Site and mechanism of uncoupling of nitric-oxide synthase: Uncoupling by monomerization and other misconceptions

Nitric oxide synthase (NOS) catalyzes the transformation of l-arginine, molecular oxygen (O2), and NADPH-derived electrons to nitric oxide (NO) and l-citrulline. Under some conditions, however, NOS catalyzes the reduction of O2 to superoxide (O2-) instead, a phenomenon that is generally referred to as uncoupling. In principle, both the heme in the oxygenase domain and the flavins in the reductase domain could catalyze O2- formation. In the former case the oxyferrous (Fe(II)O2) complex that is formed as an intermediate during catalysis would dissociate to heme and O2-; in the latter case the reduced flavins would reduce O2 to O2-. The NOS cofactor tetrahydrobiopterin (BH4) is indispensable for coupled catalysis. In the case of uncoupling at the heme this is explained by the essential role of BH4 as an electron donor to the oxyferrous complex; in the case of uncoupling at the flavins it is assumed that the absence of BH4 results in NOS monomerization, with the monomers incapable to sustain NO synthesis but still able to support uncoupled catalysis. In spite of little supporting evidence, uncoupling at the reductase after NOS monomerization appears to be the predominant hypothesis at present. To set the record straight we extended prior studies by determining under which conditions uncoupling of the neuronal and endothelial isoforms (nNOS and eNOS) occurred and if a correlation exists between uncoupling and the monomer/dimer equilibrium. We determined the rates of coupled/uncoupled catalysis by measuring NADPH oxidation spectrophotometrically at 340 nm and citrulline synthesis as the formation of [3H]-citrulline from [3H]-Arg. The monomer/dimer equilibrium was determined by FPLC and, for comparison, by low-temperature polyacrylamide gel electrophoresis. Uncoupling occurred in the absence of Arg and/or BH4, but not in the absence of Ca2+ or calmodulin (CaM). Since omission of Ca2+/CaM will completely block heme reduction while still allowing substantial FMN reduction, this argues against uncoupling by the reductase domain. In the presence of heme-directed NOS inhibitors uncoupling occurred to the extent that these compound allowed heme reduction, again arguing in favor of uncoupling at the heme. The monomer/dimer equilibrium showed no correlation with uncoupling. We conclude that uncoupling by BH4 deficiency takes place exclusively at the heme, with virtually no contribution from the flavins and no role for NOS monomerization.

Mobile proton triggered radical fragmentation of nitroarginine containing peptides

Protonated nitroarginine, [R(NO2) + H](+), which contains the nitroguanidine 'explosophore,' undergoes homolytic N - N nitro-imine bond cleavage to expel NO2(•) and form a radical cation of arginine in high yield (100% relative abundance) upon low-energy collision-induced dissociation (CID). Other ionization states of nitroarginine, including [R(NO2) - H](-), and a fixed-charge derivative of nitroarginine do not expel NO2(•) (<1%), but instead dissociate via heterolytic bond cleavage with abundant losses of small molecules (N2O and H2N2O2) from the nitroguanidine group. The effects of proton mobility on the CID reactions of nitroarginine containing peptides was investigated for peptide derivatives of leucine enkephalin, including XYGGFLR(NO2), X = D, G, K, and R, by examining the different protonation states: [M - H](-); [M + H](+); and [M + 2H](2+). For [M + H](+) containing the less basic N-terminal residues (X = D, G) and all [M + 2H](2+), mobile proton fragmentation reactions that result in peptide sequence ions dominate. In contrast, for peptides containing the basic N-terminal residues (R and K), the CID spectra of both the [M - H](-) and [M + H](+) are dominated by the losses of small even-electron neutrals from the nitroarginine side-chain. The fraction of nitroguanidine directed fragmentation of the nitroarginine side chain that results in bond homolysis to form [XYGGFLR](+•) by expulsion of NO2(•) increases by more than 10 times as the protonation state changes from [M - H](-) (<10%) to [M + 2H](2+) (ca. 90%) and by about four times as the acidity of the [M + H](+) N-terminal residue increases from R (19.0%) to D (76.5%). These results indicate that protonated peptides containing nitroarginine can undergo non-canonical mobile proton triggered radical fragmentation.

