Multiple epoxide hydrolases in human lung.

Benzo(a)pyrene 4,5-oxide (BPO) hydrolase activity and trans-stilbene oxide hydrolase activity were measured in human lung samples obtained from 12 different patients. These activities, attributable to different epoxide hydrolases, were both found in significant amounts in the cytosolic as well as the microsomal cell fractions. The catalytic properties of human lung BPO hydrolases were further investigated, and similarities between the cytosolic and microsomal forms of this enzyme were noted. Similar Km values were found for both forms (22 microM for the microsomal enzyme, 37 microM for the cytosolic enzyme). Both forms responded in similar fashion to a number of enzyme inhibitors in vitro, and the IC50 values for each inhibitor were the same for both forms. Both forms of the enzyme were inhibited uncompetitively by trichloropropene oxide, and the Kii value for the cytosolic hydrolase (11 microM) was similar to that obtained for the microsomal hydrolase (15 microM). These two forms of BPO hydrolase in the human lung are very similar, possibly identical, enzymes.