriched in the microsomal (P₃) fraction whereas most of the specific [³H]harman binding was detected in the crude mitochondrial (P₂) fraction.