Direct formation of complexes between cytochrome P-450 and nitrosoarenes

Insight into the preferential N-binding versus O-binding of nitrosoarenes to ferrous and ferric heme centers

Nitrosoarenes (ArNOs) are toxic metabolic intermediates that bind to heme proteins to inhibit their functions. Although much of their biological functions involve coordination to the Fe centers of hemes, the factors that determine N-binding or O-binding of these ArNOs have not been determined. We utilize X-ray crystallography and density functional theory (DFT) analyses of new representative ferrous and ferric ArNO compounds to provide the first theoretical insight into preferential N-binding versus O-binding of ArNOs to hemes. Our X-ray structural results favored N-binding of ArNO to ferrous heme centers, and O-binding to ferric hemes. Results of the DFT calculations rationalize this preferential binding on the basis of the energies of associated spin-states, and reveal that the dominant stabilization forces in the observed ferrous N-coordination and ferric O-coordination are dπ-pπ* and dσ-pπ*, respectively. Our results provide, for the first time, an explanation why in situ oxidation of the ferrous-ArNO compound to its ferric state results in the observed subsequent dissociation of the ligand.

The nitrosoamphetamine metabolite is accommodated in the active site of human hemoglobin: Spectroscopy and crystal structure

Amphetamine-based (Amph) drugs are metabolized in humans to their hydroxylamine (AmphNHOH) and nitroso (AmphNO) derivatives. The latter metabolites are known to bind to the Fe centers of cytochrome P450 and other heme enzymes to inhibit their activities. Although these AmphNHOH/AmphNO metabolites are present in vivo, their interactions with the blood protein hemoglobin (Hb) and the muscle protein (Mb) have been largely discounted due to a perception that the relatively small heme active sites of Hb and Mb will not be able to accommodate the large AmphNO group. We report the 2.15 Å resolution X-ray crystal structure of the AmphNO adduct of adult human hemoglobin as the Hb [α-Fe^III(H₂O)][β-Fe^II(AmphNO)] derivative. We show that the binding of AmphNO to the β subunit is enabled by an E helix movement and stabilization of ligand binding by H-bonding with the distal His63 residue. We also observe an AmphNHOH group in the Xe2 pocket in close proximity to the α heme site in this derivative. Additionally, UV-vis spectroscopy was used to characterize this and related wt and mutant Mb adducts. Importantly, our X-ray crystal structure of this Hb-nitrosoamphetamine complex represents the first crystal structure of a wild-type heme protein adduct of any amphetamine metabolite. Our results provide a framework for further studies of AmphNHOH/AmphNO interactions with Hb and Mb as viable processes that potentially contribute to the overall biological inorganic chemistry of amphetamine drugs.

Synthesis and molecular structures of N,N-dialkyl-4-nitrosoaniline adducts of formally d6 metalloporphyrins of ruthenium and cobalt

Para-aminosubstituted nitrosoarenes react with Ru ( CO )( OEP ) or [ Co ( TPP )( THF )2] SbF 6 (OEP2- = 2,3,7,8,12,13,17,18-octaethylporphyrinato dianion, TPP2- = 5,10,15,20-tetraphenylporphyrinato dianion) to generate Ru ( OEP )( ONC 6 H 4 NMe 2)2 and [ Co ( TPP )( ONC 6 H 4 NR 2)2] SbF 6 ( R = Me , Et ), respectively, in fair to high yields. These N -bound nitrosoarene complexes have been characterized by spectroscopic methods. The complexes Ru ( OEP )( ONC 6 H 4 NMe 2)2 and [ Co ( TPP )( ONC 6 H 4 NMe 2)2] ClO 4 have also been characterized by single-crystal X-ray crystallography. Their structures represent the first reported solid-state structures of Ru and Co porphyrins containing C-nitroso ligands.

NITROXYL (HNO): Chemistry, Biochemistry, and Pharmacology

Recent discoveries of novel and potentially important biological activity have spurred interest in the chemistry and biochemistry of nitroxyl (HNO). It has become clear that, among all the nitrogen oxides, HNO is unique in its chemistry and biology. Currently, the intimate chemical details of the biological actions of HNO are not well understood. Moreover, many of the previously accepted chemical properties of HNO have been recently revised, thus requiring reevaluation of possible mechanisms of biological action. Herein, we review these developments in HNO chemistry and biology.

Interaction between dioxoruthenium(VI) porphyrins and hydroxylamines: coordination of N-substituted hydroxylamine to ruthenium and X-ray crystal structures of ruthenium complexes with a unidentate nitrosoarene ligand

The interactions between dioxoruthenium(VI) porphyrins 1 with N-phenylhydroxylamine or unsubstituted hydroxylamine are described. Reaction of complexes 1 with excess PhNHOH leads to isolation of bis(nitrosobenzene)ruthenium(II) porphyrins 3 and mono(nitrosobenzene)ruthenium(II) porphyrins 4. Both the types of ruthenium complexes are characterized by 1H NMR, IR, and UV/Vis spectroscopy, and mass spectrometry. The X-ray structure determinations on [Ru(II)(TPP)(PhNO)2] (3a), [Ru(II)(2,6-Cl-TPP)(PhNO)2] (3e), and [Ru(II)(4-MeO-TPP)(PhNO)(PhNH2)] (4d) (TPP tetraarylporphyrin) disclose a unidentate nitrosoarene coordination in all these complexes, with Ru-N(PhNO) bond lengths of 2.003(3) (3a, average), 1.991(3) (3e, average), and 2.042(2) A (4d). In the case of 4d, the Ru-N(PhNH2) bond length is found to be 2.075(3) A. Mechanistic investigations reveal the formation of intermediates [Ru(II)(Por)(PhNO)(PhNHOH)] (5; Por=porphyrin), a ruthenium complex with N-substituted hydroxylamine ligand, in the "1 + PhNHOH" system. The Ru-NH(OH)Ph moiety in 5 undergoes no rapid exchange with free PhNHOH in solution at room temperature, as revealed by 1H NMR spectroscopy. Unlike the interaction between complexes 1 and PhNHOH, reaction of such complexes with NH2OH affords nitrosylruthenium(II) porphyrins [Ru(II)(Por)(NO)(OH)] (6).

