In vitro formation of codeinone from codeine by rat or guinea pig liver homogenate and its acute toxicity in mice

In vitro metabolism of codeine was investigated by using a 9000 g supernatant fraction of rat or guinea pig liver homogenate. When a mixture of [N-14CH3] and [C-6-3H]codeine was incubated with the rat liver 9000 g supernatant fraction in the presence of NAD, formation of codeinone, morphine and norcodeine was detected. Replacement of NAD with NADP abolished only the formation of codeinone. On the other hand, when the guinea pig liver homogenate was used in the presence of NAD, codeinone was the main metabolite of codeine. NADP was also ineffective in forming codeinone with the guinea pig liver homogenate. The acute toxicity of codeinone was thirty times higher than that of codeine. The roles of codeinone as a metabolic intermediate and in the acute toxicity of codeine are discussed.