The fluorescence of bilirubin upon interaction with human erythrocyte ghosts

Bilirubin fluorescence increased upon interaction with bovine serum albumin and human erythrocyte ghosts. When bound to albumin, the emission maximum occurred at 534 +/- 3 nm with maximum excitation at 462 +/- 3 nm. Upon interaction with human erythrocyte ghosts, bilirubin fluorescence increased in a biphasic manner. A rapid initial increase was followed by a slower process that required at least 40 min to reach maximum enhancement at 25 degrees C. At equilibrium, bilirubin fluorescence in erythrocyte ghosts was heterogeneous. With 470 nm excitation, maximum emission occurred in the range of 520 to 535 nm. However, decreasing the excitation to 450 nm or below, produced a red shift in the emission difference spectra. These results suggest that bilirubin exists in different states or sites when associated with plasma membranes.