Interaction of polycyclic hydrocarbons with cytochrome P-450. III. Effects of hydrocarbon binding on the interaction of some ligands with P-448(1) heme.
The binding of polycyclic hydrocarbons to P-448(1) affected the interaction of exogenous ligands, such as ethyl isocyanide and CO in the ferrous state and 1-methyl imidazole in the ferric state, with the heme to various extents depending on the structure of the hydrocarbons. The effect of the hydrocarbons on dissociation constants (Kd) on the three ligands was essentially similar. Kd of ethyl isocyanide and CO for hydrocarbon-free P-448(1) were 18 microM and 1.1 microM, respectively. The affinities of both the ligands increased about 3-fold when hydrocarbons of a small molecular size, such as phenanthrene, were bound to P-448(1). Addition of a benzene ring to the outside of the phenanthrene molecule resulted in a remarkable decrease in the affinities of both the ligands. The binding of larger hydrocarbons in size produced a stronger inhibitory effect on the interaction of ethyl isocyanide with P-448(1). Thus, Kd of the isocyanide for benz[a]anthracene-bound and dibenz[a,c]anthracene-bound P-448(1) were estimated to be 260 microM and 3 mM, respectively. Spectral changes hardly occurred on addition of 4 mM ethyl isocyanide to 3-methylcholanthrene-bound or 7,8-benzoflavone-bound P-448(1). The effect of the hydrocarbon binding on Kd of the ligands was depressed when a benzene ring which links two aromatic rings in the hydrocarbon molecule was replaced by a single C-C bond. A probable structure of the active area of P-448(1) composed of the substrate site and the heme is proposed based on the effect of the hydrocarbon binding on the interaction between the exogenous ligands and the P-448(1) heme iron reported in this series of papers. The binding of hydrocarbons to P-448(1) increased the intensity ratio of the 430 nm to the 453 nm peak of the absorption spectrum of the ethyl isocyanide compound. When a large and long hydrocarbon in shape such as dibenz[a,h]anthracene was bound to P-448(1), a red shift of the Soret absorption peak and modification of the Soret CD pattern of the CO compound were observed.
D000818:Animals; D019388:Cytochrome P-450 CYP1A2; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D003580:Cytochromes / Q000378:metabolism*; D006418:Heme / Q000378:metabolism*; D006838:Hydrocarbons / Q000378:metabolism*; D007700:Kinetics; D008024:Ligands; D008862:Microsomes, Liver / Q000378:metabolism*; D011485:Protein Binding; D011817:Rabbits; D013329:Structure-Activity Relationship
D003580:Cytochromes; D006838:Hydrocarbons; D008024:Ligands; D006418:Heme; D003577:Cytochrome P-450 Enzyme System; C017170:Cyp1a2 protein, rat; D019388:Cytochrome P-450 CYP1A2