SKF-525-A AND CYTOCHROME-P-450 LIGANDS INHIBIT WITH HIGH-AFFINITY THE BINDING OF [H-3] DEXTROMETHORPHAN AND SIGMA LIGANDS TO GUINEA-PIG BRAIN

The DM1/sigma-1 site binds dextromethorphan (DM) and sigma receptor ligands. The broad binding specificity of this site and its peculiar subcellular distribution prompted us to explore the possibility that this site is a member of the cytochrome P-450 superfamily of enzymes. We tested the effects of the liver microsomal monooxygenase inhibitor SKF 525-A (Proadifen), and other P-450 substrates on the binding of [H-3]dextromethorphan, [H-3]3-(-3-Hydroxyphenyl)-N-(1-propyl)piperidine and (+)-[H-3]1,3-Di-o-tolyl-guanidine ([H-3]DTG) to the guinea pig brain. SKF 525-A, l-lobeline and GBR-12909 inhibited the binding of the three labeled ligands with nM affinity. Each drug has identical nM K(i) values for the high-affinity site labeled by the three ligands. This indicated that they displaced the labeled ligands from the common DM1/sigma-1 site. Debrisoquine and sparteine, prototypical substrates for liver debrisoquine 4-hydroxylase, displayed K(i) values of 9-13 and 3-4 mu-M respectively against the three labeled ligands. These results, the broad specificity of the DM1/sigma-1 binding site, and its peculiar subcellular distribution, raises the possibility that this binding site is a member of the cytochrome P-450 superfamily of isozymes, rather than a neurotransmitter receptor. These findings may have important implications for the understanding of the therapeutic, side effects and toxicity of several neurotropic drugs.