Biochemical basis and metabolic interplay of redox regulation

Abstract

Accumulated evidence strongly indicates that oxidative stress, characterized by an imbalance between reactive oxygen species (ROS) production and antioxidants in favor of oxidants, plays an important role in disease pathogenesis. However, ROS can act as signaling molecules and fulfill essential physiological functions at basal levels. Each ROS would be different in the extent to stimulate and contribute to different pathophysiological effects. Importantly, multiple ROS generators can be activated either concomitantly or sequentially by relevant signaling molecules for redox biological functions. Here, we summarized the current knowledge related to chemical and biochemical features of primary ROS species and corresponding antioxidants. Metabolic pathways of five major ROS generators and five ROS clearance systems were described, including their ROS products, specific ROS enriched tissue, cell and organelle, and relevant functional implications. We provided an overview of ROS generation and induction at different levels of metabolism. We classified 11 ROS species into three types based on their reactivity and target selectivity and presented ROS homeostasis and functional implications in pathological and physiological status. This article intensively reviewed and refined biochemical basis, metabolic signaling and regulation, functional insights, and provided guidance for the identification of novel therapeutic targets.

Keywords: Antioxidants; ROS generators; Redox regulation and homeostasis.

Fig. 1

Reactive oxygen species (ROS) metabolism. A. ROS generation: Cellular ROS is generated by 5 pathways. 1) Superoxide generation: O₂^•- is derived from oxygen (O₂) after receiving an electron from various oxidases or from mitochondrial electron transport chain (ETC). 2) Reactive nitrogen species generation: O₂^•- reacts with nitric oxide (NO^•) to form peroxynitrite (ONOO⁻). At physiological pH (pKa = 6.8), ONOO⁻ is in equilibrium with peroxynitrous acid (ONOOH). In the aqueous phase, ONOO⁻ rapidly reacts with carbon dioxide (CO₂) to generate carbonate radical anion (CO₃^•-) and nitrogen dioxide (NO₂^•). In the hydrophobic phase, ONOOH can undergo homolytic fission to generate hydroxyl radical (OH^•-) and NO₂^•. 3) Hydrogen peroxide generation: O₂^•- is dismutated spontaneously or by superoxide dismutase (SOD) into H₂O₂. NOX4 and DOX1/2 can also dismutate O₂ directly to H₂O₂. 4) Hydroxyl radical generation: OH^•- can be derived from ONOOH as described in pathway 2. It can also be generated from H₂O₂ through Haber–Weiss reaction in the presence of transition metals, such as Fe²⁺, Cu²⁺, Zn²⁺. H₂O₂ can be converted to hypochlorous acid (HOCl) by myeloperoxidase (MPO), and then react with O₂^•- to form OH^•. 5) Lipid radical generation: Lipid peroxidation is usually initiated by OH^•- abstracting an allylic hydrogen (H) from unsaturated lipid and forming the carbon-centered lipid radical (L^•). Generated L^• then rapidly reacts with O [2] to form a lipid peroxyl radical (LOO^•), which abstracts a H from another unsaturated lipid (LH) molecule to form lipid hydroperoxide (LOOH). B. ROS clearance: There are five major ROS clearance pathways, including I) Superoxide dismutation: Two O₂^•- can be converted to one H₂O₂ and O₂ by SOD1-3. Ⅱ) Hydrogen peroxide decomposition: Two H₂O₂ molecules can be decomposed by catalase (CAT) into H₂O and O₂. Ⅲ) Glutathione redox cycle: H₂O₂ can be reduced by glutathione peroxidase (GPX) or Peroxiredoxin (PRDX)1/4/6 isoforms into H₂O using GSH as the electron donor. Oxidized GSSG will be reduced back to GSH by glutathione reductase (GSR) after receiving a H from NADPH. GPX can also reduce LOOH to become LOH. Ⅳ) Thioredoxin redox cycle: H₂O₂ can also be reduced by PRDX1-5 into H₂O using reduced Thioredoxin (Trx^R) as an electron donor. Oxidized Thioredoxin (Trx^O) will be reduced back to Trx^R by Thioredoxin reductase (TR). Ⅴ) glutathione-S-transferase detoxification: Some electrophilic compounds, such as xenobiotics, can be conjugated with GSH by glutathione-S-transferase (GST) forming Glutathione conjugates (GSX). Abbreviation: Arg, l-arginine; CAT, Catalase; DUOX1/2, Dual oxidase1/2; DHFR, Dihydrofolate reductase; eNOS, Endothelial nitric oxide synthase; ETC, Electron transport chain; GPX, glutathione peroxidase; GSH, Glutathione; GSSG, Glutathione disulfide; GSR, glutathione reductase; G6PD, Glucose-6-phosphate dehydrogenase; GST, glutathione-S-transferase; GSX, Glutathione conjugates; H^•, Hydrogen atom; H⁺, Hydrogen cation; HOCl, Hypochlorous acid; LH, Unsaturated lipid; L^•, Lipid radical; LOO^•, Lipid peroxyl radical; LOOH, Lipid peroxide; MPO, Myeloperoxidase; Mt, Mitochondria; NADP⁺, Nicotinamide adenine dinucleotide phosphate; NOS, Nitric oxide synthase; NOX1-5, NADPH oxidase complex1-5; ONOO-, Peroxynitrite; PRDX, Peroxiredoxin; RNS, Reactive nitrogen species; ROS, Reactive oxygen species; SOD, superoxide dismutase; TR, Thioredoxin reductase; Trx^R, Reduced Thioredoxin; Trx^O, Oxidized Thioredoxin; U-eNOS, uncoupled endothelial nitric oxide synthase; XO, xanthine oxidase.

