Protein content was determined using a Cary Eclipse Fluorescence Spectrometer (Varian, Palo Alto, CA) as described previously (17 ). Briefly, aliquots of 1 to 3 μl of whole cell lysates were mixed with 2 ml of 8 m urea in 10 mm Tris-HCl, pH 8.5. The fluorescence was measured at 295 nm for excitation and 350 nm for emission. The slits were set to 5 nm and 20 nm for excitation and emission, respectively. Tryptophan was used as a standard. The protein content was calculated from the following relationship: the fluorescence of 0.1 μg of tryptophan equals 9 μg of total protein, which reflects an average 1.1% weight content of tryptophan in whole lysates of human cells.