© Copyright 1999 by the American Chemical Society
Volume 38, Number 48
NoVember 30, 1999
Accelerated Publications Formation of a Pterin Radical in the Reaction of the Heme Domain of Inducible Nitric Oxide Synthase with Oxygen† Amy R. Hurshman,‡ Carsten Krebs,§ Dale E. Edmondson,| Boi Hanh Huynh,§ and Michael A. Marletta*,‡,⊥,# Howard Hughes Medical Institute, DiVision of Medicinal Chemistry, and Department of Biological Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109-0606, and Departments of Physics, Biochemistry, and Chemistry, Emory UniVersity, Atlanta, Georgia 30322 ReceiVed August 30, 1999; ReVised Manuscript ReceiVed October 6, 1999
ABSTRACT: The heme domain (iNOSheme) of inducible nitric oxide synthase (NOS) was expressed in Escherichia coli and purified to homogeneity. Rapid freeze-quench (RFQ) EPR was used to monitor the reaction of the reduced iNOSheme with oxygen in the presence and absence of substrate. In these reactions, heme oxidation occurs at a rate of ∼15 s-1 at 4 °C. A transient species with a g ) 2.0 EPR signal is also observed under these conditions. The spectral properties of the g ) 2.0 signal are those of an anisotropic organic radical with S ) 1/2. Comparison of the EPR spectra obtained when iNOSheme is reconstituted with N5-14N- and 15N-substituted tetrahydrobiopterin (H4B) shows a hyperfine interaction with the pterin N5 nitrogen and identifies the radical as the one-electron oxidized form (H3B‚) of the bound H4B. Substitution of D2O for H2O reveals the presence of hyperfine-coupled exchangeable protons in the H4B radical. This radical forms at a rate of 15-20 s-1, with a slower decay rate that varies (0.12-0.7 s-1) depending on the substrate. At 127 ms, H3B‚ accumulates to a maximum of 80% of the total iNOSheme concentration in the presence of arginine but only to ∼2.8% in the presence of NHA. Double-mixing RFQ experiments, where NHA is added after the formation of H3B‚, show that NHA does not react rapidly with H3B‚ and suggest that NHA instead prevents the formation of the H4B radical. These data constitute the first direct evidence for an NOS-bound H3B‚ and are most consistent with a role for H4B in electron transfer in the NOS reaction.
Nitric oxide synthase (NOS,1 EC 1.14.13.39) catalyzes the conversion of L-arginine to citrulline and ‚NO (1-3). The † This research was supported by the Howard Hughes Medical Institute and NIH Grants CA 50414 (M.A.M.), GM 29433 (D.E.E.), and GM 47295 (B.H.H.). A.R.H. was supported by NIH Grant T32GM07767, a Regents’ Fellowship from the University of Michigan, and an American Foundation for Pharmaceutical Education Fellowship. * To whom correspondence should be addressed. ‡ Division of Medicinal Chemistry, University of Michigan. § Department of Physics, Emory University. | Departments of Biochemistry and Chemistry, Emory University. ⊥ Howard Hughes Medical Institute, University of Michigan. # Department of Biological Chemistry, University of Michigan.
overall reaction is a five-electron oxidation, with NG-hydroxyL-arginine (NHA) as an intermediate (4, 5), and requires NADPH and O2 as cosubstrates. Three isoforms of NOS have 1 Abbreviations: NOS, nitric oxide synthase; iNOS, inducible NOS; eNOS, endothelial NOS; nNOS, neuronal NOS; iNOSheme, heme domain of inducible NOS; IPTG, isopropyl β-D-thiogalactopyranoside; H4B, (6R)-5,6,7,8-tetrahydro-L-biopterin; 15N-H4B, [5-15N]-(6R,S)-5,6,7,8tetrahydro-L-biopterin; HEPES, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid; DTT, dithiothreitol; Tris-HCl, tris(hydroxymethyl)aminomethane hydrochloride; NHA, NG-hydroxy-L-arginine; 15N-NHA, [15N]-NG-hydroxy-L-arginine (15N label only at the hydroxylamino nitrogen); HPLC, high-performance liquid chromatography; BSA,
10.1021/bi992026c CCC: $18.00 © 1999 American Chemical Society Published on Web 11/11/1999
15690 Biochemistry, Vol. 38, No. 48, 1999 been characterized: a particulate, constitutive enzyme from vascular endothelium (eNOS), a soluble, constitutive enzyme from neuronal cells (nNOS), and an inducible enzyme, best characterized from murine macrophages (iNOS) (6). All of the isoforms are homodimeric and bind an equivalent each of FAD and FMN (7-9) as well as iron protoporphyrin IX heme (10-12) per subunit. Full activity also requires one H4B per monomer (9, 13, 14). The roles of the enzyme-bound heme and H4B in the reaction mechanism are not fully understood. CO inhibition studies have suggested that the heme is involved in both steps of the NOS reaction (10, 15). Further evidence for the involvement of the heme in NHA oxidation comes from NOS reactions where hydrogen peroxide is substituted for NADPH and O2 (peroxide-shunt reactions). The products of the peroxide-shunt reactions are consistent with a heme ferricperoxide nucleophile as an intermediate in the NADPHdependent oxidation of NHA (16, 17). Since neither peroxide nor iodosobenzene supports the hydroxylation of arginine, the exact function of the heme in this step of the reaction