Flow cytometry offers great potential for the study of xenobiotic metabolism in intact cells. We explored this application by the use of ethoxyfluorescein ethyl ester (EFEE) and isolated rat hepatocytes, a classic system for studying such reactions. EFEE is only weakly fluorescent and it diffuses freely into viable cells, where it is metabolized to fluorescein by a process dependent upon mixed-function oxidase activity. In the current study, viable hepatocytes were first identified by flow cytometric assessment of fluorescein diacetate staining. The viable subpopulation was also identifiable on the basis of forward and right angle light scattering properties alone, and it was in this fraction that EFEE metabolism was measured. Metabolism of EFEE to fluorescein was quantified by flow cytometry. SKF 525A, alpha-naphthoflavone, and metyrapone, classic inhibitors of mixed-function oxidation, each inhibited the metabolism of EFEE. These results demonstrate the potential of EFEE for use in flow cytometric studies of drug metabolism, such as in multiparameter mechanistic assays of cellular xenobiotic metabolism and toxicity, and in the isolation by fluorescence-activated cell sorting of subpopulations which differ in this activity.
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