Influence of biotransformation of luteolin, luteolin 7-O-glucoside, 3',4'-dihydroxyflavone and apigenin by cultured rat hepatocytes on antioxidative capacity and inhibition of EGF receptor tyrosine kinase activity.

Flavonoids are known as biologically active compounds. Although this has been shown by several in vivo studies, it is still elusive whether their metabolites exert similar activities. Herein we investigated the biotransformation of four different flavonoids, 3',4'-dihydroxyflavone, apigenin, luteolin and luteolin 7-O-glucoside, by cultured rat hepatocytes using a combination of enzymatic deconjugation, HPLC separation and high-resolution mass spectrometry. These flavonoids were chosen because they are active components of many plants, e. g., artichokes. All flavonoids showed rather complex metabolite patterns dominated by phase II metabolites, mainly sulfates, methyl sulfates and methyl glucuronides, but also of combined glucuronide and sulfate conjugates. Phase I metabolism by hydroxylation was rendered likely only for apigenin to form luteolin. When culture media containing the flavonoids and their metabolites were assayed for antioxidative capacity by the DPPH assay, only compounds with hydroxy groups in position 3' and 4' of the B ring were active. Thus, during metabolism of (inactive) apigenin a strong increase in the antioxidative effect was observed while that of the other three flavonoids decreased with time. Determination of EGF receptor tyrosine kinase activity likewise revealed strong inhibition in the presence of a catechol group at ring B. However, in this case the situation was much more complex resulting in a significant increase of the inhibitory activity of 3',4'-dihydroxyflavone and apigenin, but not of luteolin and luteolin 7-O-glucoside during 22 h of incubation. These results show that the biotransformation of flavonoids is very complex and may result not only in a loss but also in a gain of biological activity depending on the individual structural features.

Studies on the interaction of the antibiotic moenomycin A with the enzyme penicillin-binding protein 1b.

The interaction of a moenomycin derivative with the enzyme penicillin binding protein 1b (PBP 1b) has been studied by means of STD NMR. The results obtained initiated the synthesis of a number of moenomycin derivatives modified in unit A including a moenomycin-ampicillin conjugate and determination of their antibiotic activities. A protocol is described that allows studying the interaction of moenomycin analogues with PBP 1b by fluorescence correlation spectroscopy.