Bilirubin sensitized photooxidation of human plasma low density lipoprotein

Previous investigations have shown that the bile pigment bilirubin can act as peroxyl radicals scavenger and transition metals trap, but also as a peroxidant, to erythrocyte ghost membranes through 1O2-driven photooxidation. In the present study we examined the changes occurring in the lipoprotein particle following bilirubin-sensitized photooxidation of isolated plasma LDL. The oxidative stress resulted in increased TBA reactivity, diene formation, free cholesterol oxidation, apo B fragmentation and enhanced uptake of the modified particle by the mouse macrophage scavenger receptors as well as the decrease binding to the native B, E-receptor on fibroblasts. The marked increase in TBARS production in D2O-enriched medium and the inhibition of lipid peroxidation of azide is consistent with singlet oxygen involvement in the oxidation process. The apo B-bound Cu2+ appears to become redox active during photooxidation since the presence of EDTA in the reaction mixture greatly reduced protein fragmentation. It was also found that BHT inhibited almost completely the lipid peroxidation, as determined by the TBA reaction but could not totally abolish the formation of 5 alpha-hydroxycholesterol, which is the main product formed by the direct attack of 1O2 on cholesterol. The results of this work strongly suggest that, through photooxidation by light-activated bilirubin, the lipoprotein particle may be modified in the blood stream as well, besides being modified in the well known oxidation site within the arterial wall. Our findings provide the rationale for extending these studies to clinical investigations, which aim at developing strategies for minimizing damage to arterial tissue following phototherapy of hyperbilirubinemic newborns or cancer patients after systemic administration of photosensitizers.