Lipoic acid. There is general agreement about the antioxidant
properties of a-lipoic acid. It scavenges hydroxyl
radicals, hypochlorous acid, and singlet oxygen.
It does not appear to scavenge hydrogen peroxide or
superoxide radical and probably does not scavenge peroxyl
radicals (Table 1). It may chelate transition
metals.
Two studies indicate that tr-lipoic acid is a potent
hydroxyl radical scavenger. In one, 12 hydroxyl radical
was generated by 2 mM HzO2 + 0.2 mM FeSO4. The
radical was detected by electron spin resonance (ESR)
using the spin-trapping agent 5,5-Dimethylpyrroline-
N-oxide (DMPO). 1 mM ot-lipoic acid completely
eliminated the DMPO-OH adduct signal. Another
study, 13 using a similar hydroxyl radical-generating
system (2.8 mM HzO2, 0.05 mM FeC13, 0.1 mM EDTA,
and 0.1 mM ascorbate) but a different assay for the
radical (deoxyribose degradation) also found ct-lipoic
acid to be a hydroxyl radical scavenger. In this study,
a rate constant of 4.7 × 101° M-is -~ was calculated;
this is an essentially diffusion-limited reaction rate.
Hence, ot-lipoic acid appears to be a highly effective
scavenger of hydroxyl radical.