Significance of Singlet Oxygen Molecule in Pathologies

Abstract

Reactive oxygen species, including singlet oxygen, play an important role in the onset and progression of disease, as well as in aging. Singlet oxygen can be formed non-enzymatically by chemical, photochemical, and electron transfer reactions, or as a byproduct of endogenous enzymatic reactions in phagocytosis during inflammation. The imbalance of antioxidant enzymes and antioxidant networks with the generation of singlet oxygen increases oxidative stress, resulting in the undesirable oxidation and modification of biomolecules, such as proteins, DNA, and lipids. This review describes the molecular mechanisms of singlet oxygen production in vivo and methods for the evaluation of damage induced by singlet oxygen. The involvement of singlet oxygen in the pathogenesis of skin and eye diseases is also discussed from the biomolecular perspective. We also present our findings on lipid oxidation products derived from singlet oxygen-mediated oxidation in glaucoma, early diabetes patients, and a mouse model of bronchial asthma. Even in these diseases, oxidation products due to singlet oxygen have not been measured clinically. This review discusses their potential as biomarkers for diagnosis. Recent developments in singlet oxygen scavengers such as carotenoids, which can be utilized to prevent the onset and progression of disease, are also described.

Keywords: biomarkers; lipid peroxidation; reactive oxygen species; singlet oxygen.

Figure 1

Relationship between energy levels of ground state triplet oxygen molecule ³O₂ and singlet oxygen molecule ¹O₂. The ground state oxygen molecule, triplet oxygen ³O₂ (¹Σ⁻_g), has two unpaired and spin-parallel electrons in π* antibonding orbitals. ³O₂ (³Σ⁻_g) in the ground state receives energy and is excited to the singlet state, forming singlet oxygen (¹O₂). ¹O₂ is the spin-flipped electron state of ³O₂ (³Σ⁻_g). There are two states of ¹O₂:¹O₂ (¹Σ⁺_g) and ¹O₂ (¹Δ_g). ¹O₂ (¹Σ⁺_g) and ¹O₂ (¹Δ_g) have energies that are 1.6 eV and 0.98 eV higher than the ground state ³O₂ (³Σ⁻_g), respectively. The lifetime of ¹O₂ (¹Σ⁺_g) is a few picoseconds, and it is rapidly converted to ¹O₂ (¹Δ_g). Since the lifetime of ¹O₂ (¹Δ_g) is several microseconds, which is significantly longer than that of ¹O₂ (¹Σ⁺_g), the ¹O₂ generated in vivo can be ¹O₂ (¹Δ_g). The arrows indicate the direction of the electron spin of the highest occupied molecular orbital (HOMO).

Figure 2

Major ¹O₂ production mechanisms. ¹O₂ production pathways; (A) photochemical reaction, (B) reactions mediated by hydrogen peroxide produced by myeloperoxidase (MPO), (C) decomposition of peroxyl radicals (Russel mechanism), (D) reaction mediated by superoxide anion (O₂^•−), (E) pathway via peroxynitrite decomposition.

Figure 3

¹O₂-mediated oxidation of amino acid, nucleic acid, and lipids. (A) Oxygenation reaction by ¹O₂. (B) Reaction products of tryptophan and methionine with ¹O₂. (C) Formation of 8-oxo-2′-deoxyguanosine (8-oxo-dG) by ¹O₂ oxidation to deoxyguanosine (dG). (D) Oxidative modification of fatty acids in membrane phospholipids by ¹O₂.

Figure 4

Structures and oxidation modification of probes for ¹O₂ detection. (A) 2,2,6,6-tetramethylpiperidine (TEMP), (B) Hydroxy-TEMP, (C) 9,10-dimethylanthracene (DMA), (D) 9-[2-(3-Carboxy-9,10-diphenyl)anthryl]-6-hydroxy-3H-xanthen-3-ones (DPAXs), (E) Singlet Oxygen Sensor Green (SOSG), (F) silicone-containing rhodamine-9,10-dimethylanthracene (Si-DMA).

