THE OXIDATION OF BLOOD-PLASMA AND LOW-DENSITY-LIPOPROTEIN COMPONENTS BY CHEMICALLY GENERATED SINGLET OXYGEN

Human blood plasma and freshly isolated LDL were exposed to singlet ox ygen (O-1(2)) by thermal decomposition of synthetic endoperoxides. Exp osure of blood plasma to 20 mm water-soluble O-1(2) generator resulted in the depletion of ascorbate (100%), urate (75%), ubiquinol-10 (65%) , protein thiols (50%), and bilirubin (25%), whereas under these condi tions the levels of alpha-tocopherol, beta-carotene, and lycopene rema ined unchanged. The following rates of depletion were obtained by kine tic analysis (moles depleted per 100 mol of O-1(2) consumed): protein thiols (5), urate (5), ascorbate (4), bilirubin (1), and ubiquinol-10 (0.008). In contrast, the rates of depletion using the lipid-soluble O -1(2) generator were faster for bilirubin (13-fold), protein thiols (9 -fold), ubiquinol-10 (8-fold), and ascorbate (5-fold), and slower for urate (2-fold). The formation of lipid hydroperoxides, including mostl y cholesteryl linoleate hydroperoxide, was observed in O-1(2)-treated plasma (0.007-0.009 mol/100 mol O-1(2)) and LDL solutions (0.086 mol/1 00 mol O-1(2)). Based on competition kinetics, we estimate that 98% of O-1(2) generated in the aqueous phase of plasma is quenched by compon ents in this phase, mostly by plasma protein (63%; 6% by protein thiol s), urate (9%; 5% by chemical quenching), and bilirubin (5%; 1% by che mical quenching). Ascorbate and ubiquinol-10 do not contribute to O-1( 2) quenching in plasma, and their oxidation is probably mediated secon dary species. The remaining O-1(2) generated in plasma (2%) diffuses i nto lipoprotein leading to the formation of lipid hydroperoxides with an efficiency of about 100-fold greater than that compared to aqueous generated O-1(2). The principal O-1(2) quenchers in LDL include apoB ( 42%), lycopene and beta-carotene (40%), and alpha-tocopherol (17%). Th e importance of carotenoids in the quenching of O-1(2) in lipoprotein suggest that the beneficial effects of these compounds in health may i n part be due to the elimination of this species in biology and medici ne.