Isolation and identification in bovine cerebral cortex of n-butyl beta-carboline-3-carboxylate, a potent benzodiazepine binding inhibitor

A substance having benzodiazepine-binding inhibitory activity has been extracted from 18 kg of gray matter of bovine cerebral cortex and purified to homogeneity. This substance inhibits competitively [3H]flunitrazepam and ethyl beta-[3H]carboline-3-carboxylate binding with high affinity (Ki, 3 nM), but it is inactive upon 3H-labeled Ro 5-4864, [3H]quinuclidinyl benzylate, [3H]prazosin, [3H]clonidine, [3H]dihydroalprenolol, and upon high-affinity [3H]muscimol binding. This inhibitor has been identified as n-butyl beta-carboline-3-carboxylate (beta-CCB) by fast atom bombardment mass spectroscopy (Mr, 268) and electron bombardment fragmentography, ultraviolet and fluorescence spectra, coelution in HPLC with standard beta-CCB, and by the exact correspondence in Ki with beta-CCB on the displacement of [3H]flunitrazepam binding. The possible artificial formation of beta-CCB has been discarded by a series of control experiments including addition of tryptophan to the starting homogenate, extraction from liver, isolation and purification by an alternative procedure avoiding organic solvents, and by the impossibility of making beta-CCB from beta-carboline-3-carboxylic acid or its methyl ester in the conditions of our extraction and purification procedures.