Fig. 2

Metabolic interplay between various ROS generators. ROS generator interplay is summarized in three major pathways: 1) NOX-ROS promoted Mt-ROS generation. In response to activation signaling, such as AngII, NOX1/2 located in PM/ER/ES can be activated via PKC-induced P47phox phosphorylation. Extracellular O₂^•- generated from PM-located NOX1/2 can be dismutated by SOD3 to form H₂O₂ which then diffuses into the cytosol via AQP, while intracellular O₂^•- generated from ER/ES-located NOX1/2 can be dismutated by SOD1 to form H₂O₂. 1a) ROS produced by Mt ETC. NOX1/2-derived H₂O₂ can activate Mt-KATP via PKC-ε signaling, leading to the K⁺ influx, decrease of Mt membrane potential (ΔΨm), inhibition of Mt complex I (Com I) and finally the increase of Mt O₂^•- production. 1b) ROS produced by NOX4 in Mt. In certain cell types such as CM and EC, NOX4 isoform is detected at the Mt. Increase expression of NOX4 by activation signaling such as AngII/Gαq pathway can directly contribute to the increase of Mt ROS. 2) Mt-ROS activated NOX-ROS generation. In Mt, O₂^•- generated by Com I is mainly released to Mt matrix and dismutated by SOD2 to H₂O₂, while O₂^•- generated by Com III can be released to either Mt matrix or intermembrane space and dismutated by SOD1. A small amount of Mt O₂^•- can be transported to the cytosol by VDAC directly and then dismutated by SOD1 to form H₂O₂. Mt-generated H₂O₂ can be leaked to the cytosol through water channel aquaporin (AQP). Mt-derived H₂O₂ then further activates NADPH oxidase by two pathways: 2a) The p47phox phosphorylation via activating PKC/cSrc; 2b) The increased expression of P22phox/NOX1/2/4 probably via c-Src activation. 3) NOX/Mt-ROS regulated eNOS/XR-ROS. Cytosolic O₂^•- from Mt or NADPH oxidase can react with NO to form ONOO⁻, resulting in eNOS uncoupling via depletion of BH4, or XR to XO transformation via oxidation of critical thiols in Cys535/992. O₂^•- is generated by XO and uncoupled eNOS. Abbreviation: AngII, Angiotensin II; AT1R, Angiotensin II receptor type 1; AQP, Aquaporin; BH2, dihydrobiopterin; BH4, Tetrahydrobiopterin; eNOS, Endothelial nitric oxide synthase; ER, Endoplasmic reticulum; ES, Endosome; ETC, Electron transportation chain; H₂O₂, hydrogen peroxide; Mt-KATP, Mitochondrial adenosine triphosphate (ATP)-sensitive potassium channel; NO, nitric oxide; NOX, NADPH oxidase; ONOO⁻, Peroxynitrite; p-P47phox, phosphorylated P47phox; PKC-ε, PKC-epsilon; PM, plasma membrane; SOD, Superoxide dismutase; U-eNOS, uncoupled eNOS; VSMC, Vascular smooth muscle cells; VDAC, Voltage-dependent anion channel; XO, Xanthine oxidase; XR, Xanthine reductase; ΔΨm, Mitochondria membrane potential.

Fig. 3

Cellular ROS homeostasis and their pathophysiological effects. Cellular ROS levels undergo consistent changes in redox status. In physiological condition, ROS is maintained at equilibrium levels to facilitate physiological redox signaling as described in the green radial network on the left. Impaired ROS production causes low redox status and suppresses physiological redox signaling. In the case of high ROS status or oxidative stress, excessive ROS would initiate pathological redox signaling and induce cellular damage and various diseases as indicated in the red radial network on the right. Type 1 ROS (O₂^•-, NO^•, H₂O₂) is firstly generated and has essential physiological functions. Type 2 ROS (ONOO⁻, ONOOH, OH^•, HOCl) and type 3 ROS (NO2^•, CO3^•-, RO^•/ROO^•) are subsequently products of Type 1 ROS and play important role in oxidative stress. ROS reactivity is defined based on descriptions in Table 1 legend. Abbreviation: CO3^•-, Carbonate radical anion; COPD, Chronic obstructive pulmonary disease; EC, Endothelial cells; HF, Heart failure; OH^•, Hydroxyl radical; H₂O₂, Hydrogen peroxide; MC, Monocytes; MI, Myocardial infarction; NO^•, Nitric oxide radical; NO₂^•, Nitrogen dioxide radical; O₂^•-, Superoxide radical anion; ONOO⁻, Peroxynitrite; RO^•, Alkoxyl radical; ROO^•, Peroxyl radical; ROS, Reactive oxygen species; VSMC, Vascular smooth muscle cells.