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

Abstract

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 transesterification 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 µM) 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 µM and 114.4, 37.5, 19.5, 7.5 and 7.6 µmol 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 µmol 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.

• Transesterification
• parabens
• Caco-2 cells
• ethanol
• presystemic metabolism
• drug-alcohol interaction