Expression and immunoaffinity purification of human inducible nitric-oxide synthase. Inhibition studies with 2-amino-5,6-dihydro-4H-1,3-thiazine

Mechanism of Inactivation of Inducible Nitric Oxide Synthase by Amidines. Irreversible Enzyme Inactivation without Inactivator Modification

Nitric oxide synthases (NOS) are hemoproteins that catalyze the reaction of L-arginine to L-citrulline and nitric oxide. N-(3-(Aminomethyl)benzyl)acetamidine (1400W) was reported to be a slow, tight-binding, and highly selective inhibitor of iNOS in vitro and in vivo. Previous mechanistic studies reported that 1400W was recovered quantitatively after iNOS fully lost its activity and modification to iNOS was not detected. Here, it is shown that 1400W is a time-, concentration-, and NADPH-dependent irreversible inactivator of iNOS. HPLC-electrospray mass spectrometric analysis of the incubation mixture of iNOS with 1400W shows both loss of heme cofactor and formation of biliverdin, as was previously observed for iNOS inactivation by another amidine-containing compound, N5-(1-iminoethyl)-L-ornithine (L-NIO). The amount of biliverdin produced corresponds to the amount of heme lost by 1400W inactivation of iNOS. A convenient MS/MS-HPLC methodology was developed to identify the trace amount of biliverdin produced by inactivation of iNOS with either 1400W or L-NIO to be biliverdin IXalpha out of the four possible regioisomers. Two mechanisms were previously proposed for iNOS inactivation by L-NIO: (1) uncoupling of the heme peroxide intermediate, leading to destruction of the heme to biliverdin; (2) abstraction of a hydrogen atom from the amidine methyl group followed by attachment to the heme cofactor, which causes the enzyme to catalyze the heme oxygenase reaction. The second mechanistic proposal was ruled out by inactivation of iNOS with d3-1400W, which produced no d2-1400W. Detection of carbon monoxide as one of the heme-degradation products further excludes the covalent heme adduct mechanism. On the basis of these results, a third mechanism is proposed in which the amidine inactivators of iNOS bind as does substrate L-arginine, but because of the amidine methyl group, the heme peroxy intermediate cannot be protonated, thereby preventing its conversion to the heme oxo intermediate. This leads to a change in the enzyme mechanism to one that resembles that of heme oxygenase, an enzyme known to convert heme to biliverdin IXalpha. This appears to be the first example of a compound that causes irreversible inactivation of an enzyme without itself becoming modified in any way.

Inducible NO synthase inhibition attenuates shear stress-induced pulmonary vasodilation in the ovine fetus

Recent studies have suggested that type II (inducible) nitric oxide (NO) synthase (NOS II) is present in the fetal lung, but its physiological roles are uncertain. Whether NOS II activity contributes to the NO-mediated fall in pulmonary vascular resistance (PVR) during shear stress-induced pulmonary vasodilation is unknown. We studied the hemodynamic effects of two selective NOS II antagonists [aminoguanidine (AG) and S-ethylisothiourea (EIT)], a nonselective NOS antagonist [nitro-L-arginine (L-NNA)], and a nonselective vasoconstrictor (U-46619) on PVR during partial compression of the ductus arteriosus (DA) in 20 chronically prepared fetal lambs (mean age 132 +/- 2 days, term 147 days). At surgery, catheters were placed in the left pulmonary artery (LPA) for selective drug infusion, an ultrasonic flow transducer was placed on the LPA to measure blood flow, and an inflatable vascular occluder was placed loosely around the DA for compression. On alternate days, a brief intrapulmonary infusion of normal saline (control), AG, EIT, L-NNA, or U-46619 was infused in random order into the LPA. The DA was compressed to increase mean pulmonary arterial pressure (MPAP) 12-15 mmHg above baseline values and held constant for 30 min. In control studies, DA compression reduced PVR by 42% from baseline values (P < 0.01). L-NNA treatment completely blocked the fall in PVR during DA compression. AG and EIT attenuated the decrease in PVR by 30 and 19%, respectively (P < 0.05). Nonspecific elevation in PVR by U-46619 did not affect the fall in PVR during DA compression. Immunostaining for NOS II identified this isoform in airway epithelium and vascular smooth muscle in the late-gestation ovine fetal lung. We conclude that selective NOS II antagonists attenuate but do not block shear stress-induced vasodilation in the fetal lung. We speculate that stimulation of NOS II activity, perhaps from smooth muscle cells, contributes in part to the NO-mediated fall in PVR during shear stress-induced pulmonary vasodilation.

Nitric oxide-generated P420 nitric oxide synthase: characterization and roles for tetrahydrobiopterin and substrate in protecting against or reversing the P420 conversion

The neuronal NO synthase (nNOS) heme binds self-generated NO, and this negatively regulates NO synthesis. Here we utilized the nNOS oxygenase domain and full-length nNOS along with various spectroscopic methods to (1) study formation of the six-coordinate ferrous NO complex and its conversion to a five-coordinate NO complex and (2) investigate the spectral and catalytic properties of the five-coordinate NO complex following its air oxidation to a ferric enzyme. NO bound quickly to ferrous nNOS oxygenase to form a six-coordinate NO complex (kon and koff values of 1.25 x 10(-)3 mM-1 s-1 and 128 s-1 at 10 degreesC, respectively) that was stable in the presence of L-arginine or tetrahydrobiopterin (BH4) but was converted to a five-coordinate NO complex in a biphasic process (k = 0.1 and 0.01 s-1 at 10 degreesC) in the absence of these molecules. Air oxidation of the ferrous six-coordinate NO complex generated an enzyme with full activity and ferrous-CO Soret absorbance at 444 nm. In contrast, oxidation of the five-coordinate NO complex generated an inactive dimer with ferrous-CO Soret absorbance at 420 nm, indicating nNOS was converted to a ferric P420 form. Incubation of ferric P420 nNOS with BH4 alone or BH4 and L-arginine resulted in time-dependent reactivation of catalysis and associated recovery of P450 character. Thus, nNOS is a heme-thiolate protein that can undergo a reversible P450-P420 conversion. BH4 has important roles in preventing P420 formation during NO synthesis, and in rescuing P420 nNOS.

