Although cytochromes P450 2C9 (CYP2C9) and 2C19 (CYP2C19) have 91% amino acid identity, they have different substrate specificities. Previous studies have suggested that several amino acid residues may be involved in substrate specificity. In this study, we focused on the roles of two amino acids, residues 72 and 241. The amino acids in these positions have opposite charges in CYP2C9 and 2C19; the former has lysines in both positions (Lys72 and Lys241), and the latter has glutamic acids (Glu72 and Glu241). Reciprocal mutants for both CYP2C19 and 2C9 were produced, and their metabolic activities and spectroscopic properties were examined using three tricyclic antidepressant (TCA) drugs: amitriptyline, imipramine, and dothiepin. Although CYP2C19 wild-type (WT) had a high metabolic activity for all three drugs, the E72K mutation decreased enzymatic activity by 29-37%, while binding affinities were diminished 2.5-to 20-fold. On the other hand, low activity and low affinity of CYP2C9 WT were recovered notably by K72E mutation. The metabolic activities and binding affinities were minimally affected by CYP2C19 E241K and CYP2C9 K241E mutations. We could also show linear correlations between metabolic activities and binding affinities, and hence we conclude that amino acid residue 72 plays a key role in TCA drug metabolism by limiting the binding affinities of CYP2C19 and CYP2C9.Key words cytochrome P450 2C19; cytochrome P450 2C9; drug metabolism; antidepressant; dissociation constant Depression is a psychiatric condition that affects 120 million people worldwide, and can interfere with independence and productivity in essentially all aspects of daily life. Depression is also associated with a high risk of self-harm, and ultimately suicide. Despite the considerable advances in depression treatments, tricyclic antidepressants (TCAs) remain an important therapeutic option for depression. TCAs such as amitriptyline (AMI), imipramine (IMI), and dothiepin (DOT) are heterocyclic chemical compounds (Fig. 1). These drugs are characterized by substantial pre-systemic first-pass metabolism and their clearance is dependent primarily on hepatic cytochrome P450 (CYP) oxidative enzymes, in particular CYP2C19.1) Because the activity of some CYP isoforms is individually influenced by genetic variations, there is wide inter-individual variability in TCA pharmacokinetics and active metabolite production, 2,3) which complicates the clinical use of these drugs. Therefore, the understanding of TCA drug metabolism at the molecular level is essential for the safe use of these drugs.CYPs are a superfamily of heme-containing enzymes that are mainly responsible for oxidative transformation of xenobiotics in humans, animals, and plants. The CYP1, 2, and 3 families in mammals have a wide range of overlapping substrates and play a role in the metabolism of both drug and endogenous substrates through N-, O-, and S-dealkylation, aromatic and aliphatic hydroxylation, epoxidation; oxidative desulfurization, and sulfoxidation. 4,5) The CYP2C subfamily in...

Cytochrome P450 reductase (CPR) is an important redox partner of microsomal CYPs. CPR is composed of a membrane anchor and a catalytic domain that contains FAD and flavin mononucleotide (FMN) as redox centers and mediates electron transfer to CYP. Although the CPR membrane anchor is believed to be requisite for interaction with CYP, its physiological role is still controversial. To clarify the role of the anchor, we constructed a mutant (Δ60-CPR) in which the N-terminal membrane anchor was truncated, and studied its effect on binding properties, electron transfer to CYP2C19, and drug metabolism. We found that Δ60-CPR could bind to and transfer electrons to CYP2C19 as efficiently as WT-CPR, even in the absence of lipid membrane. In accordance with this, Δ60-CPR could mediate metabolism of amitriptyline (AMT) and imipramine (IMP) in the absence of lipids, although activity was diminished. However, Δ60-CPR failed to metabolize omeprazole (OPZ) and lansoprazole (LPZ). To clarify the reason for this discrepancy in drug metabolism, we investigated the uncoupling reaction of the CYP catalytic cycle. By measuring the amount of H 2 O 2 by-product, we found that shunt pathways were markedly activated in the presence of OPZ/LPZ in the Δ60-CPR mutant. Because H 2 O 2 levels varied among the drugs, we conclude that the proton network in the distal pocket of CYP2C19 is perturbed differently by different drugs, and activated oxygen is degraded to become H 2 O 2 . Therefore, we propose a novel role for the membrane anchor as a suppressor of the uncoupling reaction in drug metabolism by CYP.

The investigation of cytochrome P450 (CYP) mediated metabolism reactions by determination of enzyme kinetic parameters, Michaelis constant (K m ), maximum reaction velocity (V max ), and intrinsic clearance (CL int ) is important aspects in discovery and development of drugs. The kinetic parameters can be used to predict the clearance prior to human administration and for better understanding the mechanism of clearance in vivo. In this study, the metabolic activities of three major hepatic CYP isoforms (2C19, 2D6, and 3A4) were investigated on structurally different central nervous system (CNS) acting drugs, amitriptyline, fluphenazine, and dothiepin. By using our novel in vitro evaluation system, we could compare the kinetic parameters for the metabolism of fluphenazine and dothiepin for the first time. Comparing CL int values thus obtained, we concluded that 2C19 could be predominant for metabolic activity on tricyclic antidepressants as expected, but not on phenothiazine-related antipsychotic drugs. Since the metabolism of CNS drugs is susceptible to single nucleotide polymorphisms of human gene, our results suggest that phenothiazine could be an alternative to clinical application of CNS drugs.

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