Lung cytochrome P450-dependent benzyloxyphenoxazone debenzylase and ethoxyphenoxazone deethylase activities in total microsomal and isolated alveolar type II cells: responses to changes in assay conditions with special reference to non-linear dependence at low enzyme concentration.

1.1. Responses of cytochrome P450-dependent ethoxyphenoxazone deethylase (EtOPx'ase) and benzyloxyphenoxazone debenzylase (BzOPx'ase) activities to changes in assay conditions were measured in total microsomal and isolated alveolar type II (t11) cells from rats pretreated with β-naphthoflavone.2.2. Whereas microsomal EtOPx'ase activity was unaffected by storage at −80°C for up to 4 months, BzOPx'ase activity began to decline after only 1 month.3.3. The microsomal and type II activities were unaffected by changes in pH (7.2–8.0) or salt content.4.4. The type II activities increased after sonication 2.3–2.7-fold or in the presence of 10 μM dicumarol 1.7–1.9-fold.5.5. Type II BzOPx'ase was sensitive to metyrapone (MP) whereas EtOPx'ase was sensitive to α-naphthoflavone (ANF). I50, values for the tu activities were calculated as: 0.63 μM—MP (BzOPx'ase), 80 μM—MP (EtOPx'ase), 0.024 μM—ANF (EtOPx'ase). At the highest concentration of ANF (10 μM), 50% inhibition t11, BzOPx'ase was not observed. The results were similar to those obtained with the total lung microsomal fraction.6.6. Microsomal and t11 BzOPx'ase activities exhibited non-linear dependence at low enzyme concentration. Linearity was restored by 0.5 mM dimyristoylphosphatidylcholine.