MICROSOMAL EPOXIDE HYDROLASE OF RAT-LIVER IS A SUBUNIT OF THE ANTI-ESTROGEN-BINDING SITE

A tritiated photoaffinity labelling analogue of tamoxifen, [(2-azido-4 -benzyl)-phenoxy]-N-ethylmorpholine (azido-MBPE), was used to identify the anti-oestrogen-binding site (AEBS) in rat liver tissue [Poirot, C hailleux, Fargin, Bayard and Faye (1990) J, Biol, Chem, 265, 17039-170 43]. UV irradiation of rat liver microsomal proteins incubated with tr itiated azido-MBPE led to the characterization of two photolabelled pr oteins of molecular masses 40 and 50 kDa. The amino acid sequences of proteolytic products from the 50 kDa protein were identical with those from rat microsomal epoxide hydrolase (mEH). Treatment of hepatocytes with anti-sense mRNA directed against mEH abolished AEBS in these cel ls. In addition we found that tamoxifen and N-morpholino-2-[4-(phenylm ethyl)phenoxy] ethanamine, a selective ligand of AEBS, were potent inh ibitors of the catalytic hydration of styrene oxide by mEH. However, f unctional overexpression of the human mEH did not significantly modify the binding capacity of [H-3]tamoxifen. Taken together, these results suggest that the 50 kDa protein, mEH, is necessary but not sufficient to reconstitute AEBS.