Rat oesophageal cytochrome P450 (CYP) monooxygenase system: comparison to the liver and relevance in N-nitrosodiethylamine carcinogenesis

N-nitrosodiethylamine (NDEA) is able to induce tumours in the rat oesophagu s. It has been suggested that this could be due to tissue specific expressi on of NDEA activating cytochrome P450 enzymes. We investigated this by char acterizing the oesophageal monooxygenase complex of male Wistar rats and co mparing it with that of the liver. Total amount of cytochrome P450, NADPH P 450 reductase, cytochrome b5 and cytochrome b5 reductase of the oesophageal mucosa was similar to7% of what was found in the liver. In addition, major differences were found in the cytochrome P450 isoenzyme composition betwee n these organs: CYP 2B1/2B2 and CYP3A were found only in the liver, whereas CYP1A1 was constitutively expressed only in the oesophagus. Of the two wel l-known nitrosamine metabolizing enzymes, CYP2A3 was found only in the oeso phagus whereas CYP2E1 was exclusively expressed in the liver. Catalytic stu dies, western blotting and RT-PCR analyses confirmed the expression of CYP2 A3 in the oesophagus. CYP2A enzymes are known to be good catalysts of NDEA metabolism. Oesophageal microsomes had a K-m for NDEA metabolism, which was about one-third of that of hepatic microsomes, but they showed similar act ivities when compared per nmol of total P450. NDEA activity in the oesophag us was significantly increased by coumarin (CO), which also induced oesopha geal CYP2A3. Immunoinhibition of the microsomal NDEA activity showed that u p to 70% of this reaction is catalysed by CYP2A3 in the oesophagus, whereas no inhibition of the hepatic NDEA activity could be achieved by the anti-C YP2A5 antibody. NDEA, but not N-nitrosodimethylamine (NDMA) inhibited the o esophageal metabolism of CO. The results of the present investigation show major differences in the enzyme composition of the oesophageal and hepatic monooxygenase complexes, and are in accordance with the hypothesis that the NDEA organotropism could, to a large extent, be due to the tissue specific expression of the activating enzymes.