Oxidation reactions of a bovine serum albumin-bilirubin complex. A pulse radiolysis study

Abstract

Using the technique of pulse radiolysis, oxidation studies of the bovine serum albumin-bilirubin (BSA-BR) system with radicals like CCl 3OO, N3, (SCN)2-, Br2- and OH generated in neutral and alkaline medium are reported. In a neutral solution, BSA protects the bound BR very efficiently from the attack of these radicals. The experimental k / k values for the reaction of CCl 3OO, N3 and Br2- radicals are 2. 46, 1.78 and 2.55 respectively, where k and k are the bimolecular rate constants for the formation of the semi-oxidized BSA and BR radicals respectively. The calculated ratios from our measurements of rate constants k and k are 0.16, 0.28 and 1.38 for CCl3OO, N3 and Br2- respectively. These values indicate protection of BR by BSA from free radical attack. For Br 2- radical-induced oxidation of the BSA-BR system, a radical transfer from protein to BR was observed. OH shows very fast adduct formation with both BSA and BR. The bimolecular rate constant for the formation of BR-OH adducts at PH 8 0.2 is 9.5 109 dm3 mol- 1 s- 1 (540 nm). OH adds to BSA at neutral pH with a rate constant of 3. 0 1.0 1010 dm3 mol- 1 s- 1 (305 nm). In the BSA-BR complex, BSA fully protects BR from OH attack and the (BSA-BR)-OH adduct further reacts with free BR molecule if present in solution.