Effect of flavonoids and related compounds on soybean lipoxygenase-1 activity

Lipoxygenase (EC 1.13.11.12) catalyzes the oxidation of unsaturated fatty acids containing a cis,cis-1,4-pentadiene system, and is believed to be a key enzyme in quality changes in vegetables. The objective of this study was to determine the effectiveness of flavonoids and related antioxidants in the prevention of lipoxygenase-dependent lipid oxidation.Antioxidants (BHA, BHT, TBHQ, $\alpha$-tocopherol and propyl gallate) were evaluated in a model system using soybean lipoxygenase-1 with linoleic acid as substrate. Differing effects of the antioxidants on pigment bleaching, carbonyl production and linoleate oxidation were noted in anaerobic lipoxygenase reactions. Pigment bleaching was inhibited by all of the antioxidants, with a greater effect seen on chlorophyll bleaching than on carotene bleaching. TBHQ was the most effective inhibitor, completely halting bleaching of either pigment. TBHQ and propyl gallate completely inhibited the formation of carbonyl compounds, while BHA, BHT and $\alpha$-tocopherol showed little effect. Based on the effects of the antioxidants, three possible sites of antioxidant interference in the free radical reaction were proposed. Modification of soybean lipoxygenase-1 with methylmercuric iodide or N-bromosuccinimide altered enzyme activity and antioxidant effectiveness. Based on these findings, it appears that sulfhydryl groups and tryptophan residues are essential components in the active site of lipoxygenase.Flavonoids, polyphenolic compounds that occur naturally in plants, were evaluated for antioxidant effectiveness in the lipoxygenase-linoleate model system and compared to phenolic antioxidants BHA and BHT. Flavonols (quercetin and kaempferol), flavonol glycosides (rutin and quercitrin) a flavone (apigenin), and cinnamic acids (caffeic and chlorogenic acids) were evaluated. Kaempferol, BHA and BHT inhibited lipoxygenase the most, followed by chlorogenic acid and quercetin. When plant pigments were added in conjunction with flavonoids, additional inhibition was observed. The synergistic effect of $\beta$-carotene was greatest with the flavonol glycosides, while chlorophyll a showed the highest amount of additional inhibition with the phenolics BHT and BHA. Enhancement of antioxidant activity appeared to be related to the structure of the primary antioxidant.Plant extracts containing flavonoids were effective inhibitors of lipoxygenase activity. Ethanolic extracts of over twenty food plants were evaluated. White potato, "L-1 null" soybean, red onion, green pepper and tomato showed the greatest inhibitory effect on lipoxygenase. TLC and HPLC techniques were used to identify the major phenolic constituent of white potato extract as chlorogenic acid. Soybean extract contained isoflavones, while red onion, green pepper and tomato extracts contained both flavonoids and cinnamic acids.U of I OnlyETDs are only available to UIUC Users without author permissio