Hydrolysis of di-n-butyl phthalate, butylbenzyl phthalate and di(2-ethylhexyl) phthalate in human liver microsomes

Diester phthalates are industrial chemicals used primarily as plasticizers to import flexibility to polyvinylchloride plastics. In this study, we examined the hydrolysis of di-n-butyl phthalate (DBP), butylbenzyl phthalate (BBzP) and di(2-ethylhexyl) phthalate (DEHP) in human liver microsomes. These diester phthalates were hydrolyzed to monoester phthalates (mono-n-butyl phthalate (MBP) from DBP, mono-n-butyl phthalate (MBP) and monobenzyl phthalate (MBzP) from BBzP, and mono(2-ethylhexyl) phthalate (MEHP)) by human liver microsomes. DBP, BBzP and DEHP hydrolysis showed sigmoidal kinetics in V–[S] plots, and the Hill coefficient (n) ranged 1.37–1.96. The S50, Vmax and CLmax values for DBP hydrolysis to MBP were 99.7 μM, 17.2 nmol min−1 mg−1 protein and 85.6 μL min−1 mg−1 protein, respectively. In BBzP hydrolysis, the values of S50 (71.7 μM), Vmax (13.0 nmol min−1 mg−1 protein) and CLmax (91.3 μL min−1 mg−1 protein) for MBzP formation were comparable to those of DBP hydrolysis. Although the S50 value for MBP formation was comparable to that of MBzP formation, the Vmax and CLmax values were markedly lower (<3%) than those for MBzP formation. The S50, Vmax and CLmax values for DEHP hydrolysis were 8.40 μM, 0.43 nmol min−1 mg−1 protein and 27.5 μL min−1 mg−1 protein, respectively. The S50 value was about 10% of DBP and BBzP hydrolysis, and the Vmax value was also markedly lower (<3%) than those for DBP hydrolysis and MBzP formation for BBzP hydrolysis. The ranking order of CLmax values for monoester phthalate formation in DBP, BBzP and DEHP hydrolysis was BBzP to MBzP ⩾ DBP to MBP > DEHP to MEHP > BBzP to MBP. These findings suggest that the hydrolysis activities of diester phthalates by human liver microsomes depend on the chemical structure, and that the metabolism profile may relate to diester phthalate toxicities, such as hormone disruption and reproductive effects.

► Diester phthalates were hydrolyzed to monoester phthalates in human liver.
► In vitro hydrolysis of diester phthalate showed fitted sigmoidal kinetics.
► Diester phthalate hydrolysis activities may depend on the chemical structure.