1H-NMR study of the interaction of aminopyrine with purified rat liver microsomal cytochrome P-450

The substrate binding site of human liver cytochrome P450 2C9: an NMR study

Purified recombinant human liver cytochrome P450 2C9 was produced, from expression of the corresponding cDNA in yeast, in quantities large enough for UV-visible and 1H NMR experiments. Its interaction with several substrates (tienilic acid and two derivatives, lauric acid and diclofenac) and with a specific inhibitor, sulfaphenazole, was studied by UV-visible and 1H NMR spectroscopy. At 27 degrees C, all those substrates led to an almost complete conversion of CYP 2C9 to high-spin (S = 5/2) CYP 2C9-substrate complexes characterized by a Soret peak at 390 nm; their KD values varied between 1 and 42 microM. On the contrary, sulfaphenazole led to a low-spin (S = 1/2) CYP 2C9 complex upon binding of its NH2 group to CYP 2C9 iron. Interactions of the five substrates with the enzyme were studied by paramagnetic relaxation effects of CYP 2C9-iron(III) on the 1H NMR spectrum of each substrate. Distances between the heme iron atom and substrate protons were calculated from the NMR data, and the orientation of the substrate relative to iron was determined from those distances. Finally, a model for substrate positioning in the CYP 2C9 active site was constructed by molecular modeling studies under the constraint of the iron-proton distances. It points out two structural characteristics for a compound to be selectively recognized by CYP 2C9: (i) the presence of an anionic site able to establish an ionic bond with a putative cationic residue of the protein and (ii) the presence of an hydrophobic zone between the substrate hydroxylation site and the anionic site. Sulfaphenazole was easily included in that model; its very high affinity for CYP 2C9 is due to a third structural feature, the presence of its NH2 function which binds to CYP 2C9 iron.

Proton NMR study of the interaction of benzo(a)pyrene with rat liver microsomal cytochrome P-450

1H nuclear magnetic resonance longitudinal relaxation time (T1) measurements were used to study the interaction of 1,3,6-H-benzo(a)pyrene (1,3,6-H-BaP) with microsomal cytochrome P-450 from livers of phenobarbital- and beta-naphthoflavone-treated rats. Upon addition of various amounts of cytochrome P-450 to solutions of BaP, the T1 values for the three BaP protons decreased. When the hemeprotein was converted to its carbon monoxyferrous derivative in the presence of BaP, the T1 values of the BaP protons increased. The paramagnetic contributions to the observed T1 values were calculated and resulted in distance estimates of greater than 6 A for the three BaP protons in both cases.

NMR study of the interaction of P-450 with 4-methoxypyridine

Longitudinal relaxation (T1) measurements for all lines (alpha-CH, beta-CH, O-CH3) in the 4-methoxypyridine 1H-NMR spectrum were used to study the interaction of 4-methoxypyridine with purified microsomal cytochrome P-450 from livers of phenobarbital-treated rats. The paramagnetic contribution to the observed T1(-1) value was determined from its dependence on 4-methoxypyridine concentration. In the P-450-4-methoxypyridine complex the latter is oriented so that the nitrogen of pyridine is directed towards the Fe3+.

Relationship between paracetamol binding to and its oxidation by two cytochromes P-450 isozymes--a proton nuclear magnetic resonance and spectrophotometric study

From the hepatic cytochrome P-450 isozymes b and c isolated from rats treated with phenobarbital and 3-methylcholanthrene respectively, only cytochrome P-450c was found to be active in the oxidation of paracetamol, in the presence of glutathione ultimately leading to the formation of the 3-glutathionyl conjugate. Paracetamol interacted with both cytochrome P-450b and c, as shown by difference spectrophotometry. Cytochrome P-450b was found to have a higher affinity for paracetamol than cytochrome P-450c and demonstrated a type I spectral change, whereas in the case of cytochrome P-450c a reverse type I spectral change was observed. Proton n.m.r. longitudinal relaxation rate measurements revealed that in the case of cytochrome P-450c, paracetamol was orientated with its phenolic hydroxyl group in closest proximity to the central haem iron ion. In the case of cytochrome P-450b, the acetylamino group of paracetamol most closely approached the haem iron ion.

Magnetic resonance study of the structure and functions of cytochrome P450

Cytochrome P450 is a membrane-bound enzyme providing oxidation of numerous organic compounds in organisms. The objective of this review is to show the wide possibilities that are provided by Electron Spin Resonance (ESR) and Nuclear Magnetic Resonance (NMR) techniques to the study of the structure and functions of this unique enzyme. High sensitivity of ESR spectra of cytochrome P450 to its functional state and interaction with substrates and inhibitors is illustrated. NMR and proton relaxation make it possible to obtain unique information about the structure of the active center of cytochrome P450 under physiological conditions. ESR and NMR methods allow one to obtain structural data on location of substrates, inhibitors, and their spin-labeled analogs with respect to Fe3+ ions in the enzyme-active center. Of special interest seems to be coupling of ESR with the affinity modification method. For this purpose, the spin-labeled analogs of cytochrome P450 substrates containing alkylating groups were used. As a result, an important datum has been obtained on the structure of active centers of cytochrome P450 in microsomes and in a highly purified state. In conclusion, the problems of the structure and functions of cytochrome P450, which can be most efficiently resolved with the use of magnetic resonance methods, are discussed.