Two distinct microsomal pathways involved in the metabolism of albendazole (ABZ) to albendazole-sulphoxide (SO.ABZ) by pig liver microsomes have been identified and quantified. 2. The binding of ABZ to microsomal cytochrome P-450 (Type I spectrum, Ks = 25.5 microM), the decrease of the rate of sulphoxidation by antibody against NADPH cytochrome c reductase, and the use of purified cytochrome P-450 A demonstrated the contribution of a cytochrome P-450-dependent mono-oxygenase to the metabolism of ABZ. 3. The involvement of FAD-containing mono-oxygenase (FMO) was shown by thermal pretreatment of microsomes, n-octylamine activation of the reaction, and by using purified pig liver FMO. 4. From Km and Vmax values, it would appear that the relative contributions of the two systems depend on the concentration of ABZ.