Structural requirements for human inducible nitric oxide synthase substrates and substrate analogue inhibitors

Inducible nitric oxide synthase (iNOS; EC 1.14.13.39) catalyzes the NADPH-dependent oxidation of one of the free guanidino nitrogens of L-Arg to form nitric oxide and L-citrulline. Analogues of L-Arg and the inhibitor, L-N6-(1-iminoethyl)lysine, were used to define structural elements required for the binding and catalysis of compounds. L-Arg analogues with sequentially shorter methylene spacing between the guanidino group and the amino acid portion of the molecule were not iNOS substrates but were reversible inhibitors. L-Arg analogues such as agmatine with a hydroxyl substitution at the 2-amino position were substrates. Desaminoarginine was not a substrate but a reversible inhibitor. Desaminoarginine, agmatine, and argininic acid bound to the enzyme to give type I difference spectra similar to that of L-Arg. The amidino compounds L-N6-(1-iminoethyl)lysine, L-N5-(1-iminoethyl)ornithine, and N5-(1-iminoethyl)cadaverdine, but not N6-(1-iminoethyl)-6-aminocaproic acid, were NADPH-dependent, irreversible inactivators of iNOS. For both the L-Arg and L-N6-(1-iminoethyl)lysine analogues, the 2-amino group appeared to play an important role in catalytic events leading to either substrate turnover or mechanism-based inactivation. Inactivation of iNOS by L-N6-(1-iminoethyl)lysine was NADPH- and dioxygen-dependent, but low incorporation of radiolabel with DL--4, 5-3H]-N6-(1-iminoethyl)lysine indicates that the mechanism of enzyme inactivation is not covalent modification of the protein.

Comparative functioning of dihydro- and tetrahydropterins in supporting electron transfer, catalysis, and subunit dimerization in inducible nitric oxide synthase

The nitric oxide synthases (NOS) are the only heme-containing enzymes that require tetrahydrobiopterin (BH4) as a cofactor. Previous studies indicate that only the fully reduced (i.e., tetrahydro) form of BH4 can support NO synthesis. Here, we characterize pterin-free inducible NOS (iNOS) and iNOS reconstituted with eight different tetrahydro- or dihydropterins to elucidate how changes in pterin side-chain structure and ring oxidation state regulate iNOS. Seven different enzyme properties that are important for catalysis and are thought to involve pterin were studied. Only two properties were found to depend on pterin oxidation state (i.e., they required fully reduced tetrahydropterins) and were independent of side chain structure: NO synthesis and the ability to increase heme-dependent NADPH oxidation in response to substrates. In contrast, five properties were exclusively dependent on pterin side-chain structure or stereochemistry and were independent of pterin oxidation state: pterin binding affinity, and its ability to shift the heme iron to its high-spin state, stabilize the ferrous heme iron coordination structure, support heme iron reduction, and promote iNOS subunit assembly into a dimer. These results clarify how structural versus redox properties of the pterin impact on its multifaceted role in iNOS function. In addition, the data reveal that during NO synthesis all pterin-dependent steps up to and including heme iron reduction can take place independent of the pterin ring oxidation state, indicating that the requirement for fully reduced pterin occurs at a point in catalysis beyond heme iron reduction.

Role of inducible nitric oxide synthase in regulation of pulmonary vascular tone in the late gestation ovine fetus

Nitric oxide (NO) produced by NO synthase (NOS) modulates fetal pulmonary vascular tone and contributes to the fall in pulmonary vascular resistance (PVR) at birth. Although the inducible (type II) NOS isoform is present in human and rat fetal lungs, it is uncertain whether type II NOS activity contributes to vascular NO production in the fetal lung. To determine whether type II NOS is present in the ovine fetal lung and to study the potential contribution of type II NOS on the regulation of basal PVR in the fetus, we measured the hemodynamic effects of three selective type II NOS antagonists: aminoguanidine (AG), 2-amino-5,6-dihydro-6-methyl-4H-1,3 thiazine (AMT), and S-ethylisothiourea (EIT). Studies were performed after at least 72 h of recovery from surgery in 19 chronically prepared fetal lambs (133+/-3 d; 147 d, term). Brief intrapulmonary infusions of AG (140 mg), AMT (0.12 mg), and EIT (0.12 mg) increased basal PVR by 82, 69, and 77%, respectively (P < 0.05). The maximum increase in PVR occurred within 20 min, but often persisted up to 80 min. These agents also increased mean aortic pressure but did not alter the pressure gradient between the pulmonary artery and aorta, suggesting little effect on tone of the ductus arteriosus. Acetylcholine-induced pulmonary vasodilation remained intact after treatment with selective type II NOS antagonists, but not after treatment with the nonselective NOS blocker, nitro-L-arginine. Using Northern blot analysis with poly(A)+ RNA, we demonstrated the presence of two mRNA transcripts for type II NOS (4.1 and 2.6 kb) in the fetal lung. We conclude that the type II NOS isoform is present in the ovine fetal lung, and that selective type II NOS antagonists increase PVR and systemic arterial pressure in the late-gestation fetus. We speculate that type II NOS may play a physiological role in the modulation of vascular tone in the developing fetal lung.