Figure 5

Structures and formation mechanism of hydroxyoctadecadienoic acid (HODE), an oxidation product derived from linoleic acid. ¹O₂ oxidizes linoleic acid to produce 13-(Z,E)-HODE, 9-(Z,E)-HODE, 12-(Z,E)-HODE, and 10-(Z,E)-HODE. 13-(Z,E)-HODE and 9-(Z,E)-HODE are also produced by ROS other than ¹O₂ and by enzymatic oxidation reactions via lipid oxidases. 9-(E,E)-HODE and 13-(E,E)-HODE are produced in a radical-specific manner.

Figure 6

Structures of cholesterol peroxides produced by ¹O₂-mediated oxidation reactions. ¹O₂ oxidizes cholesterol to produce 5α-hydroperoxide (cholesterol 5α-OOH), 6α-hydroperoxide (cholesterol 6α-OOH), and 6β-hydroperoxide (cholesterol 6β-OOH). Hock cleavage of cholesterol 5α-OOH converts it to cholesterol 5,6-secosterols.

Figure 7

Photosensitizers and the mechanisms of ¹O₂ production. (1) Photosensitizers (PS) absorb photons (hv) from light and transform them to the excited singlet state (¹PS^•). (2) ¹PS^• returns to the ground singlet state (¹PS) by losing energy via fluorescence emission. (3) ¹PS^• was converted to the long-lived triplet state (³PS^•). (4) ³PS^• can return to the ground singlet state (¹PS) via light emission (phosphorescence). (5) ³PS^• forms organic radicals (L^•) from organic compounds (LH) through the transfer of electrons in the type I photochemical reaction. L^• affects oxygen (³O₂) to produce ROS (superoxide anion O₂^•−, lipid peroxyl radical (LOO^•). O₂^•− leads to the production of H₂O₂ and hydroxyl radicals (OH^•), resulting in the induction of radical chain reactions. (6) In type II photochemical reactions, the energy of ³PS^• is transferred to ³O₂ to mainly produce ¹O₂, but O₂^•− is produced as a minor product via electron transfer from ³PS^•.

Figure 8

The structures of photosensitizers. (A) Protoporphyrin IX, (B) coproporphyrin, (C) hemin, (D) chlorophyll a, (E) pheophytin a, (F) pheophorbide a, (G) tryptophan, (H) riboflavin, (I) cholesta-5,7,9(11)-trien-3beta-ol (9-DDHC), (J) psoralen.

Figure 9

The structures of squalene and its peroxidation products. Squalene (Sq) is converted by ¹O₂ to six monohydroperoxides (2-, 3-, 6-, 7-, 10-, and 11-OOH-Sq). The monohydroperoxides of Sq are further photo-oxidized to 2-OOH-3-(1,2-dioxane)-Sq.

Figure 10

The structures of biosynthesis of A2E. (A) After Schiff base formation between all-trans-retinal and ethanolamine or phosphatidylethanolamine, a [1,6]-proton tautomerization to enamine follows. After further Schiff base formation with a second molecule of all-trans-retinal, a [3,3]-sigmatropic rearrangement is followed by the hydrolysis of the linked phosphatidylethanolamine adduct, resulting in the formation of N-retinyl-N-retinylidene ethanolamine (A2E). (B) ¹O₂ production process mediated by A2E in the retina. Light irradiation of A2E accumulated in lipofuscin generates ¹O₂.

Figure 11

Assumed scheme of ¹O₂ production via NADPH oxidase and myeloperoxidase (MPO) in vascular injury in diabetes mellitus. High glucose produces hydrogen peroxide (H₂O₂) by NADPH oxidase in vascular endothelial cells. ¹O₂ is generated via MPO in activated neutrophils bound to the vessel wall.

Figure 12

The structures of carotenoid and peroxidation products from lycopene and β-carotene. (A) Lycopene, (B) β-carotene, (C) astaxanthin, (D) lutein, (E) zeaxanthin.

Figure 13

Compounds with ¹O₂ scavenging activity other than carotenoids. The structures of (A) bakuchiol and (B) acetyl zingerone.