The cytochrome P450 2A (CYP2A) subfamily includes 2A3 in rats, CYP2A5 in mice, and CYP2A6 in humans (1). Coumarin, a plant alkaloid, is hydroxylated specifically by coumarin 7-hydroxylase (COH), encoded by the mouse cyp2a5 gene and the human cyp2a6 gene (2, 3). COH activity is considered as a specific marker for the catalytic function of mouse CYP2A5 and human CYP2A6. In humans, CYP2A6, a major metabolic enzyme for nicotine and procarcinogenic nitrosamines present in tobacco smoke, is predominantly expressed in the liver (4). In mice, CYP2A5, which shares 86 % amino acid sequence similarity with the human CYP2A6 (5), is also expressed in liver, but the highest CYP2A5 expression is detected in olfactory mucosa (1). In rats, CYP2A3, which is orthologous to mouse CYP2A5 and human CYP2A6, is expressed at high levels in the olfactory mucosa but is not detectable in liver (1). In mouse liver, COH activity can be strongly induced by pyrazole, while in rat liver COH is not detectable even after treatments with pyrazole (6). CYP2A5 expression or its catalytic activity (COH) is highly inducible in mouse
