Phase I - Reduction Reactions
Reduction reactions as a means for metabolizing xenobiotics are fairly rare in plants. Also, these reactions are much less common Phase I reactions than the oxidation reactions. The most common reduction reaction in plants is aryl nitroreduction (Figure 8) where a nitro group on a phenyl ring is reduced to an amino group. These reactions are catalyzed by aryl nitroreductases which require a reductant such as NADPH.
Other reduction reactions include deamination (Figure 9) where an amino group is removed or photochemical reduction of paraquat (Figure 10). Photochemical reduction of paraquat is nonenzymatic and is actually a form of bioactivation; an electron from the electron transport system in the chloroplast during the light reactions of photosynthesis reduces paraquat, creating a free-radical. This form of paraquat is very destructive to membranes because it uses its extra electron to reduce O2, creating superoxide free radical, O2-.. The free radical then causes lipid peroxidation (view an animation showing lipid peroxidation). This process is autocatalytic because once the reduced paraquat is oxidized or gives up its extra electron, it is again free to be reduced. Please refer to the Herbicides that Act Through Photosynthesis Lesson to see this in greater detail.
Reduction reactions add electrons and can either create a molecule with reduced toxicity or one with more activity.