Synthesis, Characterization, and Solid-State Molecular Structures of Nitrosoarene Complexes of Osmium Porphyrins

The reactions of (por)Os(CO) (por = TPP, TTP, OEP, TMP) with nitrosoarenes (ArNO; Ar = Ph, o-tol) in refluxing toluene generate the (por)Os(ArNO)(2) complexes in 45-76% yields (TTP = 5,10,15,20-tetra-p-tolylporphyrinato dianion, TPP = 5,10,15,20-tetraphenylporphyrinato dianion, OEP = 2,3,7,8,12,13,17,18-octaethylporphyrinato dianion, TMP = 5,10,15,20-tetramesitylporphyrinato dianion). The nu(NO) of the coordinated PhNO groups in the (por)Os(PhNO)(2) complexes occur in the 1295-1276 cm(-)(1) range, and decrease slightly in the order TPP (1295 cm(-)(1)) > TTP (1291 cm(-)(1)) > OEP (1286 cm(-)(1)) > TMP (1276 cm(-)(1)). The reaction of (TTP)Os(CO) with 1 equiv of PhNO in CH(2)Cl(2) at room temperature generates a 1:3 mixture of (TTP)Os(CO)(PhNO) and (TTP)Os(PhNO)(2) in ca. 40% isolated yield. The nu(CO) of (TTP)Os(CO)(PhNO) is at 1972 cm(-)(1) (KBr), which is 56 cm(-)(1) higher in energy than that of the precursor (TTP)Os(CO). When this mixture and excess PhNO are dissolved in toluene and the solution is heated to reflux, quantitative conversion to the (TTP)Os(PhNO)(2) product occurs. IR monitoring of the reactions of (por)Os(CO) with 1 equiv of PhNO in CH(2)Cl(2) reveal similar formations of the respective (por)Os(CO)(PhNO) intermediates for the TTP (1968 cm(-)(1); Deltanu(CO) = +74 cm(-)(1)), TMP (1966 cm(-)(1); Deltanu(CO) = +63 cm(-)(1)), and OEP (1958 cm(-)(1); Deltanu(CO) = +72 cm(-)(1)) analogues. Five of these (por)Os(ArNO)-containing complexes have been fully characterized by spectroscopic methods and by single-crystal X-ray crystallography. All the nitrosoarene ligands in these complexes are attached to the formally Os(II) centers via an eta(1)-N binding mode.

The reaction of nitroxyl (HNO) with nitrosobenzene gives cupferron (N-nitrosophenylhydroxylamine)

Nitroxyl (HNO), a penultimate product in the NOS-catalyzed conversion of L-arginine to L-citrulline, generated from Angeli's salt (AS) was determined by trapping it with nitrosobenzene (NB) to produce cupferron. The cupferron thus produced was characterized by complexation with Fe3+, Al3+, Cu2+, or Sn2+. UV/VIS spectra of the solubilized (in CHCl3) precipitates formed from NB and nitroxyl generated from AS in the presence of the iron, aluminum, copper, or tin salts were identical to those of their corresponding cupferron complexes. The identities of the Fe3+ and Cu2+ complexes formed from NB and HNO were further confirmed by their identical retention times on HPLC when compared to authentic Fe3+ and Cu2+ cupferron complexes. It was possible to detect 5 x 10(-6) M of the cupferron Fe3+ complex spectrophotometrically and to measure its production from the nitroxyl generators AS and methanesulfohydroxamic acid (MSHA) in the presence of 10(-4) M NB. The yield of cupferron was 51 and 62% of the amount of nitroxyl possible from AS or MSHA, respectively, after taking into account the relative rates of nitroxyl generation from these donors.

Nitroxyl analogs as inhibitors of aldehyde dehydrogenase. C-nitroso compounds

We previously postulated that the catalase-mediated oxidation of cyanamide leads to the formation of the unstable intermediate, N-hydroxycyanamide, which spontaneously decomposes to nitroxyl, the putative inhibitor of aldehyde dehydrogenase (EC 1.2.1.3; AlDH). Since it was not possible to provide direct evidence for the inhibition of AlDH by nitroxyl, we examined the activity of three representative substituted nitroxyls (C-nitroso compounds), viz. nitrosobenzene (NB), 1-nitrosoadamantane (NA), and 2-methyl-2-nitrosopropane (MNP), as direct inhibitors of yeast AlDH in vitro. While NB and NA were highly effective inhibitors in this system exhibiting IC50 values of 2.5 and 8.6 microM, respectively, MNP was considerably less effective with an IC50 of 0.15 mM. When tested in vivo, NA did not show any inhibitory activity on the hepatic AlDH, possibly due to the lack of site-specific delivery of the active monomeric form of this compound. However, NB at a low dose did inhibit hepatic AlDH as reflected by an increase in blood acetaldehyde levels. These results attest to the abilities of NB and NA to act as direct inhibitors of AlDH analogous to nitroxyl itself.