Mycophenolic acid (MPA), the active metabolite of the immunosuppressant mycophenolate mofetil (MMF), is primarily metabolized by glucuronidation to a phenolic glucuronide (MPAG) and an acyl glucuronide (AcMPAG). It is known that AcMPAG, which may be an immunotoxic metabolite, is deglucuronidated in human liver. AcMPAG deglucuronidation activity was detected in both human liver cytosol (HLC) and microsomes (HLM). By purification from HLC with column chromatographic purification steps, the enzyme responsible for AcMPAG deglucuronidationis identified as α/β hydrolase domain containing 1 (ABHD1). Recombinant ABHD1 expressed in Sf9 cells efficiently deglucuronidated AcMPAG with a K(m) value of 1.7 ± 1.2 μM, which was similar to those in HLM, HLC, and human liver homogenates (HLH). Immunoblot analysis revealed ABHD1 protein expression in both HLC and HLM. The AcMPAG deglucuronidation by recombinant ABHD1, HLC, and HLH were potently inhibited by AgNO(3), CdCl(2), CuCl(2), PMSF, bis-p-nitrophenylphosphate, and DTNB. The CL(int) value of AcMPAG formation from MPA, which was catalyzed by human UGT2B7, in HLH was increased by 1.8-fold in the presence of PMSF. Thus, human ABHD1 would affect the formation of AcMPAG, the immunotoxic metabolite.