Antioxidant capacity: the pathway in the body

This page shows the biochemical pathway behind the laboratory value Antioxidant capacity (ImAnOx, TAS): which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

Antioxidant capacity describes how much peroxide and how many radicals a blood sample can intercept in total. It is the sum of many dissolved scavengers such as uric acid, albumin and vitamin C, which donate electrons to oxidants and so neutralise them.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

1. Oxygen (O₂) → Superoxide respiratory chain, NOX Superoxide forms at complexes I and III of the respiratory chain and through NADPH oxidases (NOX) in immune and other cells. It is very short-lived. Source 3, 2↕ both, depending on amount Superoxide is used by immune cells to kill germs and is also the starting material for hydrogen peroxide; in large amounts it alters iron-sulphur centres of enzymes. established physiology Source 2, 4
   ⚖ When the balance tips

   too much — If superoxide builds up, it releases iron from enzymes; free iron promotes the formation of the highly reactive hydroxyl radical from hydrogen peroxide.

   too little — If little superoxide forms, immune cells lack a tool against germs, and less hydrogen peroxide is available as a signal.

   established physiology · Source 4

2. Superoxide → Hydrogen peroxide SOD · copper, zinc, manganese Superoxide dismutases (SOD) rapidly convert superoxide into hydrogen peroxide. It is more stable and crosses membranes, including from the cell into the blood. Source 4, 2↕ both, depending on amount In small amounts hydrogen peroxide is a signalling molecule that switches enzymes on and off; in larger amounts it oxidises proteins, fats and DNA. established physiology Source 2
   ⚖ When the balance tips

   too much — If hydrogen peroxide builds up, it oxidises sulphur groups in proteins beyond the signalling level; with free iron, hydroxyl radicals form.

   too little — If there is very little hydrogen peroxide, signals are missing by which cells respond, for example, to growth factors.

   established physiology · Source 2, 1

3. Peroxides in serum → Neutralised products electron donation The scavengers donate electrons to peroxides and radicals. What remains is water or stable molecules and the oxidised form of the scavenger. Source 1, 9↑ supplies Some oxidised scavengers are recovered: vitamin C restores vitamin E and is itself renewed in cells; others such as uric acid are broken down. established physiology Source 9
   ⚖ When the balance tips

   too much — If many oxidised scavengers accumulate, recovery in the cells cannot keep up and the reduced forms in the blood decrease.

   too little — If few accumulate, little is consumed and the stock of scavengers remains stable.

   established physiology · Source 9, 1

4. Peroxides in serum → Residual peroxide Whatever the sample does not break down remains as peroxide and is measured by a colour reaction. The less remains, the greater the sample's capacity. Source 5↓ depletes Peroxide that is not intercepted oxidises sulphur groups of proteins and fats in membranes; in the test, the remainder shows the amount the sample could not handle. established physiology Source 5, 1
   ⚖ When the balance tips

   too much — If a lot of peroxide remains, the sample intercepted little; the total does not show which scavenger is missing, and its meaning for individual tissues is disputed.

   too little — If little remains, capacity was large; it may come largely from uric acid, so a high total on its own says little about vitamins.

   contested · Source 5, 6

Further stations

• Oxygen (O₂) — respiration and immune cells Almost all oxygen is converted completely to water in the mitochondria. A small part picks up only one electron on the way; immune cells also form such species deliberately. Source 3, 1↑ supplies Oxygen is the final acceptor of the respiratory chain; that single electrons escape in the process is part of normal metabolism. established physiology Source 3
  ⚖ When the balance tips

  too much — If the respiratory chain is heavily loaded with electrons, for example with plenty of fuel and little demand for ATP, more superoxide escapes.

  too little — If the respiratory chain runs briskly, fewer electrons escape and less superoxide forms.

  established physiology · Source 3

• Peroxides in serum — from cells or in the test In the blood, peroxides and radicals meet a network of dissolved scavengers. In the ImAnOx test a fixed amount of hydrogen peroxide is added to the serum sample. Source 5, 1↓ depletes Peroxides and radicals in the blood remove electrons from other molecules; whatever they hit is oxidised unless a scavenger intercepts them first. established physiology Source 1
  ⚖ When the balance tips

  too much — If many peroxides reach the blood, the dissolved scavengers are used up and must be replenished from cells, liver and food.

  too little — If few reach the blood, the scavengers largely remain in their reduced, ready form.

  established physiology · Source 1

• Uric acid — breakdown product of purines Uric acid forms during the breakdown of purines, the building blocks of DNA. It is dissolved in the blood in comparatively large amounts. Source 8↕ both, depending on amount In blood plasma uric acid scavenges radicals and oxidants and makes the largest single contribution to measured capacity; inside cells, by contrast, it can itself act as an oxidant. observed in studies Source 8, 6
  ⚖ When the balance tips

  too much — If there is a lot of uric acid in the blood, measured capacity rises; in cell culture it also promotes the formation of oxidants inside cells.

