Glutathione and peroxisome redox homeostasis

Abstract

Despite intensive research on peroxisome biochemistry, the role of glutathione in peroxisomal redox homeostasis has remained a matter of speculation for many years, and only recently has this issue started to be experimentally addressed. Here, we summarize and compare data from several organisms on the peroxisome-glutathione topic. It is clear from this comparison that the repertoire of glutathione-utilizing enzymes in peroxisomes of different organisms varies widely. In addition, the available data suggest that the kinetic connectivity between the cytosolic and peroxisomal pools of glutathione may also be different in different organisms, with some possessing a peroxisomal membrane that is promptly permeable to glutathione whereas in others this may not be the case. However, regardless of the differences, the picture that emerges from all these data is that glutathione is a crucial component of the antioxidative system that operates inside peroxisomes in all organisms.

Keywords: Antioxidant; Glutathione; Membrane permeability; Peroxisomes; Redox homeostasis.

Graphical abstract

Fig. 1

– The functions of glutathione. Glutathione, the most abundant thiol in most organisms, plays a role in multiple pathways/processes such as iron-sulfur (Fe–S) cluster biogenesis and transport (yellow), detoxification of electrophilic substances (blue) and hydroperoxides (purple) and protein deglutathionylation (pink). In many of these reactions, reduced glutathione (GSH) is oxidized to GSSG, which must then be reduced back to GSH by the NADPH-dependent glutathione reductase (green). ROOH and ROH, organic hydroperoxides and the corresponding alcohols, respectively; Protein-SG and Protein-SH, glutathionylated and reduced protein, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 2

– The protective role of glutathione against oxidative damage to protein thiol groups. Glutathione-based enzymatic and non-enzymatic reactions with oxidized proteins are shown. With the exception of yeast OPT2, the identity of all the other peroxisomal glutathione pores/channels remains unknown (“?”). One of these pores might be the peroxisomal protein translocon (purple structures in the membrane), as hypothesized previously [9]. Regardless of their identities and mechanisms of transport (i.e., pore/glutathione-specific transporter, unidirectional/bidirectional), the capacity of glutathione transporters in different organisms may be different, resulting in peroxisomal glutathione pools that may display different degrees of kinetic connectivity with the cytosolic pool. In rat liver, the permeability of the peroxisomal membrane to glutathione is relatively large, as assessed in in vitro experiments [36]. In yeast and plants, the kinetic connectivity of the two pools of glutathione may be low, at least under some physiological conditions, as suggested by the presence of glutathione reductase in their peroxisomes. Hydrogen peroxide oxidizes the thiol group of protein cysteine residues to the sulfenic (-SOH), sulfinic (-SO₂H) and sulfonic (-SO₃H) derivatives. There are no repair systems for protein sulfonates. Some protein sulfinates can be repaired by sulfiredoxin but this enzyme was never found in peroxisomes. Protein disulfides (-S-S- and -SG) can be reduced by GSH by both enzymatic and non-enzymatic mechanisms (see main text for details). Cyt – cytosol; GR and GRL1 – Glutathione reductase; GSH/GSSG – reduced/oxidized glutathione; GSTk1 – Glutathione S-transferase kappa 1; GSTL2 – Glutathione S-transferase lambda-2; GTO1 – Glutathione S-transferase omega-like 1; Mat – matrix; OPT2 – Oligopeptide transporter 2; SG – glutathionylated protein derivative; SH – reduced protein thiol group; SOH – sulfenic protein derivative; SO₂H – sulfinic protein derivative; SO₃H – sulfonic protein derivative; S–S – disulfide linked protein. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 3

– Non-enzymatic oxidation/reduction kinetics of protein thiol groups in peroxisomes. The half-lives (t_1/2) of the different species were calculated assuming peroxisomal steady-state (ss) concentrations of 80 nM and 5 mM for H₂O₂ and GSH [36], respectively, and the following second order rate constants: oxidation of cysteine (PSH) to sulfenic acid (PSOH) – 2.7 M⁻¹ s⁻¹ for non-catalytic cysteines [104] and 100 M⁻¹ s⁻¹ for active-site cysteine residues ([106]; see also [36]); oxidation of cysteine sulfenic (PSOH) to sulfinic acid (PSO₂H) by H₂O₂ – 10²–10⁴ M⁻¹ s⁻¹ [106,107,132]; condensation of cysteine sulfenic acid (PSOH) with the thiol group of GSH – 6.7–10⁵ M⁻¹ s⁻¹ ([102,104]; see also [36]); and thiol-disulfide exchange – 0.1–10 M⁻¹ s⁻¹ [102]. Second-order rate constants were multiplied by the corresponding steady-state concentrations of GSH or H₂O₂ to obtain pseudo-first order constants (K), and these were converted to half-lives (t_1/2 = ln2/K). Red and green colors indicate oxidation and reduction reactions, respectively. GSH – reduced glutathione; H₂O₂ – hydrogen peroxide; PSH – reduced protein; PSSG – glutathionylated protein; PSOH – protein sulfenic derivative; PSO₂H – protein sulfinic derivative. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)