Phytoene desaturase inhibition by O-(2-phenoxy)ethyl-N-aralkylcarbamates

O-[1-Ethyl-2-(3-trifluoromethylphenoxy)]ethyl-N-benzylcarbamate exhibits a marked inhibition of carotenoid biosynthesis. Forty-one analogues were synthesized and assayed for plant-type phytoene desaturase (PDS) and zeta-carotene desaturase (ZDS) inhibition in a cell-free system using recombinant enzymes obtained from Escherichia coli transformants. The target enzyme of all carbamates synthesized in this study is PDS and not ZDS; no inhibition of ZDS was observed using a 10(-4) M inhibitor concentration. Four compounds, O-[1-ethyl-2-(3-trifluoromethylphenoxy)]ethyl-N-(2-phenylethyl)carbamate (23), O-[1-ethyl-2-(3-trifluoromethylphenoxy)]ethyl-N-(2-chlorobenzyl)carbamate (25), O-[1-ethyl-2-(3-trifluoromethylphenoxy)]ethyl-N-(2-chlorobenzyl)carbamate (26), and O-[1-methyl-2-(3-trifluoromethylphenoxy)]ethyl-N-benzylcarbamate (30), were the most potent PDS inhibitors. Their pI(50) values, the negative logarithms of the molar concentration that produces a 50% inhibition, were 7.5, representing the same inhibitory activity as norflurazon. With respect to a structure-activity relationship the oxygen atom of the phenoxy group and a carbamate structure in O-(1-ethyl-2-phenoxy)ethyl-N-aralkylcarbamates studied were found to be essential for strong PDS inhibitors. Also, introduction of an ethyl group at the alpha-position of the ethylene bridge between the phenoxy group and the carbamate was important for a strong PDS inhibitor. Substituents at the 2- and/or 3-position of the phenoxybenzene ring were found to be favorable to a strong PDS inhibition of the analogues.

Synthesis and Properties of Condensed Lumazine-Ringsystems

1,3-Dimethyllumazine-6,7-diamine (1) and its monomethyl -(3,4) and monophenylamino (5) derivates are interesting starting materials for the synthesis of imidazo[4,5-g] (6-22) and pyrazino[2,3-g]lumazines (23-26). Ringclosure proceeds under relative drastic conditions and the resulting reaction products are highly fluorescent. Methylation of 5,7-dimethyll-imidazo[4,5-g)lumazine (6) led to an isomeric mixture of the 1- (17) and 3-methyl derivatives (7) which could be separated chromatographically. Their structural assignment was based on unambiguous syntheses heating 4 and 3, respectively, with formamide under reflux for ringclosure. The newly synthesized compounds have been characterized by 1H-NMR and UV-spectra as well as elemental analyses.

Structural requirements for inhibition of the neuronal nitric oxide synthase (NOS-I): 3D-QSAR analysis of 4-oxo- and 4-amino-pteridine-based inhibitors

The family of homodimeric nitric oxide synthases (NOS I-III) catalyzes the generation of the cellular messenger nitric oxide (NO) by oxidation of the substrate L-arginine. The rational design of specific NOS inhibitors is of therapeutic interest in regulating pathological NO levels associated with sepsis, inflammatory, and neurodegenerative diseases. The cofactor (6R)-5,6,7,8-tetrahydrobiopterin (H(4)Bip) maximally activates all NOSs and stabilizes enzyme quaternary structure by promoting and stabilizing dimerization. Here, we describe the synthesis and three-dimensional (3D) quantitative structure-activity relationship (QSAR) analysis of 65 novel 4-amino- and 4-oxo-pteridines (antipterins) as inhibitors targeting the H(4)Bip binding site of the neuronal NOS isoform (NOS-I). The experimental binding modes for two inhibitors complexed with the related endothelial NO synthase (NOS-III) reveal requirements of biological affinity and form the basis for ligand alignment. Different alignment rules were derived by building other compounds accordingly using manual superposition or a genetic algorithm for flexible superposition. Those alignments led to 3D-QSAR models (comparative molecular field analysis (CoMFA) and comparative molecular similarity index analysis (CoMSIA)), which were validated using leave-one-out cross-validation, multiple analyses with two and five randomly chosen cross-validation groups, perturbation of biological activities by randomization or progressive scrambling, and external prediction. An iterative realignment procedure based on rigid field fit was used to improve the consistency of the resulting partial least squares models. This led to consistent and highly predictive 3D-QSAR models with good correlation coefficients for both CoMFA and CoMSIA, which correspond to experimentally determined NOS-II and -III H(4)Bip binding site topologies as well as to the NOS-I homology model binding site in terms of steric, electrostatic, and hydrophobic complementarity. These models provide clear guidelines and accurate activity predictions for novel NOS-I inhibitors.

Inhibition of morphine-potentiated HIV-1 replication in peripheral blood mononuclear cells with the nuclease-resistant 2-5A agonist analog, 2-5A(N6B)

Opioids potentiate HIV-1 infection in vitro at least partly by suppressing immunoresponsive processes in human lymphocytes and monocytes. For example, it appears that morphine inhibits the interferon (IFN)-alpha, -beta, and -gamma-mediated natural antiviral defense pathways in human peripheral blood mononuclear cells (PBMC). In this study, we show that restoration of a key component of the antiviral pathway reverses morphine-potentiated HIV-1 infection of human PBMC. The data show that HIV-1 replication is potentiated and RNase L activity is inhibited after morphine administration. Because HIV-1 inhibits the antiviral pathway at the level of 2',5'-oligoadenylate (2-5A) synthetase and p68 kinase, antiviral enzymes that require double-stranded RNA, we overcame this blockade by the addition of the nuclease-resistant, nontoxic 2-5A agonist, 2-5A(N6B), to PBMC in culture. Addition of 2-5A(N6B), but not zidovudine or saquinavir, to morphine-treated PBMC completely reversed the morphine-induced potentiation of HIV-1 infection. Further, 2-5A(N6B) significantly enhanced expression of both IFN-alpha and IFN-gamma. Also, increased expression of IFN-gamma was associated with a significant increase in expression of RANTES and monocyte chemotactic protein (MCP)-1, chemokines that may inhibit HIV-1 infection by blocking viral attachment to CCR2 and CCR5 co-receptors. Our results suggest that reactivation of the antiviral pathway by 2-5A agonists may be useful to inhibit opioid-potentiated HIV-1 replication.

