The activities of antioxidant nutrients in human plasma depend on the localization of attacking radical species

Abstract

The oxidation of endogenous antioxidant nutrients in human plasma was determined to examine their activities against free radicals generated in the aqueous and lipid compartments of plasma. Free radicals were induced at a constant rate in the aqueous compartment by the hydrophilic radical generator, 2,2′-azobis-(2-amidino-propane)dihydrochloride (AAPH; 10-20 mmol/L) and in the lipid compartment by the lipophilic radical generator, 2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile) (MeO-AMVN; 1-2 mmol/L). The depletion of endogenous plasma antioxidant nutrients (lutein, cryptoxanthin, β-carotene, lycopene, α-tocopherol, ascorbic acid, uric acid) was determined after incubation with either AAPH or MeO-AMVN at 37°C using HPLC. The oxidation of the aqueous and lipid compartments of plasma was selectively monitored by a fluorimetric method using either the hydrophilic probe, 2′,7′-dichlorodihydrofluorescein (DCFH) or the lipophilic probe, 4,4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diazas-indacene-3- undecanoic acid (BODIPY 581/591). When plasma was incubated with AAPH, the rates of consumption of the antioxidant nutrients were as follows: ascorbic acid > α-tocopherol > uric acid > lycopene > lutein > cryptoxanthin > β-carotene. When plasma was incubated with MeO-AMVN, α-tocopherol and carotenoids were depleted at similar rates and ahead of the major water-soluble antioxidants. Our study indicates that the antioxidant nutrients present in both the lipid and aqueous compartments can remove free radicals generated in plasma, and their activity depends on the localization of the attacking radical species.