Alternariol (AOH) is a mycotoxin produced by Alternaria species. In vitro studies suggest the genotoxic, mutagenic, and endocrine disruptor effects of AOH, and an increased incidence of esophageal cancer has been reported related to higher AOH exposure. Human serum albumin (HSA) is the most abundant plasma protein in the circulation, it is able to affect toxicokinetic properties of numerous xenobiotics. HSA forms stable complexes with several mycotoxins, however, the interaction of AOH with albumin has not been examined. In this study, the complex formation of AOH with HSA was tested, employing fluorescence spectroscopy, ultrafiltration, and molecular modeling. Each spectroscopic measurement shows the formation of stable AOH-HSA complexes (K = 4 × 105 L/mol). Investigations with site markers (in spectroscopic and ultrafiltration models) as well as modeling studies suggest that AOH occupies Sudlow’s site I as a high-affinity binding site in HSA. The binding affinity of AOH towards bovine, porcine, and rat albumins was also tested, suggesting that AOH binds to rat albumin with considerably higher affinity than other albumins tested. Our results demonstrate the strong interaction of AOH with serum albumins, suggesting the potential in vivo importance of these interactions.

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Alternariol is an Alternaria mycotoxin that appears in fruits, tomatoes, oilseeds, and corresponding products. Chronic exposure to it can induce carcinogenic and xenoestrogenic effects. Cyclodextrins (CDs) are ring-shaped molecules built up by glucose units, which form host–guest type complexes with some mycotoxins. Furthermore, insoluble CD polymers seem suitable for the extraction/removal of mycotoxins from aqueous solutions. In this study, the interactions of alternariol with β- and γ-CDs were tested by employing fluorescence spectroscopic and modeling studies. Moreover, the removal of alternariol from aqueous solutions by insoluble β-CD bead polymer (BBP) was examined. Our major observations/conclusions are the following: (1) CDs strongly increased the fluorescence of alternariol, the strongest enhancement was induced by the native γ-CD at pH 7.4. (2) Alternariol formed the most stable complexes with the native γ-CD (logK = 3.2) and the quaternary ammonium derivatives (logK = 3.4–3.6) at acidic/physiological pH and at pH 10.0, respectively. (3) BBP effectively removed alternariol from aqueous solution. (4) The alternariol-binding ability of β-CD polymers was significantly higher than was expected based on their β-CD content. (5) CD technology seems a promising tool to improve the fluorescence detection of alternariol and/or to develop new mycotoxin binders to decrease alternariol exposure.

Chrysin (5,7-dihydroxyflavone) is a flavonoid aglycone, which is found in nature and in several dietary supplements. During the biotransformation of chrysin, its conjugated metabolites chrysin-7-sulfate (C7S) and chrysin-7-glucuronide (C7G) are formed. Despite the fact that these conjugates appear in the circulation at much higher concentrations than chrysin, their interactions with serum albumin have not been reported. In this study, the complex formation of chrysin, C7S, and C7G with human (HSA) and bovine (BSA) serum albumins was investigated employing fluorescence spectroscopic, ultrafiltration, and modeling studies. Our major observations/conclusions are as follows: (1) Compared to chrysin, C7S binds with a threefold higher affinity to HSA, while C7G binds with a threefold lower affinity; (2) the albumin-binding of chrysin, C7S, and C7G did not show any large species differences regarding HSA and BSA; (3) tested flavonoids likely occupy Sudlow’s Site I in HSA; (4) C7S causes significant displacement of Sudlow’s Site I ligands, exerting an even stronger displacing ability than the parent compound chrysin. Considering the above-listed observations, the high intake of chrysin (e.g., through the consumption of dietary supplements with high chrysin contents) may interfere with the albumin-binding of several drugs, mainly due to the strong interaction of C7S with HSA.

Quercetin is an abundant flavonoid in nature and is used in several dietary supplements. Although quercetin is extensively metabolized by human enzymes and the colonic microflora, we have only few data regarding the pharmacokinetic interactions of its metabolites. Therefore, we investigated the interaction of human and microbial metabolites of quercetin with the xanthine oxidase enzyme. Inhibitory effects of five conjugates and 23 microbial metabolites were examined with 6-mercaptopurine and xanthine substrates (both at 5 μM), employing allopurinol as a positive control. Quercetin-3′-sulfate, isorhamnetin, tamarixetin, and pyrogallol proved to be strong inhibitors of xanthine oxidase. Sulfate and methyl conjugates were similarly strong inhibitors of both 6-mercaptopurine and xanthine oxidations (IC50 = 0.2–0.7 μM); however, pyrogallol inhibited xanthine oxidation (IC50 = 1.8 μM) with higher potency vs. 6-MP oxidation (IC50 = 10.1 μM). Sulfate and methyl conjugates were approximately ten-fold stronger inhibitors (IC50 = 0.2–0.6 μM) of 6-mercaptopurine oxidation than allopurinol (IC50 = 7.0 μM), and induced more potent inhibition compared to quercetin (IC50 = 1.4 μM). These observations highlight that some quercetin metabolites can exert similar or even a stronger inhibitory effect on xanthine oxidase than the parent compound, which may lead to the development of quercetin–drug interactions (e.g., with 6-mercaptopurin or azathioprine).