8.1 Oxidative Stress, Reactive Oxygen/Nitrogen Species (ROS/RNS) & Antioxidant Defenses

Why oxidative stress is a Domain II mechanism skill

Handbook II.1 asks you to develop a mechanistic hypothesis and to know established molecular pathways, organ physiology, genetic-variant phenotypes (knockouts and SNPs), and when omics is an appropriate test of that hypothesis (II.1 A–F). Handbook II.3 asks which attributes—genetic polymorphisms, life-stage, sex, and background disease—change susceptibility. Independent OpenExamPrep material in this section treats reactive oxygen species (ROS) and reactive nitrogen species (RNS) as a working example of that skill, not as a slogan you write on every unexplained lesion. This chapter is not an ABT product and does not claim official approval, review, or partnership with ABT.

A ROS story is complete only when you can name the initiating oxidant, the metal or enzyme that converts it, the macromolecule that is damaged, the defense that failed or was induced, and the in-life or anatomic outcome. “Oxidative stress” written into a report without those pieces is a label, not a mechanism.

II.1 A–B are the experimental craft. Start from an apical effect (centrilobular necrosis, delayed pulmonary fibrosis, dilated cardiomyopathy). Propose a molecular initiating chemistry (CYP2E1 reductive dehalogenation, redox cycling, Fenton iron). Test it in vitro (isolated mitochondria, primary hepatocytes, ARE-luciferase reporters, iron chelation, CYP2E1 inhibition) and in vivo (time course of GSH before necrosis, CYP2E1 knockout or induction, Nrf2-null challenge, metabolite identification). Omics enter only after that hypothesis exists (II.1 F).

Superoxide, hydrogen peroxide, hydroxyl radical, and peroxynitrite

Molecular oxygen accepts electrons one at a time. The first product is superoxide (O2•−). Sources you should be able to name include the mitochondrial electron-transport chain (especially complex I and complex III when the chain is over-reduced or blocked), NADPH oxidases (NOX), xanthine oxidase, and one-electron reduction of redox-cycling xenobiotics. Superoxide is a modest one-electron reductant and a precursor. It does not wander the nucleus oxidizing random guanines the way hydroxyl radical does.

Copper/zinc superoxide dismutase (SOD1) in cytosol, manganese SOD (SOD2) in the mitochondrial matrix, and extracellular SOD (SOD3) convert two superoxide anions to hydrogen peroxide (H2O2) and oxygen: 2 O2•− + 2 H+ → H2O2 + O2. H2O2 is not a free radical. It crosses membranes more readily than superoxide, supports physiologic signaling at low flux, and becomes injurious when it meets redox-active iron or copper.

The Fenton reaction is Fe2+ + H2O2 → Fe3+ + •OH + OH−. Copper(I) does the analogous chemistry. The hydroxyl radical (•OH) reacts at diffusion-limited rates with DNA, protein, and lipid at the site where the metal is bound. That is why loosely chelatable iron and copper are not trivia: they localize the lesion. Superoxide can reduce Fe3+ back to Fe2+ (a Haber–Weiss cycle), so a catalytic metal plus a modest peroxide flux is enough to keep producing hydroxyl radical.

Nitric oxide (NO•) from nitric-oxide synthases combines with superoxide at a near-diffusion-limited rate to form peroxynitrite (ONOO−). Peroxynitrite and its conjugate acid nitrate protein tyrosines (3-nitrotyrosine), oxidize thiols, and can yield hydroxyl-like oxidants. RNS injury is what happens when NO production and superoxide production occupy the same compartment—not a separate universe from ROS.

Antioxidant defenses: SOD, catalase, GPx, GSH, and Nrf2/Keap1

Catalase in peroxisomes converts 2 H2O2 → 2 H2O + O2 and matters most at high peroxide concentrations. Glutathione peroxidases (GPx), several of them selenoproteins, reduce H2O2 and lipid hydroperoxides using reduced glutathione (GSH): H2O2 + 2 GSH → 2 H2O + GSSG. Glutathione reductase then uses NADPH to regenerate GSH. Drain NADPH—as paraquat does while it redox-cycles—and the GSH cycle stalls even if the tripeptide is still present on a bulk assay.

