The functional role of threonine-205 in the mechanism-based inactivation of P450 2B1 by two ethynyl substrates: the importance of the F helix in catalysis

A Metabolic Activation-Based Chemoproteomic Platform to Profile Adducted Proteins Derived from Furan-Containing Compounds

Human exposure to widespread furan-containing compounds (FCCs) has drawn much attention due to the high risk of their toxicities. Identifying adducted proteins resulting from the metabolic activation of FCCs is the core to learning the mechanism of FCCs' toxic action. We succeeded in establishing a metabolic activation-based chemoproteomic platform to map FCC-derived protein adducts in cultured primary hepatocytes treated with FCCs and to pinpoint the modification sites, using click chemistry but without alkynylation or azidation of FCCs to be investigated. The proposed platform was systematically verified by biomimetic synthesis, liver microsomal incubation, and primary hepatocyte culture. A mixture of furan, 2-methylfuran, and 2,5-dimethylfuran as model was tested by use of the established platform. A total of hepatic 171 lysine-based adducted proteins and 145 cysteine-based adducted proteins by the reactive metabolites of the three FCCs were enriched and identified (Byonic score ≥ 100). The target proteins were found to mainly participate in ATP synthesis. The technique was also successfully applied to furan-containing natural products. The established platform made it possible to profile covalently adducted proteins, because of potential exposure to a vast inventory of over two million of FCCs documented.

Acetylenes: cytochrome P450 oxidation and mechanism-based enzyme inactivation

The oxidation of carbon-carbon triple bonds by cytochrome P450 produces ketene metabolites that are hydrolyzed to acetic acid derivatives or are trapped by nucleophiles. In the special case of 17α-ethynyl sterols, D-ring expansion and de-ethynylation have been observed as competing pathways. The oxidation of acetylenic groups is also associated with mechanism-based inactivation of cytochrome P450 enzymes. One mechanism for this inactivation is reaction of the ketene metabolite with cytochrome P450 residues essential for substrate binding or catalysis. However, in the case of monosubstituted acetylenes, inactivation can also occur by addition of the oxidized acetylenic function to a nitrogen of the heme prosthetic group. This addition reaction is not mediated by the ketene metabolite, but rather occurs during oxygen transfer to the triple bond. In some instances, a detectable intermediate is formed that is most consistent with a ketocarbene-iron heme complex. This complex can progress to the N-alkylated heme or revert back to the unmodified enzyme. The ketocarbene complex may intervene in the formation of all the N-alkyl heme adducts, but is normally too unstable to be detected.

Formation of Both Heme and Apoprotein Adducts Contributes to the Mechanism-Based Inactivation of Human CYP2J2 by 17α-Ethynylestradiol

17α-Ethynylestradiol (EE), a major component of many oral contraceptives, affects the activities of a number of the human cytochrome P450 (P450) enzymes. Here, we characterized the effect of EE on CYP2J2, a major human P450 isoform that participates in metabolism of arachidonic acid. EE inactivated the hydroxyebastine carboxylation activity of CYP2J2 in a reconstituted system. The loss of activity is time and concentration dependent and requires NADPH. The K_I and k_inact values for the inactivation were 3.6 μM and 0.08 minute^-1, respectively. Inactivation of CYP2J2 by EE was due to formation of a heme adduct as well as an apoprotein adduct. Mass spectral analysis of CYP2J2 partially inactivated by EE showed two distinct protein masses in the deconvoluted spectrum that exhibited a mass difference of approximately 312 Da, which is equivalent to the sum of the mass of EE and one oxygen atom. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis revealed a heme adduct with MH⁺ ion at m/z 875.5, corresponding to alkylation of an iron-depleted prosthetic heme by EE plus one oxygen atom. The reactive intermediate responsible for covalently modifying both the prosthetic heme and apoprotein was characterized by trapping with glutathione (GSH). LC-MS/MS analysis revealed two GSH conjugate isomers with MH⁺ ions at m/z 620, which were formed by reaction between GSH and EE with the oxygen being added to either the internal or terminal carbon of the ethynyl moiety. High-pressure liquid chromatography analysis revealed that three other major metabolites were formed during EE metabolism by CYP2J2.

Roles of Residues F206 and V367 in Human CYP2B6: Effects of Mutations on Androgen Hydroxylation, Mechanism-Based Inactivation, and Reversible Inhibition

The crystal structures of human CYP2B6 indicate that Phe206 and Val367 are in close proximity to the substrate binding site and suggest that both residues may play important roles in substrate metabolism and inhibitor binding. To test this hypothesis, we investigated the effects of mutating these residues to Ala on the regiospecificity of CYP2B6 for the metabolism of testosterone and androstenedione. For testosterone metabolism, 16β-OH-testosterone formation by the F206A mutant was <5% of the wild type (WT), whereas the V367A mutant exhibited a doubling of 16α-OH-testosterone formation with a 50% decrease in 16β-OH-testosterone formation compared with the WT. Significant alterations in the regiospecificity for androstenedione metabolism were also observed. To investigate the roles of these two residues in the metabolic activation of mechanism-based inactivators, tert-butylphenylacetylene (BPA) and bergamottin (BG) were used to test the susceptibility to inactivation. Although the rates of inactivation of both mutants by BG were not significantly decreased compared with the WT, the efficiency of inactivation by BPA of both mutants was more than an order of magnitude lower. Our results demonstrate that Phe206 plays a crucial role in determining the specificity of CYP2B6 for the 16β-hydroxylation of testosterone and androstenedione and that it also plays an important role in BG binding and mechanism-based inactivation by BPA. In addition, Val367 dramatically enhances the catalytic activity of CYP2B6 toward androstenedione and plays an important role in mechanism-based inactivation by BPA. The results presented here show the important roles of Phe206 and Val367 in interactions of CYP2B6 with substrates and inactivators/inhibitors and are consistent with the crystal structures.