Minimal pharmacophoric elements and fragment hopping, an approach directed at molecular diversity and isozyme selectivity. Design of selective neuronal nitric oxide synthase inhibitors

Fragment hopping, a new fragment-based approach for de novo inhibitor design focusing on ligand diversity and isozyme selectivity, is described. The core of this approach is the derivation of the minimal pharmacophoric element for each pharmacophore. Sites for both ligand binding and isozyme selectivity are considered in deriving the minimal pharmacophoric elements. Five general-purpose libraries are established: the basic fragment library, the bioisostere library, the rules for metabolic stability, the toxicophore library, and the side chain library. These libraries are employed to generate focused fragment libraries to match the minimal pharmacophoric elements for each pharmacophore and then to link the fragment to the desired molecule. This method was successfully applied to neuronal nitric oxide synthase (nNOS), which is implicated in stroke and neurodegenerative diseases. Starting with the nitroarginine-containing dipeptide inhibitors we developed previously, a small organic molecule with a totally different chemical structure was designed, which showed nanomolar nNOS inhibitory potency and more than 1000-fold nNOS selectivity. The crystallographic analysis confirms that the small organic molecule with a constrained conformation can exactly mimic the mode of action of the dipeptide nNOS inhibitors. Therefore, a new peptidomimetic strategy, referred to as fragment hopping, which creates small organic molecules that mimic the biological function of peptides by a pharmacophore-driven strategy for fragment-based de novo design, has been established as a new type of fragment-based inhibitor design. As an open system, the newly established approach efficiently incorporates the concept of early "ADME/Tox" considerations and provides a basic platform for medicinal chemistry-driven efforts.

Structure-based design and synthesis of N(omega)-nitro-L-arginine-containing peptidomimetics as selective inhibitors of neuronal nitric oxide synthase. Displacement of the heme structural water

The neuronal isoform of nitric oxide synthase (nNOS), the enzyme responsible for the production of nitric oxide in the central nervous system, represents an attractive target for the treatment of various neurodegenerative disorders. X-ray crystal structures of complexes of nNOS with two nNOS-selective inhibitors, (4S)-N-{4-amino-5-[(2-aminoethylamino]pentyl}-N'-nitroguanidine (1) and 4-N-(Nomega-nitro-l-argininyl)-trans-4-amino-l-proline amide (2), led to the discovery of a conserved structural water molecule that was hydrogen bonded between the two heme propionates and the inhibitors (Figure 2). On the basis of this observation, we hypothesized that by attaching a hydrogen bond donor group to the amide nitrogen of 2 or to the secondary amine nitrogen of 1, the inhibitor molecules could displace the structural water molecule and obtain a direct interaction with the heme cofactor. To test this hypothesis, peptidomimetic analogues 3-5, which have either an N-hydroxyl (3 and 5) or N-amino (4) donor group, were designed and synthesized. X-ray crystal structures of nNOS with inhibitors 3 and 5 bound verified that the N-hydroxyl group had, indeed, displaced the structural water molecule and provided a direct interaction with the heme propionate moiety (Figures 5 and 6). Surprisingly, in vitro activity assay results indicated that the addition of a hydroxyl group (3) only increased the potency slightly against the neuronal isoform over the parent compound (1). Rationalizations for the small increase in potency are consistent with other changes in the crystal structures.

Exploring the binding conformations of bulkier dipeptide amide inhibitors in constitutive nitric oxide synthases

A series of L-nitroarginine-based dipeptide inhibitors are highly selective for neuronal nitric oxide synthase (nNOS) over the endothelial isoform (eNOS). Crystal structures of these dipeptides bound to both isoforms revealed two different conformations, curled in nNOS and extended in eNOS, corresponding to higher and lower binding affinity to the two isoforms, respectively. In previous studies we found that the primary reason for selectivity is that Asp597 in nNOS, which is Asn368 in eNOS, provides greater electrostatic stabilization in the inhibitor complex. While this is the case for smaller dipeptide inhibitors, electrostatic stabilization may no longer be the sole determinant for isoform selectivity with bulkier dipeptide inhibitors. Another residue farther away from the active site, Met336 in nNOS (Val106 in eNOS), is in contact with bulkier dipeptide inhibitors. Double mutants were made to exchange the D597/M336 pair in nNOS with N368/V106 in eNOS. Here we report crystal structures and inhibition constants for bulkier dipeptide inhibitors bound to nNOS and eNOS that illustrate the important role played by residues near the entry to the active site in isoform selective inhibition.

