Species Differences in Liver Microsomal Hydrolysis of Acyl Glucuronide in Humans and Rats

1. Acyl glucuronides (AGs) are known as one of the causes of idiosyncratic drug toxicity (IDT). Although AGs can be enzymatically hydrolysed by beta-glucuronidase and esterase, much information on their characteristics and species differences is lacking. This study was aimed to clarify species differences in AG hydrolysis between human and rat liver microsomes (HLM and RLM). 2. To evaluate the AG hydrolysis profile, and the contribution of beta-glucuronidase and esterase towards AG hydrolysis in HLM and RLM, nonsteroidal anti-inflammatory drugs (NSAIDs) were used. AGs were incubated with 0.1 M Tris-HCl buffer (pH 7.4) and 0.3 mg/mL HLM or RLM in the absence or presence of P-glucuronidase inhibitor, D-saccharic acid 1,4-lactone (D-SL) and esterase inhibitor, phenylmethylsulfonyl fluoride (PMSF). 3. AGs of mefenamic acid (MEF-AG) and etodolac (ETO-AG) showed significantly higher AG hydrolysis rates in RLM than in HLM. Esterases were found to serve as AG hydrolases dominantly in HLM, whereas both esterases and beta-glucuronidase equally contribute to AG hydrolysis in RLM. However, MEF-AG and ETO-AG were hydrolysed only by beta-glucuronidase. 4. We demonstrated for the first time that the activity of AG hydrolases towards NSAID-AGs differs between humans and rats.