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The antioxidant and prooxidant activities of gallic acid and its propyl, methyl, and (where solubility allowed) lauryl esters were investigated. Gallic acid (GA), propyl gállate (PG), and gallic acid methyl ester (GM) were able to scavenge hypochlorous acid at a rate sufficient to protect -1-antiproteinase against inactivation by this molecule. When dissolved in ethanol, gallic acid lauryl ester (GL), PG, and GM decreased the peroxidation of ox brain phospholipids. GA had only a weak inhibitory effect. GM, GL, and PG reacted with CCI3O2* ** §(trichloromethyl peroxyl radical) with rate constants of 1.23 X 107, 2.33 X 107, and 1.67 X 107 M-1 s_1, respectively. Gallic acid was much less reactive with a rate constant of 4.47 X 10s M'1 s_1. By contrast to these antioxidant properties, GA, GM, and PG accelerated damage to the sugar deoxyribose in the presence of ferric-EDTA and H2O2. GA also accelerated DNA damage by a ferric-bleomycin system. GM was less effective but GL had no effect. Reaction of NBT and cytochrome c by O2•" was only slightly inhibited by PG, GM, and GA, indicating that their rates of reaction with O2" are low. Our data confirm the antioxidant actions of gallic acid lauryl, propyl, and methyl esters. However, they also show that both the prooxidant and antioxidant actions of "proposed antioxidants" should be fully characterized.

It has been suggested that taurine, hypotaurine and their metabolic precursors (cysteic acid, cysteamine and cysteinesulphinic acid) might act as antioxidants in vivo. The rates of their reactions with the biologically important oxidants hydroxyl radical (.OH), superoxide radical (O2.-), hydrogen peroxide (H2O2) and hypochlorous acid (HOCl) were studied. Their ability to inhibit iron-ion-dependent formation of .OH from H2O2 by chelating iron ions was also tested. Taurine does not react rapidly with O2.-, H2O2 or .OH, and the product of its reaction with HOCl is still sufficiently oxidizing to inactivate alpha 1-antiproteinase. Thus it seems unlikely that taurine functions as an antioxidant in vivo. Cysteic acid is also poorly reactive to the above oxidizing species. By contrast, hypotaurine is an excellent scavenger of .OH and HOCl and can interfere with iron-ion-dependent formation of .OH, although no reaction with O2.- or H2O2 could be detected within the limits of our assay techniques. Cysteamine is an excellent scavenger of .OH and HOCl; it also reacts with H2O2, but no reaction with O2.- could be measured within the limits of our assay techniques. It is concluded that cysteamine and hypotaurine are far more likely to act as antioxidants in vivo than is taurine, provided that they are present in sufficient concentration at sites of oxidant generation.

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