Impaired endothelium-dependent responses and enhanced influence of Rho-kinase in cerebral arterioles in type II diabetes

BACKGROUND AND PURPOSE

Although the incidence of type II diabetes is increasing, very little is known regarding vascular responses in the cerebral circulation in this disease. The goals of this study were to examine the role of superoxide in impaired endothelium-dependent responses and to examine the influence of Rho-kinase on vascular tone in the cerebral microcirculation in type II diabetes.

METHODS

Diameter of cerebral arterioles (29+/-1 microm; mean+/-SE) was measured in vivo using a cranial window in anesthetized db/db and control mice.

RESULTS

Dilatation of cerebral arterioles in response to acetylcholine (ACh; 1 and 10 micromol/L), but not to nitroprusside, was markedly reduced in db/db mice (eg, 10 micromol/L ACh produced 29+/-1% and 9+/-1% in control and db/db mice, respectively). Superoxide levels were increased (P<0.05) in cerebral arterioles from db/db mice (n=6) compared with controls (n=6). Vasodilatation to ACh in db/db mice was restored to normal by polyethylene glycol-superoxide dismutase (100 U/mL). Y-27632 (1 to 100 micromol/L; a Rho-kinase inhibitor) produced modest vasodilatation in control mice but much greater responses in db/db mice. N(G)-nitro-L-arginine (100 micromol/L; an inhibitor of NO synthase) significantly enhanced Y-27632-induced dilatation in control mice to similar levels as observed in db/db mice.

CONCLUSIONS

These findings provide the first evidence for superoxide-mediated impairment of endothelium-dependent responses of cerebral vessels in any model of type II diabetes. In addition, the influence of Rho-kinase on resting tone appears to be selectively enhanced in the cerebral microcirculation in this genetic model of type II diabetes.

Renal d-Amino Acid Oxidase Mediates Chiral Inversion ofNG-Nitro-d-arginine

N(G)-nitro-d-arginine (d-NNA), i.v. injected into rats, produced a pressor response, and was presumed to act via chiral inversion into N(G)-nitro-l-arginine (l-NNA), an inhibitor of nitric oxide synthase. We examined the possible role of renal d-amino acid oxidase (DAAO) in the chiral inversion of d-NNA to l-NNA. In pentobarbital-anesthetized rats, l-NNA was detected via capillary electrochromatography in the blood immediately after i.v. injection of d-NNA. The time course of appearance of l-NNA paralleled the increase in blood pressure elicited by d-NNA. Unilateral renal ligation partially, and bilateral ligation completely, blocked the pressor response as well as the conversion of d-NNA to l-NNA. Furthermore, injection into conscious rats of sodium benzoate, a selective DAAO inhibitor, completely blocked the pressor response to naive d-NNA, but not pressor response to d-NNA preincubated with homogenates of the kidney. Homogenates of the kidneys, liver (lesser degree), and brain (much lesser degree) converted d-NNA to l-NNA, and the chiral inversion was blocked by the addition of benzoate. Moreover, d-NNA chiral inversion correlates with the activity of DAAO. Our results reveal a novel pathway of chiral inversion of d-amino acids where the renal DAAO plays an essential role that accounts for the biological activity of d-NNA.

Structures of the Neuronal and Endothelial Nitric Oxide Synthase Heme Domain with d-Nitroarginine-Containing Dipeptide Inhibitors Bound

