Clinical Relevance of Biomarkers of Oxidative Stress

Abstract

Significance: Oxidative stress is considered to be an important component of various diseases. A vast number of methods have been developed and used in virtually all diseases to measure the extent and nature of oxidative stress, ranging from oxidation of DNA to proteins, lipids, and free amino acids.

Recent advances: An increased understanding of the biology behind diseases and redox biology has led to more specific and sensitive tools to measure oxidative stress markers, which are very diverse and sometimes very low in abundance.

Critical issues: The literature is very heterogeneous. It is often difficult to draw general conclusions on the significance of oxidative stress biomarkers, as only in a limited proportion of diseases have a range of different biomarkers been used, and different biomarkers have been used to study different diseases. In addition, biomarkers are often measured using nonspecific methods, while specific methodologies are often too sophisticated or laborious for routine clinical use.

Future directions: Several markers of oxidative stress still represent a viable biomarker opportunity for clinical use. However, positive findings with currently used biomarkers still need to be validated in larger sample sizes and compared with current clinical standards to establish them as clinical diagnostics. It is important to realize that oxidative stress is a nuanced phenomenon that is difficult to characterize, and one biomarker is not necessarily better than others. The vast diversity in oxidative stress between diseases and conditions has to be taken into account when selecting the most appropriate biomarker.

FIG. 1.

Publications on oxidative stress biomarkers in different diseases. Searches were performed using oxidative stress biomarkers patients and the specific disease MeSH term using Web of Science. (A) Indicates the number of hits of all diseases combined per 10,000, normalized to a search with patients and the diseases in question. (B) Shows the number of hits per disease, which is proportional to the circle size, for the years 2005–2015.

FIG. 2.

Redox pathways associated with putative biomarkers of oxidative stress. The processes that lead to oxidative modifications of proteins, lipids, and nucleotides are highly complex. Enzymes, such as XO, NOX, and NOS, can produce ROS and RNS. These ROS can furthermore serve as substrates for other enzymes to generate additional types of ROS, such as the generation of HOCl from H₂O₂ by MPO. Cellular systems and enzymes, including the GSH and thioredoxin system, together with peroxiredoxins (T/Prx), counterbalance the production of ROS. In addition, increased levels of ROS activate Nrf2 to transcribe genes that are involved in counteracting these ROS. Oxidative stress affects cGMP signaling through its effects on nitric oxide (^•NO) production, scavenging, and on the ^•NO receptor sGC. cGMP, cyclic guanosine monophosphate; GSH, glutathione; H₂O₂, hydrogen peroxide; HOCl, hypochlorous acid; MPO, myeloperoxidase; NOS, nitric oxide synthase; NOX, NADPH oxidase; RNS, reactive nitrogen species; ROS, reactive oxygen species; sGC, soluble guanylate cyclase; XO, xanthine oxidase.

FIG. 3.

Cluster analysis of ROS biomarkers in disease. Different diseases were clustered according to described ROS biomarkers in Refs. (33, 100, 181) and studies described in this review. Some disease conditions cluster as might be expected, such as ischemia/reperfusion and heart failure, and amyotrophic lateral sclerosis and multiple sclerosis. A comprehensive analysis of ROS markers and pattern analysis in diseases might uncover common disease mechanisms or new measures of disease progression or treatment outcome. Cluster analysis was performed using Genesis software (https://genome.tugraz.at/genesisclient/genesisclient_description.shtml) as described in Mengozzi et al. (111).

FIG. 4.

Regioisomers of isolevuglandins. Specific IsoLG regioisomers differ by the relative orientation of their keto- and aldehyde moieties (D₂-IsoLG vs. E₂-IsoLG) and the position of the double bonds and hydroxyl group on the side chains (5-, 8-, 12-, or 15-IsoLG) (37, 141, 147, 148). Theoretical considerations from peroxidation chemistry suggest that the 5- and 15-IsoLG series should predominate over the 8- and 12-IsoLG series (198). It is important to recognize that one of the eight stereoisomers of both 15-D₂-IsoLG and 15-E₂-IsoLG is chemically identical to levuglandin D₂ and E₂, respectively, which are generated nonenzymatically from prostaglandin H₂ (149, 150). IsoLG, isolevuglandins.

FIG. 5.

Structure of 3-nitrotyrosine. Tyrosine nitration involves the replacement of the C₃ hydrogen atom of the tyrosine aromatic ring with a nitro group (R-NO₂). The 3-nitrotyrosine is depicted as part of a polypeptide/protein.

FIG. 6.

Formation of nitrotyrosine. In pathway 1, peroxynitrite is formed by the reaction of ^•NO with the superoxide anion radical (O₂^•−). The enzymatic generation of both these radicals is increased during inflammation. Radical–radical combination of the two species occurs exceedingly fast (rate constant 1 × 10¹⁰ M⁻¹s⁻¹), meaning that ^•NO can outcompete the dismutation of O₂^•− by SODs (138). Under physiological conditions in which CO₂ is present, nitration via peroxynitrite is increased (3) due to the formation of the adduct ONOOCO₂⁻. This adduct undergoes homolysis to the secondary free radicals, nitrogen dioxide (^•NO₂) and carbonate anion radical (CO₃^•−) (132). CO₃^•− is able to perform step 1 of the nitration process by oxidizing tyrosine to tyrosine radical, which then reacts with the ^•NO₂. In pathway 2, MPO catalyzes, in the presence of H₂O₂ and nitrite (NO₂⁻), the production of both the tyrosine radical and ^•NO₂ (11, 132). CO₂, carbon dioxide; ONOOCO₂, nitrosoperoxocarbonate; SODs, superoxide dismutases.

FIG. 7.

Protein cysteine oxidation states. Cysteine residues in proteins can exist in different oxidation states, ranging from reduced free thiols to reversible oxidized forms (disulfides, S-nitrosothiols, sulfenic acids, and sulfinic acids) to irreversible sulfonic acids. *Reversibility of protein cysteine sulfinic acids has so far been demonstrated only for some sulfinylated peroxiredoxins and requires the enzymatic activity of sulfiredoxin.

FIG. 8.

Structure of methionine sulfoxide. Methionine contains a sulfur atom that is also susceptible to oxidation and can give rise to methionine sulfoxide. The methionine sulfoxide is depicted as part of a polypeptide/protein.

FIG. 9.

Structure of 8-oxo-2′-deoxyguanosine and 8-oxo-guanosine. Oxidation of DNA and RNA commonly occurs in the guanosine moiety, leading to 8-oxo-2′-deoxyguanosine and 8-oxo-guanosine, respectively.

FIG. 10.

Protein thiol-disulfide oxidoreductases regulate the redox state of protein thiols. Different thiol compounds can be present under different redox states, depending on the overall redox state of the cell, from the more reduced (left) to more oxidized (right), and the significance of a specific biomarker will depend on its intracellular localization, tissue expression, and redox potential.