Experimental evidence of the reciprocal oxidation of Bovine Serum Albumin and Linoleate in aqueous solution, initiated by HO* free radicals.

An investigation of radiation-induced oxidation of aqueous bovine serum albumin (BSA) in the presence of linoleate (LH) at pH 10.5 has been carried out in order to better understand the respective oxidative processes involved in both lipid and protein phases. Solutions containing BSA (15 μmol L−1) and linoleate (15–600 μmol L−1) below the critical micellar concentration (cmc = 2000 μmol L−1), have been irradiated by γ-rays (137Cs) at radiation doses ranging from 10 to 400 Gy (dose rate 9.5 Gy min−1). It can be noticed that, in the absence of BSA, the main hydroperoxides formed from HO•-induced linoleate oxidation below the cmc, do not exhibit a conjugated dienic structure. This was also verified in the presence of BSA. Selected chemical markers of oxidation have been monitored: non-conjugated dienic hydroperoxides and conjugated dienes (without hydroperoxide function) for linoleate oxidation, and carbonyl groups for BSA oxidation. We have shown that for the lowest linoleate concentration (15 μmol L−1) in the presence of BSA (15 μmol L−1), the formation of conjugated dienes was not observed, meaning that LH was not exposed to HO• radicals attack. However, non-conjugated dienic lipid hydroperoxides were simultaneously detected, indicating that LH was secondarily oxidised by BSA oxidised species. Moreover, the oxidation of linoleate was found to be enhanced by the presence of BSA. For the highest linoleate concentration (600 μmol L−1), the expected protection of BSA by LH was not observed, even if LH monomers were responsible for the total scavenging of HO• radicals. In this latter case, the formation of non-conjugated dienic lipid hydroperoxides was lower than expected. Those results showed that BSA was not oxidised by the direct action of HO• radicals but was undergoing a secondary oxidation by non-dienic lipid hydroperoxides and/or lipid radical intermediates, coming from the HO•-induced linoleate oxidation.