In a continuing effort to unravel the structural basis for isoform-selective inhibition of nitric oxide synthase (NOS) by various inhibitors, we have determined the crystal structures of the nNOS and eNOS heme domain bound with two D-nitroarginine-containing dipeptide inhibitors, D-Lys-D-Arg(NO)2-NH(2) and D-Phe-D-Arg(NO)2-NH(2). These two dipeptide inhibitors exhibit similar binding modes in the two constitutive NOS isozymes, which is consistent with the similar binding affinities for the two isoforms as determined by K(i) measurements. The D-nitroarginine-containing dipeptide inhibitors are not distinguished by the amino acid difference between nNOS and eNOS (Asp 597 and Asn 368, respectively) which is key in controlling isoform selection for nNOS over eNOS observed for the L-nitroarginine-containing dipeptide inhibitors reported previously [Flinspach, M., et al. (2004) Nat. Struct. Mol. Biol. 11, 54-59]. The lack of a free alpha-amino group on the D-nitroarginine moiety makes the dipeptide inhibitor steer away from the amino acid binding pocket near the active site. This allows the inhibitor to extend into the solvent-accessible channel farther away from the active site, which enables the inhibitors to explore new isoform-specific enzyme-inhibitor interactions. This might be the structural basis for why these D-nitroarginine-containing inhibitors are selective for nNOS (or eNOS) over iNOS.

Computer modeling of selective regions in the active site of nitric oxide synthases: implication for the design of isoform-selective inhibitors

Selective inhibition of nitric oxide synthase (NOS) isoforms has great therapeutic potential in the treatment of certain disease states arising from the pathological overproduction of nitric oxide. In this study three structures of each NOS isoform were employed to examine selective regions in the active site using the GRID/CPCA approach. In the GRID calculations, 10 probes covering hydrophobic, steric, and hydrogen-bond-acceptor and -donor interactions were used to calculate the molecular interaction fields (MIFs) in the active site. The side chain flexibility of the residues and the grid spacings were considered at the same time. Consensus principal component analysis (CPCA) was applied to analyze the MIFs differences in the active site between the NOS isoforms. By combining the cutout tool with GRID/CPCA pseudofield differential plots, several selective regions in the active site were identified. The selectivity analysis showed that the most important determinants for NOS inhibitor selectivity are hydrophobic and charge-charge interactions. Twenty-five inhibitors of NOS were then docked into the active site using the program AutoDock3.0. The regions identified as being important for selectivity by this method are in excellent agreement with inhibitor structure-activity relationships. A rational usage of the selective region described in this work should make it possible to develop NOS isoform-selective inhibitors.

Potent and Selective Conformationally Restricted Neuronal Nitric Oxide Synthase Inhibitors

Selective inhibition of the isoforms of nitric oxide synthase (NOS) in pathologically elevated synthesis of nitric oxide has great therapeutic potential. We previously reported nitroarginine-containing dipeptide amides and some peptidomimetic analogues as potent and selective inhibitors of neuronal NOS (nNOS). Here we report conformationally restricted dipeptides derived from the dipeptide L-Arg(NO2)-L-Dbu-NH2 (8). The selectivities for nNOS over endothelial NOS and inducible NOS of the most potent nNOS inhibitor (10a) among these compounds are comparable to that of the parent compound. An unsubstituted amide bond is necessary for potency against nNOS. The stereochemistry of compound 10a was optimum for potency and selectivity and thus provides the binding conformation of the parent compound with nNOS.

Effects of iNOS-related NO on hearts exposed to liposoluble iron

Inducible nitric oxide synthase (iNOS) protects heart against ischemia/reperfusion injury. However, it is unknown whether the beneficial effects of iNOS are mediated by the interaction of NO with radical oxygen species (ROS). To address this issue, we examined the effects of liposoluble iron-induced ROS generation in isolated perfused hearts from rats treated with lipopolysaccharide (LPS). LPS administration (10 mg/kg, i.p., 6 h before heart removal) induced iNOS expression and increased NO production as indicated by a 3-fold elevation of nitrite level in coronary effluents relative to control hearts. An enhanced expression of hemeoxygenase 1 protein was also observed in septic hearts compared to control. Iron-induced perfusion and contractile deficits were ameliorated by LPS with more important coronary than myocardial benefits. In iron-loaded hearts, oxidative stress as measured by the 2,3 dihydroxybenzoic acid/salicylic acid concentration ratio in cardiac tissue was 23% lower in septic than in control heart although the difference did not reach significance. In addition, the presence of the NO synthase inhibitor N-nitro-L-arginine in the perfusion medium totally blocked NO production but did not reverse the protective effects of LPS. The results indicate that LPS protects from iron-induced cardiac dysfunction by mechanisms independent on ex vivo NO production and suggest that NO acts as a trigger rather than a direct mediator of the cardioprotective effects of LPS in heart exposed to iron.

