Uptake and metabolism of new synthetic lipophilic derivatives, hydroxytyrosyl ethers, by human hepatoma HepG2 cells.

As a response to the increasing demand by the food industry for new synthetic lipophilic antioxidants, hydroxytyrosyl methyl, ethyl, propyl and butyl ethers have been synthesized from hydroxytyrosol, with similar or even higher antioxidant activity than free hydroxytyrosol. The uptake and metabolism of hydroxytyrosyl ethers with different alkyl side chain lengths (methyl, ethyl, propyl, and butyl) was studied after incubation for 2 and 18 h with HepG2 cells as a model of the human liver. LC-DAD and LC-MS were used for the identification of metabolites in culture media, cell lysates and samples hydrolyzed with beta-glucuronidase and sulfatase. In vitro conjugation reactions of pure phenols were also performed. The results show an extensive uptake and metabolism by HepG2 cells after 18 h of incubation. A direct relationship between the lipophilic nature of the compound and the biotransformation yield was observed. Similar ratio of methyl and glucuronide forms were detected after 2 h of incubation while at 18 h high amounts of methylglucuronides and glucuronide metabolites were identified together with low amounts of methyl conjugates. In conclusion, alkyl hydroxytyrosyl ethers could be metabolized by the liver, their metabolic rate being higher for the more lipophilic compounds.