= Antioxidant
An \b[antioxidant] is a substance acting to oppose <oxidation> or inhibit reactions driven by oxygen or peroxides in a specified system. ChEBI classifies <antioxidant> as a chemical role: the label describes what a substance does under relevant conditions, rather than one shared molecular structure.<Antioxidant in ChEBI>[CHEBI:22586, definition and chemical-role classification]{ref}
A <radical-trapping antioxidant> provides one possible mechanism. In a simplified reaction scheme, a donor $\mathrm{AH}$ intercepts a <radical (chemistry)> $\mathrm{R^{\bullet}}$:
$$
\mathrm{R^{\bullet}+AH\longrightarrow RH+A^{\bullet}}.
$$
This step is protective in the model only if the resulting species and subsequent reactions propagate less unwanted <oxidation> than the intercepted route. Removing one <radical (chemistry)> while producing another does not, by itself, prove <radical chain termination>. Other protective mechanisms need not follow this scheme.
An original <antioxidant reaction competition> model makes the importance of concentration explicit. Suppose a radical has just two consumption routes: interception by $\mathrm{AH}$ and attack on substrate $\mathrm S$. Their <reaction rates> are
$$
v_A=k_A c_A c_R,\qquad v_S=k_S c_S c_R.
$$
At fixed concentrations, the fraction of radical-consumption events assigned to interception is
$$
f_A=\frac{k_Ac_A}{k_Ac_A+k_Sc_S}.
$$
With $k_Ac_A=9\,\mathrm{s^{-1}}$ and $k_Sc_S=1\,\mathrm{s^{-1}}$, this fraction is $0.90$. Decreasing antioxidant concentration tenfold gives $f_A=0.9/1.9\simeq0.474$. The compound has not changed identity; its competition with the substrate has changed. Depletion, additional reactions and transfer between phases require a richer <chemical kinetics> model.
Experimental interpretation must specify the substrate, medium, time interval and measured outcome. Reducing an indicator is a chemical observation and need not establish protection of cells. A primary study found that <kaempferol> and <resveratrol> could produce <formazan> from <MTT> without cells.<Interference of plant extracts, phytoestrogens and antioxidants with the MTT tetrazolium assay by Regina Bruggisser and others>[Abstract, cell-free tests]{ref} Such a response motivates a <tetrazolium cell-free control>, rather than interpreting a stronger color as increased <cell viability>.
An antioxidant assay therefore measures a defined reaction or consequence under its own conditions. Extrapolation to a different reaction environment requires evidence about concentration, reaction partners and the outcome of interest.
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