Synthesis and evaluation of dipeptide amides containing N omega-nitroarginine and D-2,4-diaminobutyric acids as inhibitors of neuronal nitric oxide synthase

Selective inhibition of the isoforms of nitric oxide synthase (NOS) could be beneficial in the treatment of certain disease states arising from the overproduction of nitric oxide by NOS. Recently, we reported dipeptide amides containing a basic amine side chain as potent and selective inhibitors of neuronal NOS (Huang H. et al. (1999) J. Med. Chem., 42, 3147). The most potent nNOS inhibitor among these compounds is L-ArgNO2-L-Dbu-NH2 (1) (Ki = 130 nM), which also exhibits the highest selectivity over eNOS (> 1500-fold). The D,D-dipeptide, D-Lys-D-ArgNO2-NH2 (3) also shows high potency and selectivity. Here the dipeptide amides containing ArgNO2 and D-Dbu (9-12) were synthesized and evaluated. They are all modest inhibitors of nNOS, but poor inhibitors of eNOS and iNOS. D-Dbu-D-ArgNO2-NH2 (12) exhibits decreased inhibitory potency as compared with 3. A hypothesis regarding the binding at the active site of nNOS is proposed to explain the potency differences between the L- and D-form dipeptide amides.

Crystal structure of nitric oxide synthase bound to nitro indazole reveals a novel inactivation mechanism

Nitric oxide is generated under normal and pathophysiological conditions by three distinct isoforms of nitric oxide synthase (NOS). A small-molecule inhibitor of NOS (3-Br-7-nitroindazole, 7-NIBr) is profoundly neuroprotective in mouse models of stroke and Parkinson's disease. We report the crystal structure of the catalytic heme domain of endothelial NOS complexed with 7-NIBr at 1.65 A resolution. Critical to the binding of 7-NIBr at the substrate site is the adoption by eNOS of an altered conformation, in which a key glutamate residue swings out toward one of the heme propionate groups. Perturbation of the heme propionate ensues and eliminates the cofactor tetrahydrobiopterin-heme interaction. We also present three crystal structures that reveal how alterations at the substrate site facilitate 7-NIBr and structurally dissimilar ligands to occupy the cofactor site.

NO and prostaglandin interactions during hemodynamic stress in the fetal ovine pulmonary circulation

Nitric oxide (NO) and prostacyclin (PGI(2)) are potent fetal pulmonary vasodilators, but their relative roles and interactions in the regulation of the perinatal pulmonary circulation are poorly understood. We compared the separate and combined effects of nitric oxide synthase (NOS) and cyclooxygenase (COX) inhibition during acute hemodynamic stress caused by brief mechanical compression of the ductus arteriosus (DA) in chronically prepared fetal lambs. Nitro-L-arginine (L-NNA; NOS antagonist), meclofenamate (Mec; COX inhibitor), combined drugs (L-NNA-Mec), or saline (control) was infused into the left pulmonary artery (LPA) before DA compression. In controls, DA compression decreased pulmonary vascular resistance (PVR) by 43% (P < 0.01). L-NNA, but not Mec, treatment completely blocked vasodilation and caused a paradoxical increase in PVR (+31%; P < 0.05). The effects of L-NNA-Mec and L-NNA on PVR were similar. To determine if the vasodilator effect of PGI(2) is partly mediated by NO release, we studied PGI(2)-induced vasodilation before and after NOS inhibition. L-NNA treatment blocked the PGI(2)-induced rise in LPA blood flow by 73% (P < 0.001). We conclude that NO has a greater role than PGs in fetal pulmonary vasoregulation during acute hemodynamic stress and that PGI(2)-induced pulmonary vasodilation is largely mediated by NO release in the fetal lung.

N(omega)-arginine dimethylation modulates the interaction between a Gly/Arg-rich peptide from human nucleolin and nucleic acids

We studied the interaction between a synthetic peptide (sequence Ac-GXGGFGGXGGFXGGXGG-NH(2), where X = arginine, N(omega),N(omega)-dimethylarginine, DMA, or lysine) corresponding to residues 676-692 of human nucleolin and several DNA and RNA substrates using double filter binding, melting curve analysis and circular dichroism spectroscopy. We found that despite the reduced capability of DMA in forming hydrogen bonds, N(omega),N(omega)-dimethylation does not affect the strength of the binding to nucleic acids nor does it have any effect on stabilization of a double-stranded DNA substrate. However, circular dichroism studies show that unmethylated peptide can perturb the helical structure, especially in RNA, to a much larger extent than the DMA peptide.

