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 als o as a prooxidant, to erythrocyte ghost membranes through O-1(2)-drive n photooxidation. In the present study we examined the changes occurri ng in the lipoprotein particle following bilirubin-sensitized photooxi dation of isolated plasma LDL. The oxidative stress resulted in increa sed TBA reactivity, diene formation, free cholesterol oxidation, apo B fragmentation and enhanced uptake of the modified particle by the mou se macrophage scavenger receptors as well as the decrease binding to t he native B, E-receptor on fibroblasts. The marked increase in TBARS p roduction in D2O-enriched medium and the inhibition of lipid peroxidat ion of azide is consistent with singlet oxygen involvement in the oxid ation process. The apo B-bound Cu2+ appears to become redox active dur ing photooxidation since the presence of EDTA in the reaction mixture greatly reduced protein fragmentation. It was also found that BHT inhi bited almost completely the lipid peroxidation, as determined by the T BA reaction but could not totally abolish the formation of 5 alpha-hyd roxycholesterol, which is the main product formed by the direct attack of O-1(2) on cholesterol. The results of this work strongly suggest t hat, through photooxidation by light-activated bilirubin, the lipoprot ein particle may be modified in the blood stream as well, besides bein g modified in the well known oxidation site within the arterial wall. Our findings provide the rationale for extending these studies to clin ical investigations, which aim at developing strategies for minimizing damage to arterial tissue following phototherapy of hyperbilirubinemi c newborns or cancer patients after systemic administration of photose nsitizers.