Abstract

The psychotropic β-carboline alkaloids, showing high affinity for 5-hydroxytryptamine, dopamine, benzodiazepine, and imidazoline receptors and the stimulation of locus coeruleus neurons, are formed endogenously from tryptophan-derived indolealkylamines through the Pictet-Spengler condensation with aldehydes in both plants and mammals. Cytochromes P450 1A1 (18.5), 1A2 (20), and 2D6 (100) catalyzed theO-demethylation of harmaline, and CYP1A1 (98.5), CYP1A2 (35), CYP2C9 (16), CYP2C19 (30), and CYP2D6 (115) catalyzed that of harmine (relative activities). The dehydrogenation/aromatization of harmaline to harmine was not carried out by aromatase (CYP19), CYP1A2, CYP2C9, CYP2D6, CYP3A4, pooled recombinant cytochromes P450, or human liver microsomes (HLMs). Kinetic parameters were calculated for theO-demethylations mediated by each isozyme and by pooled HLMs. K_cat (min⁻¹) andK_m (μM) values for harmaline were: CYP1A1, 10.8 and 11.8; CYP1A2, 12.3 and 13.3; CYP2C9, 5.3 and 175; CYP2C19, 10.3 and 160; and CYP2D6, 39.9 and 1.4. Values for harmine were: CYP1A1, 45.2 and 52.2; CYP1A2, 9.2 and 14.7; CYP2C9, 11.9 and 117; CYP2C19, 21.4 and 121; and CYP2D6, 29.7 and 7.4. Inhibition studies using monoclonal antibodies confirmed that CYP1A2 and CYP2D6 were the major isozymes contributing to both harmaline (20% and 50%, respectively) and harmine (20% and 30%) O-demethylations in pooled HLMs. The turnover numbers for CYP2D6 are among the highest ever reported for a CYP2D6 substrate. Finally,CYP2D6-transgenic mice were found to have increased harmaline and harmine O-demethylase activities as compared with wild-type mice. These findings suggest a role for polymorphic CYP2D6 in the pharmacology and toxicology of harmine and harmaline.