Potentiation of penile tumescence by T-1032, a new potent and specific phosphodiesterase type V inhibitor, in dogs

We examined the mechanism underlying the potentiation of penile tumescence by methyl 2-(4-aminophenyl)-1, 2dihydro-1-oxo-7-(2-pyridinylmethoxy)-4-(3,4, 5-trimethoxyphenyl)3-isoquinoline carboxylate sulfate (T-1032), a new potent and selective phosphodiesterase type V inhibitor. In vivo, pelvic nerve stimulation induced a penile tumescence together with increase of total nitric oxide metabolite levels within the corpus cavernosa of anesthetized dogs. Intravenous (1-100 microg/kg) and intraduodenal (3, 30, 300 microg/kg) treatment with T-1032 dose dependently potentiated the tumescence. The potency of T-1032 was equivalent to that of sildenafil. T-1032 did not influence the intracavernous pressure when the pelvic nerve stimulation was absent. The potentiation of tumescence was more pronounced by intracavernous than i.v. injection. Intracavernous N(G)-nitro-L-arginine, a nitric-oxide synthase inhibitor, but not N(G)-nitro-D-arginine diminished the effects of T-1032 on the tumescence. Furthermore, i.v. T-1032 augmented the tumescence induced by sodium nitroprusside (SNP) but not by vasoactive intestinal polypeptide (VIP). In vitro, in isolated preparations of canine corpus cavernosum precontracted with phenylephrine, SNP (0. 01-100 microM) and VIP (0.01-1 microM) produced a dose-dependent relaxation accompanied by an increase in cGMP and cAMP levels, respectively. T-1032 augmented the relaxation induced by SNP but not by VIP. These data suggest that oral treatment with T-1032 has potential to improve erectile dysfunction through the inhibition of phosphodiesterase type V in the smooth muscles of corpus cavernosa.

Reduction-oxidation (redox) state regulation of matrix metalloproteinase activity in human fetal membranes

OBJECTIVE

The mechanisms underlying membrane rupture at term and preterm are obscure. Collagenolytic activity of matrix metalloproteinases in amniochorionic membranes increases during spontaneous term and preterm labor associated with intra-amniotic infection. We sought to test the hypothesis that reduction-oxidation homeostasis, which is altered in inflammatory states, directly regulates amniochorionic matrix metalloproteinases.

STUDY DESIGN

Membranes were collected from 7 patients undergoing elective cesarean delivery at term, rinsed thoroughly, and immediately incubated in phosphate-buffered sodium chloride solution at 37 degrees C for 24 hours. Matrix metalloproteinase activity in the culture medium was assayed by substrate-gel electrophoresis and normalized against the dry weight of the tissue incubated. Superoxide anions were generated in the presence of membranes by a xanthine (2 mmol/L) and xanthine oxidase (20 mU/mL) mixture and monitored by reduction of ferri-cytochrome c to ferro-cytochrome c. Incubations were performed in the presence of xanthine alone, a xanthine-xanthine oxidase mixture, superoxide dismutase (500 U/mL), a xanthine-xanthine oxidase-superoxide dismutase mixture, nitro-L-arginine (a nitric oxide synthase inhibitor, 1 mmol/L), xanthine-xanthine oxidase-nitro-L-arginine, S-nitroso-N -acetylpenicillamine (a nitric oxide donor, 10 mmol/L), xanthine-xanthine oxidase-S-nitroso-N -acetylpenicillamine, N -acetylcysteine (a thiol-containing antioxidant, 0.1, 1, or 10 mmol/L), lipopolysaccharide (100 ng/mL), or lipopolysaccharide-N -acetylcysteine. Intracellular generation of superoxide anions was monitored by the reduction of nitroblue tetrazolium to formazan.

