Glutathione revisited: a better scavenger than previously thought

Abstract

Glutathione (GSH) is the classical example of a scavenging antioxidant. It forms the first line of defense and efficiently scavenges reactive species, e.g., hypochlorous acid (HOCl), before they inflict damage to biomolecules. Scavenging antioxidant activity is best established in competition assays (that closely mimics molecular mechanism of the biological effect). In this type of assay, the antioxidant competes with a molecule that functions as an easy read-out detector for a reactive species. It is generally assumed that the scavenging antioxidant activity reflects the reaction rate constant of the antioxidant with the reactive species (ka). However, critical appraisal of several competition assays of GSH with HOCl as reactive species, reveals that ka does not determine the scavenging antioxidant activity. Assays using acetylcholine esterase, alpha1-antiprotease, methionine, and albumin as detector are compared. The total number of molecules of the reactive species scavenged by GSH plus that by partially oxidized forms of the GSH, reflect the scavenging activity of GSH. The contribution of the partially oxidized forms of GSH depends on the reactivity of the competing molecule. In several assays the partially oxidized forms of GSH have a substantial contribution to the scavenging activity of GSH. In contrast to the prevailing perception, not the reaction rate but rather the total number of molecules of the reactive species scavenged reflects the true scavenging activity of an antioxidant like GSH.

Keywords: acetylcholinesterase; alpha 1-antitrypsin; antioxidant; glutathione; hypochlorous acid; rate constant.

FIGURE 1

The stoichiometry and the capacity of the reaction of either 1 mM of glutathione (GSH) or 1 mM of 1,4-dithiothreitol with increasing concentrations of HOCl (up to 10 mM). The inset shows the bar diagrams are obtained for 1,4-dithiothreitol. Note that three molecules of 1,4-dithiothreitol are consumed, but only after the consumption of almost four molecules HOCl is detected. This leads to a stoichiometry value of 3 and a capacity of 4. Similarly, GSH has a stoichiometry of 1 and a capacity of 4.

FIGURE 2

Putative reaction scheme of GSH with OCl^- (after Winterbourn and Brennan, 1997). The initial reaction is the formation of glutathione sulfenyl chloride (GSCl). Further suggested products are depicted as oxidized condensation products of the sulfonyl chloride with the amino group of the glutamyl residue of GSH.