GSH is γ-glutamyl-cysteinyl-glycine. Hepatocytes hold it in the millimolar range. Rate-limiting synthesis is glutamate-cysteine ligase (GCLC/GCLM). GSH conjugates electrophiles via glutathione S-transferases (GSTs) and keeps protein thiols reduced. A fall in the GSH:GSSG ratio is a redox readout. It is not, by itself, an adverse outcome.

Nrf2 (NFE2L2) is the inducible transcriptional defense against electrophiles and oxidants. Under basal conditions Keap1 binds Nrf2 and presents it to a Cul3 ubiquitin ligase, so Nrf2 is degraded. Oxidative or electrophilic modification of Keap1 cysteines (classically Cys151, Cys273, and Cys288) impairs that degradation. Nrf2 accumulates, heterodimerizes with small Maf proteins, and binds antioxidant response elements (AREs). Induced genes include heme oxygenase-1 (HMOX1), NAD(P)H:quinone oxidoreductase 1 (NQO1), GCLC, many GSTs, and thioredoxin-system components. A chemical that both generates ROS and induces Nrf2 is the expected adaptive loop, not a contradiction. Nrf2-null animals convert a borderline oxidant stress into frank injury—the knockout half of II.1 E.

Lipid peroxidation and 8-oxo-dG

Polyunsaturated fatty acids (PUFAs) have bis-allylic hydrogens that hydroxyl radical or lipid oxyl radicals abstract. The carbon-centered radical adds oxygen to a lipid peroxyl radical, which abstracts a hydrogen from a neighboring PUFA—chain propagation. Termination and decomposition yield malondialdehyde (MDA), 4-hydroxynonenal (4-HNE), and, more specifically, F2-isoprostanes. Membranes lose order; proteins and DNA acquire carbonyl and aldehyde adducts; mitochondrial inner membrane damage feeds the permeability-transition story in section 8.2.

8-oxo-7,8-dihydro-2′-deoxyguanosine (8-oxo-dG) (also written 8-OHdG) is the workhorse oxidative DNA lesion at guanine C8. It mispairs with adenine and produces G→T transversions if unrepaired. OGG1 base-excision repair removes it. Urinary 8-oxo-dG is used in biomonitoring; artifactual oxidation during DNA work-up is a famous pitfall. A single 8-oxo-dG number without a second marker or pathology is a weak standalone claim.

Omics as hypothesis tools (II.1 F, II.6 E)

Transcriptomics (microarray or RNA-seq) can show an Nrf2-class signature, a p53 DNA-damage signature, or a hypoxic HIF signature. It reports mRNA, not enzyme activity and not that the animal is clinically injured. Proteomics can add abundance and some post-translational marks (protein carbonyls, 3-nitrotyrosine peptides). Metabolomics sits closer to phenotype: GSH/GSSG, NADPH, lipid-peroxidation products, TCA intermediates.

Handbook II.1 F is the decision to apply omics to test a stated hypothesis (“if this is CYP2E1-dependent lipid peroxidation, then CYP2E1 protein, F2-isoprostanes, and an Nrf2 transcript battery should move before necrosis”). Handbook II.6 E—picked up again in section 8.3—is the translation rule: an omics pattern is not an organism-level effect until you connect it to function, histopathology, or a population outcome. Use enrichment to design the next experiment (SOD2 heterozygote, iron chelation, CYP2E1 inhibition), not as the last sentence of a risk conclusion.

Genetic variants, knockouts, and SNPs as susceptibility (II.3)

GSTM1*0 is a common homozygous gene deletion. GSTM1-null individuals make no GSTM1 protein. For some electrophilic metabolites (certain polycyclic aromatic hydrocarbon diol-epoxides), that can increase adduct burden. For other substrates another GST isoform compensates. Do not treat “GSTM1-null equals universally sensitive” as a law.

