Green tea polyphenolic antioxidants oxidize hydrogen sulfide to thiosulfate and polysulfides: A possible new mechanism underpinning their biological action

Abstract

Matcha and green tea catechins such as (−)-epicatechin (EC), (−)-epigallocatechin (EGC) and (−)-epigallocatechin gallate (EGCG) have long been studied for their antioxidant and health-promoting effects. Using specific fluorophores for H 2 S (AzMC) and polysulfides (SSP4) as well as IC-MS and UPLC-MS/MS-based techniques we here show that popular Japanese and Chinese green teas and select catechins all catalytically oxidize hydrogen sulfide (H 2 S) to polysulfides with the potency of EGC > EGCG >> EG. This reaction is accompanied by the formation of sulfite, thiosulfate and sulfate, consumes oxygen and is partially inhibited by the superoxide scavenger, tempol, and superoxide dismutase but not mannitol, trolox, DMPO, or the iron chelator, desferrioxamine. We propose that the reaction proceeds via a one-electron autoxidation process during which one of the OH-groups of the catechin B-ring is autooxidized to a semiquinone radical and oxygen is reduced to superoxide, either of which can then oxidize HS − to thiyl radicals (HS • ) which react to form hydrogen persulfide (H 2 S 2 ). H 2 S oxidation reduces the B-ring back to the hydroquinone for recycling while the superoxide is reduced to hydrogen peroxide (H 2 O 2 ). Matcha and catechins also concentration-dependently and rapidly produce polysulfides in HEK293 cells with the potency order EGCG > EGC > EG, an EGCG threshold of ~300 nM, and an EC 50 of ~3 μM, suggesting green tea also acts as powerful pro-oxidant in vivo . The resultant polysulfides formed are not only potent antioxidants, but elicit a cascade of secondary cytoprotective effects, and we propose that many of the health benefits of green tea are mediated through these reactions. Remarkably, all green tea leaves constitutively contain small amounts of H 2 S 2 .

Oxidation of Hydrogen Sulfide by Quinones: How Polyphenols Initiate Their Cytoprotective Effects

“…This has been suggested to be overcome by trace-metal catalysis [ 2 ], although catechins can also undergo autoxidation by O 2 in the absence of metal catalysts, albeit slowly [ 21 ]. The latter is consistent with our observations of polysulfide production from H 2 S catalyzed by tea catechins [ 16 ]. Two or three vicinal hydroxyl (catechol) groups of the B ring render these compounds electron-rich and give catechins their antioxidant function [ 2 , 22 ], and their antioxidant efficiency increases as the number of hydroxyl groups increases [ 1 , 2 , 23 ].…”

“…The side-chain modifications which stabilize the semiquinone of the parent compounds appear to render a variety of neutraceutical polyphenols susceptible to autoxidation and H 2 S oxidation, which we have shown previously for tea catechins [ 16 ]. Although 2,5-dihydroxy-1,4-benzoquinone has para hydroxyl groups, it does not oxidize H 2 S because it has exceptional resonance stabilization that renders the molecule relatively unreactive [ 25 ].…”

“…We have previously demonstrated that green tea polyphenols oxidize H 2 S to thiosulfate in addition to polysulfides [ 16 ]. Here, we used the AgNP assay to examine thiosulfate production by various benzenes.…”

“…Tea polyphenols may also autoxidize and thereby become pro-oxidants [ 11 , 12 ]. We recently showed that these autoxidized green teas and their catechins catalytically oxidized hydrogen sulfide (H 2 S) to polysulfides (H 2 S n ; where n = 2–5) and other sulfoxides, especially thiosulfate [ 16 ]. As these products are themselves potent direct antioxidants [ 17 ] and will initiate Nrf2-mediated transcription of antioxidant defenses [ 18 , 19 ], we proposed that many of the health benefits of teas could be attributed to their autoxidation and subsequent effects on cellular sulfur metabolism.…”

“…Two or three vicinal hydroxyl (catechol) groups of the B ring render these compounds electron-rich and give catechins their antioxidant function [ 2 , 22 ], and their antioxidant efficiency increases as the number of hydroxyl groups increases [ 1 , 2 , 23 ]. This also correlates with their ability to oxidize H 2 S [ 16 ].…”