Metabolism of Parabens (4-Hydroxybenzoic Acid Esters) by Hepatic Esterases and UDP-Glucuronosyltransferases in Man

Abstract

  Parabens (alkyl esters of 4-hydroxybenzoic acid) are widely used as preservatives in drugs, cosmetic products, and foodstuffs. Safety concern has recently increased due to the potential health risk associated to exposure to large amounts of these substances. Biotransformation of parabens mainly includes hydrolysis of the ester bond and glucuronidation reactions. The hydrolysis and glucuronidation of a series of six parabens differing by the nature of the alkyl group were investigated in human liver microsomes and plasma, and the major human UDP-glucuronosyltransferase (UGT) isoforms involved in the reaction were identified. Methyl- and ethylparaben were stable in human plasma, with 95% of the initial concentration found after 24 h. On the other hand, propyl-, butyl- and benzylparaben concentrations decreased by 50% under similar conditions. By contrast, a rapid hydrolysis was measured with human liver microsomes depending of the alkyl chain length, t_1/2 varying from 22 min for methylparaben to 87 min for butylparaben. All parabens were actively glucuronidated by liver microsomes, when compared to 4-hydroxybenzoic acid. They were mainly substrates of the human recombinant UGT1A1, UGT1A8, UGT1A9, UGT2B7, UGT2B15 and UGT2B17. In conclusion, the parabens were readily metabolized in human liver through esterase hydrolysis and glucuronidation by several UGT isoforms. This result suggests that these parabens do not accumulate in human tissue.