ETHYLMORPHINE O-DEETHYLATION IN ISOLATED RAT HEPATOCYTES - INVOLVEMENT OF CODEINE O-DEMETHYLATION ENZYME-SYSTEMS

The O-dealkylation of ethylmorphine (EM) and codeine (CD) to morphine (M) co-segregates with debrisoquine/sparteine genetic polymorphism in man. CD O-demethylation is catalysed by cytochrome P450 2D1 (CYP2D1) i n rats. In the present study, the O-deethylation of EM was examined an d compared with that of CD in suspensions of freshly-isolated hepatocy tes prepared by a collagenase method from Wistar rats with and without CYP2D1 inhibitors. Isolated hepatocytes were also prepared from Dark Agouti (DA) rats deficient in CYP2D1, and were incubated with EM or CD . EM, CD and their metabolites were quantified by HPLC with UV detecti on. EM had a similar pattern of metabolism to that of CD in suspension s of hepatocytes from Wistar rats. Both EM and CD were O-dealkylated t o form M plus morphine-3-glucuronide (M3G) and N-demethylated to form norethylmorphine (NEM) or norcodeine (NCD), respectively, which were f urther metabolized to normorphine (NM) and finally glucuronidated to n ormorphine 3-glucuronide (NM3G). As compared to hepatocytes from Wista r rats, DA rats were characterized by a markedly decreased formation ( 70-75% reduction) of M plus M3G from both EM and CD. Quinine, quinidin e, propafenone and sparteine all inhibited EM O-deethylation as well a s CD O-demethylation. Quinine was the most potent inhibitor of both th ese O-dealkylations (K-i = 0.2 mu M for both EM and CD, respectively). Quinine as well as the other inhibitors inhibited both EM and CD O-de alkylation competitively and with small differences in K-i versus EM a nd CD, respectively. The metabolism of EM to M plus M3G and that of CD to M plus M3G was highly correlated when results from the various sep arate cell suspensions were plotted. In conclusion all findings indica ted that the enzyme responsible for O-demethylation of CD, CYP2D1 was also responsible for the O-deethylation of EM to M.