Abstract
Humic acid, and the acid-extracted residue obtained from it, stimulated the production of superoxide radicals (O2 .−) generated in the xanthine-xanthine oxidase system. Several synthetic humic acids, prepared by the oxidation of simple phenolic substances, also stimulated the production of O2 .− but the degree of stimulation depended on the initial phenol. Fulvic acid and water-extractable soil organic matter were less effective in stimulating O2 .− production than was humic acid. The activity of superoxide dismutase, an enzyme which destroys O2 .−, was also enhanced by HA. In contrast, fulvic acid and water-extractable soil organic matter had little effect on the activity of the dismutase.
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