CYP2E1 oxidizes ethanol, acetone, benzene, chloroform, and carbon tetrachloride. Published CYP2E1 SNPs, including the well-studied 5′-flanking RsaI c2 allele, have inconsistent human epidemiology. The clearer susceptibility story is phenotypic induction: chronic ethanol, fasting, acetone, and poorly controlled diabetes increase CYP2E1 protein and can potentiate zone-3 bioactivation. CYP2E1-null mice are protected from classic CCl4 necrosis—the experimental converse.

SOD2 heterozygosity increases mitochondrial oxidative injury without requiring a xenobiotic. Life-stage changes neonatal GSH and CYP ontogeny. Sex changes hormonal regulation of CYPs and, in the male rat, the α2u-globulin load taught in section 8.3. Background disease—fatty liver, hemochromatosis increasing catalytic iron—completes the II.3 attribute list. A polymorphism is a hypothesis about susceptibility, not a default uncertainty factor.

Worked ROS toxicants: paraquat, carbon tetrachloride, doxorubicin

Paraquat (1,1′-dimethyl-4,4′-bipyridinium) accepts an electron from NADPH-dependent reductases to a radical cation, then donates that electron to oxygen, regenerating the parent dication and superoxide—futile redox cycling. Alveolar epithelium concentrates paraquat via the polyamine uptake system (shared with putrescine and spermidine). NADPH and GSH collapse locally. The clinical picture is delayed, progressive pulmonary fibrosis. The mechanism is not CYP2E1 bioactivation and is not a liver-zonation story.

Carbon tetrachloride is the opposite architecture. Hepatic CYP2E1 in zone 3 reductively dehalogenates CCl4 to the trichloromethyl radical (•CCl3). Low centrilobular oxygen tension favors that reductive path. •CCl3 adds oxygen to CCl3OO• and starts lipid peroxidation and centrilobular necrosis. Ethanol induction of CYP2E1 potentiates CCl4—a mixture term you will name formally in section 8.3. Vitamin E status and GSH tone modify severity; they do not rewrite the initiating enzyme.

Doxorubicin has a quinone that undergoes one-electron reduction (NADPH-cytochrome P450 reductase; mitochondrial NADH dehydrogenase) to a semiquinone that redox-cycles to superoxide. Iron catalyzes hydroxyl-radical formation. Cardiomyocytes are mitochondrion-rich and catalase-poor, which is why the heart is the ROS-target organ in this story. A second, on-target mechanism—poisoning of topoisomerase IIβ (TOP2B) in cardiomyocytes—also explains delayed dilated cardiomyopathy. On an item, say which data would distinguish redox cycling (iron chelation, SOD mimics, lipid peroxidation) from TOP2B (enzyme engagement and DNA-break markers that do not require catalytic iron). Cumulative, delayed cardiomyopathy is the clinical package; do not collapse it to “the drug is a general ROS generator in every organ.”

Scenario

A solvent study shows centrilobular necrosis, increased F2-isoprostanes, early GSH depletion, and a transcriptomic Nrf2 signature in wild-type mice, with worse necrosis after ethanol pretreatment and protection by a CYP2E1 inhibitor. That package is CYP2E1-dependent radical lipid peroxidation. A paraquat study with polyamine-transporter-dependent alveolar uptake, NADPH collapse, and fibrosis without a CYP2E1 requirement is redox cycling in lung. Calling both “ROS toxicity” erases the mechanism Domain II is testing. A third cohort’s RNA-seq heat map lights Nrf2 genes at a dose with normal histopathology and normal liver function: that is an adaptive transcript hypothesis, not an organism-level adverse outcome (II.6 E).

Traps

• Treating hydrogen peroxide as a free radical.
• Forgetting that hydroxyl radical hits where iron or copper sits, not at a distant receptor.
• Using a lone 8-oxo-dG value or a heat map as an adverse outcome.
• Equating GSTM1-null with universal chemical sensitivity.
• Assigning paraquat to CYP2E1, or carbon tetrachloride to the polyamine transporter.
• Treating every doxorubicin injury as redox cycling while ignoring TOP2B, or the reverse.