Human hepatic microsomal epoxide hydrolase. A structural comparison with the rat enzyme.

Purified human microsomal epoxide hydrolase has been compared to the rat enzyme by means of NHzterminal amino acid sequencing, amino acid composition, and peptide mapping procedures.Automated Edman degradation of human epoxide hydrolase identified methionine as the NHZ-terminal amino acid of the enzyme; in addition, the next 18 residues of the N H 2terminal region were also identified.The NHz-terminal segment of human epoxide hydrolase is homologous (68%) to the previously reported partial NHz-terminal sequence of the purified rat microsomal enzyme and, like the rat counterpart, is highly hydrophobic.Amino acid compositions were identical for the purified native enzyme, a chromatographically purified sample, and the reduced and alkylated enzyme.Comparison of the amino acid composition of the human enzyme with those of the rat and rabbit enzymes showed similar but not identical compositions.Peptide maps of the purified human and rat epoxide hydrolase were obtained by both chemical cleavage and enzymatic cleavage methods.Cleavage at acid-sensitive aspartyl-prolyl bonds produced identical peptide maps for both enzymes.Cleavage at methionine residues by cyanogen bromide treatment produced similar but not identical peptide maps for the human and rat enzymes.Peptide mapping by limited proteolysis in sodium dodecyl sulfate with Staphylococcus aureus V8 protease also showed very similar but not identical fragmentation patterns for both enzymes.Peptide mapping by limited proteolysis in sodium dodecyl sulfate with chymotrypsin produced different patterns for the human and rat epoxide hydrolase.Fragmentation patterns using aIl of the above peptide mapping procedures were identical for rat microsomal epoxide hydrolase purified from either phenobarbital-treated or isosafrole-treated rats.These studies provide evidence for structural similarities between the human and rat hepatic epoxide hydrolase and suggest that the human enzyme, like the rat enzyme, exists as a single enzyme in the endoplasmic reticulum of the cell.Epoxide hydrolase and the cytochrome P-450 containing monooxygenase system in the endoplasmic reticulum of mammals play crucial roles in the metabolism of many endogenous substrates and exogenous substances such as drugs, mutagens, carcinogens, and other toxic compounds (1-4).Epoxide hy-