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

Abstract

Abstract 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 [ZZ-BR]/[HSA] from 0.5 to 2. The BR isomer compositions in the photostationary state were constant at [BR]/[HSA] = 0.5—0.7. With increasing [ZZ-BR]/[HSA] from 0.8 to 2, the ZZ-BR composition in the photostationary state increased from 58 to 75%, but the ZE-BR composition decreased from 39 to 22%. The quantum yields in the isomerization of ZZ-BR to ZE-BR (ΦZZ→ZE) and a cyclized product, lumirubin (LR), (ΦZZ→LR), remained unvaried up to [ZZ–BR]/[HSA] = 1, but ΦZZ→ZE decreased while ΦZZ→LR increased along with a further increase of [ZZ-BR]/[HSA]. 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; ΦZZ→ZE in the second-class site (= 0.035) was as low as 1/3 of that in the first-class site (= 0.11), but ΦZZ→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).