RESULTS

Basal matrix metalloproteinase 9 and matrix metalloproteinase 2 levels were detected in all samples. Superoxide anions significantly increased matrix metalloproteinase 9 activity but did not increase matrix metalloproteinase 2 activity, which effect was reversed by the addition of superoxide dismutase. N-acetylcysteine reduced basal activity of both matrix metalloproteinase 9 and matrix metalloproteinase 2 to 20%. Importantly, N-acetylcysteine completely inhibited intracellular formazan formation in cultured membranes both in the absence and in the presence of lipopolysaccharide. Neither nitric oxide synthase inhibition nor the nitric oxide donor S-nitroso-N -acetylpenicillamine had any effect on fetal membrane matrix metalloproteinase activity.

CONCLUSION

Matrix metalloproteinase activity in human fetal membranes is reduction-oxidation (redox)-regulated. Matrix metalloproteinase 9 activity in human fetal membranes is directly increased by superoxide anion, a byproduct of macrophages and neutrophils. Neither nitric oxide donors nor nitric oxide synthase inhibitors significantly affect matrix metalloproteinase activity in human fetal membranes. The glutathione precursor N-acetylcysteine dramatically inhibits amniochorionic matrix metalloproteinase activity in addition to inhibiting intrinsic superoxide generation within the tissue. Thus thiol-reducing agents, such as N-acetylcysteine, may be beneficial in preventing preterm premature rupture of the membranes.

N(omega)-Nitroarginine-containing dipeptide amides. Potent and highly selective inhibitors of neuronal nitric oxide synthase

Selective inhibition of the isoforms of nitric oxide synthase (NOS) could be therapeutically useful in the treatment of certain disease states arising from the overproduction of nitric oxide (NO). Recently, we reported the dipeptide methyl ester, D-Phe-D-Arg(NO)()2-OMe (19), as a modest inhibitor of nNOS (K(i) = 2 microM), but with selectivity over iNOS as high as 1800-fold (Silverman, R. B.; Huang, H.; Marletta, M. A.; Martasek, P. J. Med. Chem. 1997, 40, 2813-2817). Here a library of 152 dipeptide amides containing nitroarginine and amino acids other than Phe are synthesized and screened for activity. Excellent inhibitory potency and selectivity for nNOS over eNOS and iNOS is achieved with the dipeptide amides containing a basic amine side chain (20-24), which indicates a possible electrostatic (or hydrogen bonding) interaction at the enzyme active site. The most potent nNOS inhibitor among these compounds is L-Arg(NO)()2-L-Dbu-NH(2) (23) (K(i) = 130 nM), which also exhibits the highest selectivity over eNOS (>1500-fold) with a 192-fold selectivity over iNOS. These compounds do not exhibit time-dependent inhibition. The order and the chirality of the amino acids in the dipeptide amides have profound influences on the inhibitory potency as well as on the isoform selectivity. These dipeptide amide inhibitors open the door to the design of potent and highly selective inhibitors of nNOS.

Bilateral kidney ligation abolishes pressor response to N(G)-nitro-D-arginine

We have shown that N(G)-nitro-D-arginine (D-NNA) is 50% as potent as N(G)-nitro-L-arginine (L-NNA) in causing pressor response and 2-3% as potent as L-NNA in inhibiting endothelium-dependent relaxation in vitro. These results suggest in vivo activation of D-NNA. Furthermore, the potency of D-NNA was markedly increased after it had been incubated with homogenate of the kidney, but not plasma or homogenate of the aorta, lungs or liver. This study examined if bilateral ligation of the kidneys attenuated the biological action of D-NNA. I.v. bolus of D-NNA (16 mg/kg), L-NNA (3 mg/kg) and norepinephrine (0.25-16 microg/kg) increased arterial pressure in sham-operated rats. Bilateral ligation of the kidneys abolished pressor response to D-NNA, but not L-NNA and norepinephrine. I.v. bolus D-NNA in sham-operated rats, but not kidney-ligated rats, inhibited relaxation response to acetylcholine in pre-constricted aortic rings ex vivo. These results indicate that the kidney is the primary organ which activates D-NNA.