Thr302 is the site for the covalent modification of human cytochrome P450 2B6 leading to mechanism-based inactivation by tert-butylphenylacetylene

The mechanism-based inactivation of human CYP2B6 by tert-butylphenylacetylene (BPA) in the reconstituted system was investigated. The inactivation of CYP2B6 by BPA is time-, concentration-, and NADPH-dependent and exhibits a K(I) of 2.8 μM, a k(inact) of 0.7 min(-1), and a t(1/2) of 1 min. The partition ratio is ∼5. Unlike CYP2B1 and CYP2B4, in addition to the formation of an apoprotein adduct and a glutathione conjugate, a small heme adduct was observed when CYP2B6 was incubated with BPA. The mass increase of the adducted apoprotein and GSH conjugate is 174 Da, equivalent to the mass of one molecule of BPA plus one oxygen atom. To identify the adducted residue, BPA-inactivated CYP2B6 was digested with trypsin, and the digest was then analyzed by liquid chromatography-tandem mass spectrometry. A mass shift of 174 Da was used for the SEQUEST database search, and the identity of the modified residue was confirmed by MS/MS fragmentation of the modified peptide. Two residues, Lys274 and Thr302, were identified as having been modified. Further mutagenesis studies have demonstrated that the residue that is modified to result in inactivation is Thr302, not Lys274. Docking studies show that in the enzyme-substrate complex, Thr302 is in close contact with the triple bond of BPA with a distance of 3.8 Å between the terminal carbon of BPA and the oxygen in the hydroxyl group of Thr302. In conclusion, Thr302 of CYP2B6 is covalently modified by a reactive metabolite of BPA, and this modification is responsible for the mechanism-based inactivation.

Mechanism-based inactivation of CYP2B1 and its F-helix mutant by two tert-butyl acetylenic compounds: covalent modification of prosthetic heme versus apoprotein

The mechanism-based inactivation of cytochrome CYP2B1 [wild type (WT)] and its Thr205 to Ala mutant (T205A) by tert-butylphenylacetylene (BPA) and tert-butyl 1-methyl-2-propynyl ether (BMP) in the reconstituted system was investigated. The inactivation of WT by BPA exhibited a k(inact)/K(I) value of 1343 min(-1)mM(-1) and a partition ratio of 1. The inactivation of WT by BMP exhibited a k(inact)/K(I) value of 33 min(-1)mM(-1) and a partition ratio of 10. Liquid chromatography/tandem mass spectrometry analysis (LC/MS/MS) of the WT revealed 1) inactivation by BPA resulted in the formation of a protein adduct with a mass increase equivalent to the mass of BPA plus one oxygen atom, and 2) inactivation by BMP resulted in the formation of multiple heme adducts that all exhibited a mass increase equivalent to BMP plus one oxygen atom. LC/MS/MS analysis indicated the formation of glutathione (GSH) conjugates by the reaction of GSH with the ethynyl moiety of BMP or BPA with the oxygen being added to the internal or terminal carbon. For the inactivation of T205A by BPA and BMP, the k(inact)/K(I) values were suppressed by 100- and 4-fold, respectively, and the partition ratios were increased 9- and 3.5-fold, respectively. Only one major heme adduct was detected following the inactivation of the T205A by BMP. These results show that the Thr205 in the F-helix plays an important role in the efficiency of the mechanism-based inactivation of CYP2B1 by BPA and BMP. Homology modeling and substrate docking studies were presented to facilitate the interpretation of the experimental results.

Structure-function analysis of cytochromes P450 2B

In the last 4 years, breakthroughs were made in the field of P450 2B (CYP2B) structure-function through determination of one ligand-free and two inhibitor-bound X-ray crystal structures of CYP2B4, which revealed many of the structural features required for binding ligands of different size and shape. Large conformational changes of several plastic regions of CYP2B4 can dramatically reshape the active site of the enzyme to fit the size and shape of the bound ligand without perturbing the overall P450 fold. Solution biophysical studies using isothermal titration calorimetry (ITC) have revealed the large difference in the thermodynamic parameters of CYP2B4 in binding inhibitors of different ring chemistry and side chains. Other studies have revealed that the effects of site-specific mutations on steady-state kinetic parameters and mechanism-based inactivation are often substrate dependent. These findings agree with the structural data that the enzymes adopt different conformations to bind various ligands. Thus, the substrate specificity of an individual enzyme is determined not only by active site residues but also non-active site residues that modulate conformational changes that are important for substrate access and rearrangement of the active site to accommodate the bound substrate.

Roles of the threonine 407, aspartic acid 417, and threonine 419 residues in P450 2B1 in metabolism

We have previously observed that the quadruple (S407T-N417D-A419T-K473M) and triple (S407T-N17D-A419T) mutants of the chimeric construct of P450 2B1/2B2 do not undergo mechanism-based inactivation by 17alpha-ethynylestradiol (17EE) and tert-butyl 1-methyl-2-propynyl ether (tBMP). The ability of these mutants to metabolize 17EE, benzphetamine, and testosterone has been investigated. The profile for 17EE metabolism by both mutants was characteristic of both wild-types. The two mutants metabolized testosterone to form androstenedione with no formation of the hydroxy products as was seen with both the wild-types. Benzphetamine metabolism by the mutants showed that both mutants exhibited an increased tendency to catalyze demethylation rather than debenzylation. In the presence of the alternate oxidants cumene hydroperoxide and tert-butyl hydroperoxide, the wild-type 2B1 was not inactivated by 17EE. Metabolism of 17EE by 2B1 supported by these alternate oxidants revealed differences in the metabolites that may be related to the inability of 2B1 to be inactivated under these conditions.