Dihydrolipoyl dioleoylglycerol antioxidant capacity in phospholipid vesicles

Antioxidants have critical roles in maintaining cellular homeostasis and disease-state prevention. The multi-functional agent α-lipoic acid offers numerous beneficial effects to oxidatively stressed tissues. α-Lipoic acid was enzymatically incorporated into a triglyceride in conjunction with oleic acid, creating lipoyl dioleoylglycerol, and chemically reduced to form dihydrolipoyl dioleoylglycerol. The triglyceride forms of lipoic acid stabilized dioleoylphosphatidylcholine unilamellar liposomal vesicles, as judged by calcein-cobalt leakage. Stabilization resulted from increased packing density of phospholipid acyl chains. Scavenging activity against the 2,2'-azobis(2-amidinopropane) dihydrochloride (AAPH) radical was monitored by oxidation of 4,4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza-s-indacene-3-undecanoic acid (C(11)-Bodipy). Dihydrolipoyl dioleoylglycerol in vesicles demonstrated strong antioxidant capacity in comparison to the conventional Trolox standard. Fluorescence quenching measurements indicated the lipoyl moiety of dihydrolipoyl dioleoylglycerol is positioned near the vesicle aqueous/lipid boundary. Treatment of intact vesicles with a nonpenetrating sulfhydryl reagent indicated that 80% of the dihydrolipoyl dioleoylglycerol was available for reaction. Molecular modeling of lipoyl dioleoylglycerol and dihydrolipoyl dioleoylglycerol in a phospholipid layer confirmed the existence of an extended configuration for the molecules that accounts for the interfacial location of the lipoyl moiety, which may allow the antioxidant to readily react with radical species approaching membranes from the aqueous phase.