The chemical biology of the persulfide (RSSH)/perthiyl (RSS·) redox couple and possible role in biological redox signaling

Highlights

• Hydropersulfides (RSSH) are readily oxidized to the corresponding perthiyl radical (RSS·).

• RSS· does not react with O₂ or NO.

• Hydropersulfides are superior reductants compared to thiols (RSH).

• Perthiyl is less reactive than a thiyl (RS·) species.

• The RSSH/RSS· redox couple is biologically accessible and possibly relevant.

Abstract

The recent finding that hydropersulfides (RSSH) are biologically prevalent in mammalian systems has prompted further investigation of their chemical properties in order to provide a basis for understanding their potential functions, if any. Hydropersulfides have been touted as hyper-reactive thiol-like species that possess increased nucleophilicity and reducing capabilities compared to their thiol counterparts. Herein, using persulfide generating model systems, the ability of RSSH species to act as one-electron reductants has been examined. Not unexpectedly, RSSH is relatively easily oxidized, compared to thiols, by weak oxidants to generate the perthiyl radical (RSS·). Somewhat surprisingly, however, RSS· was found to be stable in the presence of both O₂ and NO and only appears to dimerize. Thus, the RSSH/RSS· redox couple is readily accessible under biological conditions and since dimerization of RSS· may be a rare event due to low concentrations and/or sequestration within a protein, it is speculated that the general lack of reactivity of individual RSS· species may allow this couple to be utilized as a redox component in biological systems.

Introduction

Hydrogen sulfide (H₂S) is an endogenously synthesized small molecule bioregulator with numerous physiological activities [1], [2], [3]. For example, H₂S has been reported to be involved in the etiology of atherosclerosis [4] and cancer [5], regulate transcription via NRF2/KEAP1 [6] or NF-⎕B [7], control vascular tone [8], inhibit phosphodiesterase activity [9], protect against neurodegenerative diseases [10] and protect from ischemia-reperfusion toxicity [11], just to name a few. Current interest in the chemical biology of H₂S has been motivated primarily by the reports of its biological activity and potential therapeutic utility. Despite the increasing recognition that H₂S is an important physiological mediator/regulator, the biochemical targets and chemical mechanism(s) associated with these actions are, for the most part, unknown.

In a biological system, H₂S can react with disulfides (RSSR), creating an equilibrium involving H₂S, RSSR, thiols (RSH) and hydropersulfides (RSSH, note: this abbreviation will be used to denote all generic hydropersulfide species) (Reaction 1) [12], [13].

RSSR+H₂S⇌RSSH+RSH

The existence of this equilibrium reaction indicates the potential for RSSH formation from H₂S when RSSR is present. Moreover, a pathway for H₂S biosynthesis, conversion of cystine (Cys-SS-Cys) to H₂S and pyruvate, involves the intermediacy of cysteine hydroperpersulfide (Cys-SSH) [14]. Thus, H₂S and RSSH are intimately linked chemically and, presumably biologically. In support of this idea, numerous recent studies indicate that RSSH are ubiquitous and highly prevalent in mammals with concentrations of glutathione hydropersulfide (GSSH) as high as 150 micromolar in mouse brain and 50 micromolar in heart and liver [15]. Also, several labs report the prevalence of numerous protein hydropersulfides [15], [16], [17]. Indeed, it has been speculated that RSSH and polysulfides (RS_nH or RS_nR, n>2) are crucial biological effectors and that H₂S can serve as a marker for their presence [18]. Although H₂S is likely to have its own functions, it is becoming increasingly evident that RSSH (and derived compounds) can be important biological effectors, regulators and signaling species for example, [19]. Thus, the chemical biology of RSSH becomes a topic of vital importance in the attempt to delineate the mechanisms by which H₂S and related species elicit their biological actions.

Recent work from this lab and others has shown that RSSH exhibit enhanced thiol-like chemistry compared to the corresponding thiol [20], [21], [22]. That is, RSSH are more nucleophilic and reducing (both one- and two-electron) compared to thiols and it is speculated that these properties may be important to their physiological utility. Of particular interest to the current study is the ability of RSSH to act as a one-electron reductant. One possible reason for the superior one-electron reducing capability of RSSH species versus the corresponding thiol is the presumed stability of the oxidized product, the perthiyl radical (RSS·), compared to that of a thiyl radical (RS·) (vide infra). Thus, in order to further evaluate and predict the possible physiological utility of hydropersulfides as one-electron donors or redox regulators, the formation, stability and reactivity of RSS· has been examined.