Non-enzymatic production of nitric oxide (NO) from NO synthase inhibitors

The gaseous signal molecule, nitric oxide (NO*), is generated enzymatically by NO synthase (NOS) from L-arginine. Overproduction of NO contributes to cell and tissue damage as sequelae of infection and stroke. Strategies to suppress NO synthesis rely heavily on guanidino-substituted L-arginine analogs (L-NAME, L-NA, L-NMMA, L-NIO) as competitive inhibitors of NOS, which are often used in high doses to compete with millimolar concentrations of intracellular arginine. We show that these analogs are also a source for non-enzymatically produced NO. Enzyme-independent NO release occurs in the presence of NADPH, glutathione, L-cysteine, dithiothreitol and ascorbate. This non-enzymatic synthesis of NO can produce potentially toxic, micromolar concentrations of NO and can oppose the effects of NOS inhibition. NO production driven by NOS inhibitors was demonstrated ex vivo in the central nervous and peripheral tissues of gastropod molluscs Aplysia and Pleurobranchaea using electron paramagnetic resonance and spin-trapping techniques. These results have important implications for therapeutic regulation of NO homeostasis.

Selective inhibition of neuronal nitric oxide synthase by N omega-nitroarginine-and phenylalanine-containing dipeptides and dipeptide esters

A series of N omega-nitroarginine (ArgNO2)- and phenylalanine-containing dipeptides and dipeptide esters were synthesized as potential selective inhibitors of neuronal nitric oxide synthase (nNOS). All of the dipeptides and dipeptide esters are competitive inhibitors of nNOS, macrophage nitric oxide synthase (iNOS), and endothelial nitric oxide synthase (eNOS), except for the ones that contain D-ArgNO2 (8-10, 12, 13), which are uncompetitive inhibitors of iNOS but competitive inhibitors of nNOS and eNOS. None of the dipeptides or dipeptide esters tested (1, 2, 12, 13) exhibited time-dependent inhibition of any of the NOS isoforms, unlike N omega-nitro-L-arginine itself, which does, although it is reversible. The order of the amino acids in the dipeptide or dipeptide ester is important to selectivity, and the selectivity depends on the chirality of the amino acids. In the case of the corresponding benzyl esters (5 vs 6), both dipeptides favor iNOS over nNOS and eNOS inhibition. All of the dipeptide methyl esters containing a D-amino acid, however, exhibit an inhibitory preference for nNOS over iNOS and eNOS. The most impressive selectivities observed are 1800- and 800-fold for 12 and 13, respectively, in favor of nNOS over iNOS; unfortunately, the selectivities of these compounds for nNOS over eNOS are only 2.5 and 5.3, respectively.

Pharmacokinetics and steady-state tissue distribution of L- and D-isomers of nitroarginine in rats

Nitric oxide synthase (NOS) inhibitors, such as nitro-L-arginine (L-NNA), have been used in vivo as mechanistic probes of the NOS system and as potential therapeutic agents for reversing the hypotension developed in septic shock. Little information is available regarding the pharmacokinetic and biodistribution pattern of these compounds. We have examined the in vivo disposition, as well as steady-state biodistribution, of NNA isomers in rats. Plasma and tissue concentrations of L-NNA were determined by HPLC. After intravenous administration of a bolus dose of 20 mg/kg in rats, plasma concentrations of both L- and D-NNA declined biexponentially, with average half-lives of 12 min and 20 hr for L-NNA, and 15 min and 15 hr for the D-enantiomer, respectively. In contrast to L-NNA, the D-isomer had a higher systemic clearance (170 +/- 20 vs. 70.9 +/- 8.2 ml/hr/kg; p = 0.0004) and shorter mean residence time (17.3 +/- 3.7 vs. 31.7 +/- 3.7 hr; p = 0.0039). Based on these pharmacokinetic characteristics, steady-state plasma concentrations of NNA isomers were achieved within 6-8 hr through the use of a loading dose and maintenance infusion. NNA concentrations achieved in many tissues exceeded plasma concentrations, indicating binding of the drug to tissue components. The tissue-to-plasma distribution coefficient (Kp) for L-NNA was variable among various tissues and ranged from 0.67 for testes to 3.17 for liver. Both kidneys and skeletal muscle had Kp values higher than 2, whereas distribution to the cerebrospinal fluid was minimal (Kp = 0.09). Steady-state distribution of the D-isomer of NNA was similar to that of L-NNA.