One hundred milligrams of frozen lung tissue was homogenized in buffer A (0.32 M sucrose, 5 m M Tris–HCl pH 7.5, 120 m M KCl, 1 m M EDTA, 0.2 m M phenylmethylsulfonylfluoride, 1 μg ml −1 leupeptin, 1 μg ml −1 pepstatin and 1 μg ml −1 aprotinin) and spun at 4 °C, 12 890 g for 10 min.