##Identification of cytochrome P450 enzymes involved in the metabolism of the alkaloids californine and protopine in human liver microsomes

Abstract

Californine and protopine are main alkaloids of Eschscholtzia californica. Preparations of this traditional medicinal plant are used as phytopharmaceuticals and as herbal drugs of abuse. In vivo studies in Wistar rats and in vitro studies using rat liver microsomes have revealed that Ndemethylation (CNDM) and demethylenation of californine (CDM) and demethylenation of protopine (PDM) are the main metabolic steps catalyzed by cytochrome P450 (CYP). The aim of the study presented here was to identify the human hepatic CYP enzymes involved in these reactions using human cDNA-expressed CYPs and pooled human liver microsomes. CNDM was catalyzed almost exclusively by CYP3A4, as indicated by an intrinsic clearance, corrected for the relative activity factor, of almost 100%. This was confirmed by the marked inhibition of this reaction by the CYP3A4 specific inhibitor ketoconazole. CDM was catalyzed mainly by CYP2C19 and CYP3A4 showing similar affinities (apparent Km values of 2.7�M or 3.0�M, respectively) but higher capacity of CYP2C19 and therefore higher percentage of total clearance. PDM was catalyzed mainly by CYP3A4 with a contribution of about 60%. CYP1A2, CYP2C19 and CYP2D6 accounted to equal shares for the other 40% to this reaction. The calculated contributions could be confirmed by chemical inhibition studies. Implications in possible pharmacokinetic interactions are discussed.