is less clear. Recent results with H4B-free iNOS have shown that H4B is absolutely required for the reaction with arginine (18). Additionally, the products of NHA oxidation are different in the presence and absence of H4B, implicating a role for the pterin cofactor in this step as well. To further elucidate the function of these cofactors in the NOS reaction, we have expressed and purified the heme domain of iNOS. The heme domain of NOS binds both the heme and H4B cofactors, as well as substrate (19-21). These properties suggest an intact active site and make iNOSheme a useful tool for mechanistic studies. In particular, it is ideal for examining the formation of intermediates that may form in the NOS reaction, since there is no spectral contribution from the flavins in the reductase domain that might interfere with the observation of any transient species. Furthermore, although the absence of the flavins requires the use of an alternate source of reducing equivalents, such as sodium dithionite, tighter control over the number of electrons delivered to the active site is achieved. We report here the observation of a novel EPR signal during the reaction of reduced iNOSheme with oxygen. Characterization of this signal suggests that it is due to an NOS-bound trihydropterin radical (H3B‚). The possible involvement of this radical in the NOS reaction, particularly in electron transfer, and the implications for NOS catalysis are discussed. EXPERIMENTAL PROCEDURES Materials and General Methods. Escherichia coli JM109 competent cells were purchased from Promega. Terrific broth (TB) was from Gibco-BRL. Ampicillin and IPTG were from Boehringer-Mannheim. iNOS cDNA in pBluescript II KS was a gift from Dr. Solomon H. Snyder (Johns Hopkins University). The pCWori plasmid (ampicillin resistance, tactac promoter) was a gift from Dr. Michael R. Waterman (Vanderbilt University). H4B and 15N-H4B (labeled at N5) were purchased from Schircks Laboratories (Jona, Switzerbovine serum albumin; SDS-PAGE, sodium dodecyl sulfate-polyacrylamide gel electrophoresis; RFQ, rapid freeze-quench; EPR, electron paramagnetic resonance; EDTA, ethylenediamine-N,N,N′,N′tetraacetic acid; NDA, 2,3-naphthalenedicarboxaldehyde.
Accelerated Publications land) and were prepared either in 100 mM HEPES (pH 7.5) containing 100 mM DTT or anaerobically in 100 mM HEPES (pH 7.5) with no DTT. Coomassie Blue R-250 and Bradford protein reagent dye were purchased from Bio-Rad. Reaction vials and silicone/Teflon septa were obtained from Pierce Chemical Co. Centrifugal filtration units (Ultrafree15 and Ultrafree-0.5, Biomax-30K NMWL membrane) were from Millipore. NHA was purchased either from Alexis Corp. or from Cayman Chemical Co. (Ann Arbor, MI) and was found to contain less than 2% citrulline contamination as analyzed by HPLC. 15N-NHA (labeled only at the hydroxylamino nitrogen) was synthesized as previously described (5). Sodium dithionite was purchased from Aldrich; solutions (∼4 mg/mL) were prepared anaerobically in 100 mM HEPES (pH 7.5) and were standardized against potassium ferricyanide before use (22). All other reagents were purchased from Sigma. Expression of iNOS Heme Domain. The cloning and expression of iNOSheme will be reported in detail elsewhere. Briefly, the iNOS heme domain expression vector (pCWiNOSheme) was constructed by PCR amplification from the iNOS cDNA, addition of appropriate restriction sites, and ligation into the pCW plasmid. This heme domain construct includes amino acids 1-490 of the iNOS sequence and also contains a C-terminal histidine tag (6×His). Optimal expression of iNOSheme was obtained in JM109 cells grown in Terrific broth (47 g/L TB and 4 mL/L glycerol). Expression cultures (1.5 L of TB containing 50 µg/mL ampicillin) were inoculated (1:100) from an overnight culture of JM109-pCWiNOSheme and were grown at 37 °C to an A600 of ∼0.5. The cultures were then cooled to 25 °C, induced by the addition of IPTG (0.4 mM final concentration), and harvested by centrifugation (10 min at 15900g) 21 h after induction. Purification of iNOS Heme Domain. Fresh cell pellets from 9 L of culture were resuspended in 200 mL of lysis buffer (50 mM sodium phosphate, pH 8.0, 300 mM NaCl, 10 mM imidazole, 10% glycerol, 10 µg/mL benzamidine, 5 µg/mL leupeptin, and 1 µg/mL each of pepstatin, chymostatin, and antipain) and lysed in a French pressure cell press (SLMAminco, Rochester, NY). Supernatant was prepared by centrifugation for 1 h at 40000g and was loaded at 1 mL/ min on a 10 mL nickel column (Ni-NTA agarose from QIAGEN). All buffers for the nickel column step contained 50 mM sodium phosphate (pH 8.0), 300 mM NaCl, 10% glycerol, and various amounts of imidazole. The column was washed with 150 mL of buffer containing 10 mM imidazole, and then the protein was eluted with a linear gradient of imidazole (10-500 mM) in 125 mL. Fractions containing iNOSheme were red in color and were pooled on the basis of the A280/A428 ratio (ratio of peak
