PHARMACOLOGICAL ANALYSIS OF STEROL DELTA-8-DELTA-7 ISOMERASE PROTEINSWITH [H-3]IFENPRODIL

Sterol Delta 8-Delta 7 isomerases (Sls) catalyze the shift of the doub le band from C8-9 to C7-8 in the B-ring of sterols. Surprisingly, the isoenzymes in fungi (ERG2p) and vertebrates [emopamil binding protein (EBP)] are structurally completely unrelated, whereas the sigma(1) rec eptor, a mammalian protein of unknown function, bears significant simi larity with the yeast ERG2p. Here, we compare the drug binding propert ies of Sis and related proteins with [H-3]ifenprodil as a common high affinity radioligand (K-d = 1.4-19 nM), demonstrating an intimate phar macological relationship among ERG2p, sigma(1) receptor, and EBP. This renders Sis a remarkable example for structurally diverse enzymes wit h similar pharmacological profiles and the propensity to bind drugs fr om different chemical groups with high affinity. We identified a varie ty of experimental drugs with nanomolar affinity for the human EBP (K- i = 0.5-14 nM) such as MDL28315, AY9944, triparanol, and U18666A. Thes e compounds, as well as the fungicide tridemorph and the clinically us ed drugs tamoxifen, clomiphene, amiodarone, and opipramol, inhibit the in vitro activity of the recombinant human EBP (IC50 = 0.015-54 mu M) . The high affinity of the human EBP for H-3-tamoxifen (K-d = 3 +/- 2 nM) implies that the EBP carries the previously described microsomal a ntiestrogen binding site. Interactions of the EBP with structurally di verse lipophilic amines suggest that novel compounds of related struct ure should be counterscreened for inhibition of the enzyme to avoid in terference with sterol Delta 8-Delta 7 isomerization.