Pseudomonas putida cytochrome P-450. Binding of a spin-labeled analog of the inhibitor metyrapone

Interaction of a spin-labelled cholesterol derivative with the cytochrome P-450scc active site

The cholesterol analogue 25-doxyl-27-nor-cholesterol (CNO), was found to be a substrate for cytochrome P-450scc. Upon incubation with the cytochrome P-450scc electron transfer system, CNO is transformed to pregnenolone (Km = 33 microM, Vmax = 0.32 min-1). The pregnenolone formation from endogenous cholesterol is strongly inhibited by CNO (50% at 5 microM). It binds tightly to cytochrome P-450scc as evidenced by a reversed type I spectral absorbance change (Kd = 5.9 microM) which is paralleled by a greater hyperfine splitting of the room-temperature CNO ESR spectrum due to an enhanced probe immobilization (Kd = 1.9 microM). This finding is in accord with a rotational correlation time of about 10(-7) s, which is close to the tumbling rate of the protein. At 110 K the CNO-bound cytochrome P-450scc displays the ESR g-values gx = 2.404/2.456, gy = 2.245 and gz = 1.916; these are different from those of cholesterol-liganded cytochrome P-450scc and may thus serve as a marker for cytochrome P-450scc. Our data indicate that the stereospecificity of the cytochrome P-450scc side-chain-cleaving activity is not dependent on the nature of the cholesterol side-chain termination (C25 to C27). The substrate binding site is however rather sensitive to a modification of the side chain. The doxyl ring confers a stronger affinity of the substrate to the enzyme. Upon binding it becomes embedded in the protein matrix, and we estimate that its final position is 0.6-1.0 nm from the heme moiety.

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.

Effect of cimetidine on delta-aminolevulinic acid synthase and microsomal heme oxygenase in rat liver

Cimetidine is a well known inhibitor of the heme-containing enzyme cytochrome P-450. We have found that it also inhibits delta-aminolevulinic acid synthase (ALA-S) and microsomal heme oxygenase, the rate-limiting enzymes for heme synthesis and heme degradation respectively. Cytochrome P-450 content was decreased but microsomal heme concentration remained unaltered for a period of 30 min after in vivo cimetidine administration to rats. In vitro incubation of cimetidine with each of the above enzymes revealed no direct effect of cimetidine on ALA-S but about 50% inhibition of heme oxygenase and 20% reduction in cytochrome P-450 content. This suggests that a metabolite of cimetidine inhibits ALA-S activity in vivo, while the drug itself or a metabolite inhibits heme oxygenase both in vivo and in vitro. A rise in ALA-S activity seen after its early inhibition and its return to approximate control values after 60 min suggest a reversible inhibition of ALA-S by a metabolite of cimetidine and may correspond to its clearance from the animal. An elevation in microsomal heme content paralleled the rise in ALA-S activity while microsomal heme oxygenase activity returned to only 65% of control value 60 min after cimetidine treatment. Cytochrome P-450 content did not change after its initial decrease, suggesting that irreversible alteration had occurred.

The interaction of cimetidine with rat liver microsomes

The binding of cimetidine to rat liver microsomes in M/15 phosphate buffer, pH 7.9, has been investigated by difference spectroscopy and also by equilibrium partition studies, the latter method providing the more definitive characterization of the interaction in the pharmacologically relevant, low micromolar range of drug concn. In addition, the effect of cimetidine on the rate of dilution-induced displacement of [3H]cimetidine from rat liver microsomes has been used to justify consideration of the binding results in terms of two distinct and independent classes of microsomal site, governed by dissociation constants of 8.3 and 104 microM under the above conditions. By demonstrating unequivocally the existence of the stronger interaction, this investigation has provided an acceptable experimental basis for considering the undesired side effect of cimetidine in concomitant use with a number of other drugs to be the consequence of its inhibition of their monooxygenase-dependent metabolism.

Binding of a metyrapone spin label to microsomal cytochrome P-450

Probing of the active site of microsomal cytochrome P-450 was carried out with a spin label derived from 2-methyl-1,2-bis(3-pyridyl)-1-propanone (metyrapone). Its optical binding spectra to cytochrome P-450 resemble the spectra with metyrapone. The electron paramagnetic resonance spectrum of the spin label in microsomes indicates binding with strong immobilization. Since the apparent optical and EPR binding constants agree very well (Ks approximately 2-10(-5) M), and metyrapone is found to displace the spin label, we conclude, that the spin label binds to the active site of cytochrome P-450. Addition of detergents or an increase in temperature mobilizes the bound spin label slightly. The EPR signal accounts for only 60% of the bound spin label due to its dipolar interaction with the low-spin ferric heme of cytochrome P-450. From this finding, the distance between nitroxide and iron is evaluated to be 11 A. This supports the model that one of the pyridine nitrogens of metyrapone is coordinated to the iron of cytochrome P-450. The bound spin label shows virtually no interaction with ferricyanide as if the active site of cytochrome P-450 is a hydrophobic pocket not accessible to ions.