PROBING THE CYTOCHROME-P-450 2B1 ACTIVE-SITE WITH DIAMANTOID COMPOUNDS

Hydrocarbone diamantane has been shown to be a specific substrate with a high affinity for the binding site of PB-inducible cytochrome P-450 2B1 (Hodek et al. 1988). Using a difference spectroscopy approach, a battery of diamantane analogues and diamantane oxygen containing deriv atives were examined for their interaction with P-450 2B1 active site. Of the compounds (diamantane and its analogues, adamantane and triama ntane) tested, diamantane had the lowest value of a spectral dissociat ion constant K-s = 0.5 mu mol/l, indicating that diamantane was accomm odated well to the cytochrome P-450 2B1, hence values of 0.46 nm and 0 .66 nm for the width and length of the diamantane molecule, respective ly, were used to describe of the dimensions the cytochrome P-450 bindi ng site. Adamantane (K-s = 1.3 mu mol/l) is relatively small and thus it binds loosely whereas triamantane (K-s = 4.3 mu mol/l) is bulky eno ugh to fit the binding site. This conclusion has been confirmed by spe ctral competition experiments as well as metabolic studies. Of all oxy gen containing derivatives diamantane 1,6-dicarboxylic acid dimethyles ter only exhibited a pronounced ligand interaction with cytochrome P-4 50. Using molecular dimensions of this derivative the distance of 0.56 nm from the heme iron to the center of the substrate binding site was estimated.