Chloroplasts isolated from leaves of spinach‐beet ( Beta vulgaris L. ssp. vulgaris ) do not catalyse the hydroxylation of p ‐coumaric acid in the dark unless a reductant (such as ascorbate, NADH or NADPH) is added. Superoxide dismutase has no effect on this reaction. Illuminated chloroplasts catalyse the hydroxylation in the absence of added reductant. This reaction is completely inhibited by superoxide dismutase, but catalase has little effect. Both hydroxylation in the light and hydroxylation in the dark in the presence of reductants are inhibited by diethyldithiocarbamate, EDTA, cyanide and 2‐mercaptoethanol. It is proposed that O 2 .− generated by illuminated chloroplasts is involved in the provision of a reductant to the enzyme phenolase.
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