No abstract

The effect of water- and steam-cooking on the content of vitamin C, polyphenols, carotenoids, tocopherols and glucosinolates, as well as on the antioxidant activity of broccoli, are reported. Flavonoids, phenolic acids, vitamins C and E, beta-carotene, lutein, and glucosinolates in domestically processed broccoli were quantified using high-performance liquid chromatography (HPLC) methods; total polyphenols were determined with Folin-Ciocalteu reagent. The antioxidant capacities of broccoli extracts were evaluated using the Trolox equivalent antioxidant capacity (TEAC) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) methods. The results indicated that steam-cooking of broccoli results in an increase in polyphenols, as well as the main glucosinolates and their total content as compared with fresh broccoli, whereas cooking in water has the opposite effect. Steam-cooking of broccoli has no influence on vitamin C, whereas cooking in water significantly lowers its content. Both, water- and steam-cooking of broccoli results in an increase in beta-carotene, lutein, and alpha- and gamma-tocopherols as compared with fresh broccoli. Similar effects of steaming and water-cooking of broccoli on their antioxidant activity were observed.

Polyphenolic antioxidants exhibited a dose-dependent toxicity against human promyelocytic leukemia cells (HL-60). Their action was accompanied by malondialdehyde formation, and was partly prevented by desferrioxamine and the antioxidant N,NP P-diphenyl-p-phenylene diamine. This points to a prooxidant character of their cytotoxicity. A quantitative structure-activity relationship (QSAR) has been obtained to describe the cytotoxicity of 13 polyphenolic antioxidants belonging to three different groups (flavonoids, derivatives of gallic and caffeic acid): log cL 50 (W WM) = (2.7829 þ 0.2339)+(1.2734 þ 0.4715) E pa2 (V)3 3(0.3438 þ 0.0582) log P (r 2 = 0.8129), where cL 50 represents the concentration for 50% cell survival, E pa2 represents the voltammetric midpoint potential, and P represents the octanol/ water partition coefficient. Analogous QSARs were obtained using enthalpies of single-electron oxidation of these compounds, obtained by quantum-mechanical calculations. These findings clearly point to two important characteristics determining polyphenol cytotoxicity, namely their ease of oxidation and their lipophilicity.z 1999 Federation of European Biochemical Societies.

The chemical reactivity, isomerization, and glutathione conjugation of quercetin o-quinone were investigated. Tyrosinase was used to generate the unstable quercetin o-quinone derivative which could be observed upon its subsequent scavenging by glutathione. Identification of the products revealed formation of 6-glutathionyl-quercetin and 8-glutathionyl-quercetin adducts. Thus, in particular, glutathione adducts in the A ring of quercetin were formed, a result which was not expected a priori. Quantum mechanical calculations support the possibility that the formation of these glutathione adducts can be explained by an isomerization of quercetin o-quinone to p-quinone methides. Surprisingly, additional experiments of this study reveal the adduct formation to be reversible, leading to interconversion between the two quercetin glutathione adducts and possibilities for release and further electrophilic reactions of the quercetin quinone methide at cellular sites different from those of its generation.