Transesterification of p-hydroxybenzoate esters (parabens) by human intestinal (Caco-2) cells

p-Hydroxybenzoate ester (paraben) preservatives are used in numerous orally administered products. The recognized route of metabolism for parabens is hydrolysis to p-hydroxybenzoic acid followed by conjugation and excretion. However, in the presence of alcohols, a presystemic transesterification pathway not previously reported for the human intestine can occur. Using human intestinal (Caco-2) cells, it was observed that hydrolysis of parabens to p-hydroxybenzoic acid is reduced markedly by ethanol concentrations that can occur in the human intestine, 0.25-0.5% (v/v). Ethanol concentrations of 1.0-2.5% (v/v) were optimal for trans esterification to ethylparaben in Caco-2 cell homogenates. The kinetics of the transesterification reaction with regard to ethanol concentration (0-20%), time, pH (3-9), protein concentration (1-5 mg ml(-1)) and substrate concentration (6.25-200 mu M) as well as the effects of different alcohols were studied. The K-m and V-max values for transesterification with ethanol for methyl, propyl, butyl, heptyl and octyl parabens were 449.7, 165.7, 86.1, 24.2 and 45.9 mu M and 114.4, 37.5, 19.5, 7.5 and 7.6 mu mol h(-1) mg(-1) Caco-2 cell protein, respectively. The V-max values for transesterification of methylparaben with ethanol, propan-1-ol, butan-1-ol were 114.4, 5.1 and 4.9 mu mol h(-1) mg(-1), respectively. Collectively, the kinetic data demonstrate that the enzyme responsible for the transesterification reaction has a preference for short-chain esters and represents the first report of transesterification in human intestinal cells. An implication of this mechanism is that alcohol-containing in vitro biosystems or protocols for the study of parabens disposition could generate transesterified artefacts. The clinical or toxicological implication is that, following co-ingestion of ester compounds with ethanol, transesterification could provide the basis for a previously unrecognized drug-alcohol interaction.