Pharmacological analysis of sterol Δ8=Δ7 isomerase proteins with [³H]fenprodil

Sterol Δ8-Δ7 isomerases (SIs) catalyze the shift of the double bondfrom C_8-9 to C_7-8 in the B-ring of sterols. Surprisingly, the isoenzymes in fungi (ERG2p) and vertebrates [emopamil binding protein (EBP)] are structurally completely unrelated, whereas the σ₁ receptor, a mammalian protein of unknown function, bears significantly similarity with the yeast ERG2p. Here, we compare the drug binding properties of SIs and related proteins with [³H]ifenprodil as a common high affinity radioligand (K(d) = 1.4-19 nM), demonstrating an intimate pharmacological relationship among ERG2p, σ₁ receptor, and EBP. This renders SIs a remarkable example for structurally diverse enzymes with similar pharmacological profiles and the propensity to bind drugs from different chemical groups with high affinity. We identified a variety of experimental drugs with nanomolar affinity for the human EBP (K(i) = 0.5-14 nM) such as MDL28815, AY9944, triparanol, and U18666A. These compounds, as well as the fungicide tridemorph and the clinically used drugs tamoxifen, clomiphene, amiodarone, and opipramol, inhibit the in vitro activity of the recombinant human EBP (IC₅₀ = 0.015- 54 μM). The high affinity of the human EBP for ³H-tamoxifen (K(d) = 3 ± 2 nM) implies that the EBP carries the previously described microsomal antiestrogen binding site. Interactions of the EBP with structurally diverse lipophilic amines suggest that novel compounds of related structure should be counterscreened for inhibition of the enzyme to avoid interference with sterol Δ8-Δ7 isomerization.