Ring-opening hydrolases for simvastatin in plasma, liver and intestinal microsomes of rats

The characterization of the hydrolase responsible for the ring-opening hydrolysis of simvastatin (SV) to its open acid form (SVA) was carried out using rat plasma, liver and intestinal microsomes. The optimum pHs for the ring-opening hydrolase in plasma, liver and intestine were found to be between pH 7-8. The values of Vmax for plasma, liver and intestine were 0.33, 0.97 and 0.24 nmol/min/mg protein, respectively, and the corresponding values of Km were 21.9, 203.3 and 567.6μM, respectively. The value of Vmax/Km for plasma hydrolase showed the largest value of 0.0154ml/min/mg protein, which is 3.2- and 38-fold larger than those for the liver and intestine, respectively. The enzymatic hydrolysis of SV in the presence of plasma was inhibited by addition of diisopropyl fluorophosphate (DFP), bis (p-nitrophenyl) phosphate (BNPP), high concentration of physostigmine (Phys) and α-naphthyl acetate (α-NA). In liver, the enzymatic hydrolysis of SV was inhibited by DFP, BNPP, α-NA and EDTA. These results suggested that carboxylesterase might be, in part, responsible for the hydrolysis of SV in the plasma and liver. However, the difference in the inhibition patterns between plasma and liver indicated that different esterase isozymes might be responsible for the hydrolysis. On the other hand, the hydrolysis of SV by the intestine was not affected at all by the inhibitors and substrates mentioned above except EDTA, suggesting that the hydrolase in the intestine is different from those present in the plasma and liver.