Effect of the Binding Sites of Human Serum Albumin on the Efficiency and Photostationary State Isomer Ratios of the Photoisomerization of Bilirubin

Article abstract of DOI:10.1246/bcsj.67.2758

The quantum yields of the isomerization and photostationary state isomer ratios of the photoisomerization of (4Z,15Z)-bilirubin IXα (ZZ-BR) were determined in an aqueous buffered solution in the presence of human serum albumin (HSA) at a molar ratio of / from 0.5 to 2.The BR isomer compositions in the photostationary state were constant at
/=0.5-0.7.With increasing / from 0.8 to 2, the ZZ-BR composition in the photostationary state increased from 58 to 75percent, but the ZE-BR composition decreased from 39 to 22percent.The quantum yields in the isomerization of ZZ-BR to ZE-BR (Φ_ZZ->ZE) and a cyclized product, lumirubin (LR), (Φ_ZZ->LR), remained unvaried up to /=1, but Φ_ZZ->ZE decreased while Φ_ZZ->LR increased along with a further increase of /.These results are explained by the existence of two binding sites, a first-class site and a second-class site, for bilirubin binding to HSA; Φ_ZZ->ZE in the second-class site (=0.035) was as low as 1/3 of that in the first-class site (=0.11), but Φ_ZZ->LR in the second-class site (=4.2*10^-3) was nearly two-times higher than that in the first-class site (=2.4*10^-3).