Reaction mixtures contained 0.1 μM P450 from HL27, 150 μM phenytoin or HPPH, an NADPH generating system consisting of 1 mM NADPH, 2.5 mM glucose-6-phosphate, and 0.5 U/ml G6PDH, in 150 mM Tris-HCl pH 7.4. Sulfaphenazole and troleandomycin were added from stocks prepared in methanol and dimethyl sulfoxide, respectively, such that final solvent concentration was maintained at 1% (v/v) in assays. Control incubations contained the equivalent amount of methanol/dimethyl sulfoxide alone. Assays were initiated by the addition of the NADPH generating system and stopped after 45 min by extraction with methyl-t-butyl ether. Organic extracts were desiccated and resuspended in mobile phase prior to HPLC analysis of HPPH production as described in Experimental Procedures. No product was detected in -NADPH controls run in the same assay. Data represent the means of duplicate determinations.