  too little — If there is little uric acid in the blood, the largest single contributor is missing and measured capacity is lower.

  observed in studies · Source 8

• Albumin — free thiol group Albumin is the most abundant protein in the blood. Its free sulphur group (Cys34) and the metals it binds make it a scavenger of oxidants. Source 7↓ depletes Albumin intercepts peroxides via its thiol group and binds copper and iron, which would otherwise form radicals; in this way it deprives oxidants of their effect. established physiology Source 7
  ⚖ When the balance tips

  too much — If a lot of albumin is present, more thiol groups are available and more metal ions are bound.

  too little — If little albumin is present or its thiol group is already oxidised, the plasma intercepts less peroxide.

  established physiology · Source 7

• Vitamin C — ascorbic acid Vitamin C is dissolved in the watery part of the blood. It donates electrons readily and becomes dehydroascorbic acid in the process. Source 9↓ depletes Vitamin C scavenges radicals in the watery environment and returns oxidised vitamin E in the lipoproteins to its active form. established physiology Source 9
  ⚖ When the balance tips

  too much — If a lot of vitamin C is present, it can itself form radicals together with free iron; in the blood, however, iron is tightly bound.

  too little — If little vitamin C is available, vitamin E is recovered more slowly and the watery plasma scavenges fewer radicals.

  established physiology · Source 9

• Vitamin E — in lipoproteins Vitamin E sits, fat-soluble, in lipoproteins and membranes. There it interrupts chain reactions in which fats are oxidised. Source 9↓ depletes Vitamin E donates an electron to a lipid radical and so stops the chain; the resulting vitamin E radical is unreactive and is recovered by vitamin C. established physiology Source 9
  ⚖ When the balance tips

  too much — If there is a lot of vitamin E in the lipoproteins, chain reactions are interrupted earlier; it contributes little to the capacity of watery serum.

  too little — If little vitamin E is available, chain reactions in the fats of the lipoproteins run longer before they stop.

  established physiology · Source 9

Cofactors in this pathway

• Vitamin C — Donates electrons to radicals and recovers oxidised vitamin E Source 9In the ORY catalogue as a laboratory value: Vitamin C (Ascorbinsäure)
• Vitamin E — Interrupts the chain reaction of lipid oxidation in lipoproteins and membranes Source 9In the ORY catalogue as a laboratory value: Vitamin E (Tocopherol)
• Zinc and copper — Metal centres of the SOD in the cytoplasm and outside the cell that converts superoxide Source 4In the ORY catalogue as a laboratory value: Zink
• Manganese — Metal centre of the SOD in the mitochondrion Source 4
• Uric acid — Largest single contributor to the measured capacity of plasma Source 6, 8
• Albumin — Intercepts peroxides with its thiol group and binds metal ions Source 7

Sources

1. Sies H. Oxidative stress: a concept in redox biology and medicine. Redox Biol 2015 · PubMed 25588755
2. Sies H, Jones DP. Reactive oxygen species (ROS) as pleiotropic physiological signalling agents. Nat Rev Mol Cell Biol 2020 · PubMed 32231263
3. Murphy MP. How mitochondria produce reactive oxygen species. Biochem J 2009 · PubMed 19061483
4. Wang Y, Branicky R, Noë A, Hekimi S. Superoxide dismutases: Dual roles in controlling ROS damage and regulating ROS signaling. J Cell Biol 2018 · PubMed 29669742
5. Sies H. Total antioxidant capacity: appraisal of a concept. J Nutr 2007 · PubMed 17513413
6. Benzie IF, Strain JJ. The ferric reducing ability of plasma (FRAP) as a measure of "antioxidant power": the FRAP assay. Anal Biochem 1996 · PubMed 8660627
7. Roche M, Rondeau P, Singh NR et al. The antioxidant properties of serum albumin. FEBS Lett 2008 · PubMed 18474236
8. Sautin YY, Johnson RJ. Uric acid: the oxidant-antioxidant paradox. Nucleosides Nucleotides Nucleic Acids 2008 · PubMed 18600514
9. Sies H, Stahl W. Vitamins E and C, beta-carotene, and other carotenoids as antioxidants. Am J Clin Nutr 1995 · PubMed 7495226

Related pathways

• 8-OHdG — superoxide
• Glutathione — hydrogen peroxide
• Selenium — hydrogen peroxide
• Iron and ferritin — Vitamin C (Ascorbinsäure), Zink
• Copper — Vitamin C (Ascorbinsäure), Zink

As of 2026-10-05. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
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