Structural and functional features of the 37-kDa 2-5A-dependent RNase L in chronic fatigue syndrome

A 2',5'-oligoadenylate (2-5A)-dependent 37-kDa form of RNase L has been reported in extracts of peripheral blood mononuclear cells (PBMC) from individuals with chronic fatigue syndrome (CFS). In the current study, analytic gel permeation FPLC, azido photoaffinity labeling, two-dimensional (2-D) gel electrophoresis, and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) have been used to examine the biochemical relationship between the 80-kDa RNase L in healthy control PBMC and the 37-kDa RNase L in PBMC from individuals with CFS. Like the 80-kDa RNase L, the 37-kDa RNase L is present as a catalytically inactive heterodimer complex with the RNase L inhibitor (RLI). Formation of a 37-kDa RNase L-RLI complex indicates that the 37-kDa RNase L is structurally similar to the 80-kDa RNase L at the N-terminus, which contains the 2-5A binding domain. The enzymatically active monomer form of 37-kDa RNase L resolved by 2-D gel electrophoresis has a pI of 6.1. RT-PCR and Southern blot analyses demonstrated that the 37-kDa RNase L is not formed by alternative splicing. In-gel tryptic digestion of the 37-kDa RNase L that was excised from 2-D gels and subsequent MALDI-MS analysis identified three peptide masses that are identical to three predicted peptide masses in the 80-kDa RNase L. The electrophoretic mobility of 2-5A azido photolabeled/immunoprecipitated 37-kDa RNase L was the same under reducing and nonreducing conditions. The results presented show that the 37-kDa form of RNase L in PBMC shares structural and functional features with the native 80-kDa RNase L, in particular in the 2-5A binding and catalytic domains.

Structural basis for pterin antagonism in nitric-oxide synthase. Development of novel 4-oxo-pteridine antagonists of (6R)-5,6,7,8-tetrahydrobiopterin

Pathological nitric oxide (NO) generation in sepsis, inflammation, and stroke may be therapeutically controlled by inhibiting NO synthases (NOS). Here we targeted the (6R)-5,6,7,8-tetrahydro-l-biopterin (H(4)Bip)-binding site of NOS, which, upon cofactor binding, maximally increases enzyme activity and NO production from substrate l-arginine. The first generation of H(4)Bip-based NOS inhibitors employed a 4-amino pharmacophore of H(4)Bip analogous to antifolates such as methotrexate. We developed a novel series of 4-oxo-pteridine derivatives that were screened for inhibition against neuronal NOS (NOS-I) and a structure-activity relationship was determined. To understand the structural basis for pterin antagonism, selected derivatives were docked into the NOS pterin binding cavity. Using a reduced 4-oxo-pteridine scaffold, derivatives with certain modifications such as electron-rich aromatic phenyl or benzoyl groups at the 5- and 6-positions, were discovered to markedly inhibit NOS-I, possibly due to hydrophobic and electrostatic interactions with Phe(462) and Ser(104), respectively, within the pterin binding pocket. One of the most effective 4-oxo compounds and, for comparisons an active 4-amino derivative, were then co-crystallized with the endothelial NOS (NOS-III) oxygenase domain and this structure solved to confirm the hypothetical binding modes. Collectively, these findings suggest (i) that, unlike the antifolate principle, the 4-amino substituent is not essential for developing pterin-based NOS inhibitors and (ii), provide a steric and electrostatic basis for their rational design.

Synthesis and fluorescence characterization of pteridine adenosine nucleoside analogs for DNA incorporation

Two fluorescent adenosine analogs, 4-amino-6-methyl-8-(2-deoxy-beta-d-ribofuranosyl)-7(8H)-pteridone (6MAP) and 4-amino-2,6-dimethyl-8-(2'-deoxy-beta-d-ribofuranosyl)-7(8H)-pteridone (DMAP), have been synthesized as phosphoramidites. These probes are site-selectively incorporated into oligonucleotides using automated DNA synthesis. Relative quantum yields are 0.39 for 6MAP and 0.48 for DMAP as monomers and range from >0.01 to 0.11 in oligonucleotides. Excitation maxima are 310 (6MAP) and 330 nm (DMAP) and the emission maximum for each is 430 nm. Fluorescence decay curves of each are monoexponential exhibiting lifetimes of 3.8 and 4.8 ns for 6MAP and DMAP, respectively. When these probes are incorporated into oligonucleotides they display quenching of fluorescence intensity, increases in the complexity of decay curves, and decreases in mean lifetimes. Because these changes are apparently mediated by interactions with neighboring bases, spectral changes that occur as probe-containing oligonucleotides meet and react with other molecules provide a means of monitoring these interactions in real time. These probes are minimally disruptive to DNA structure as evidenced by melting temperatures of probe-containing oligonucleotides that are very similar to those of controls. Digestion of probe-containing oligonucleotides with P1 nuclease confirms probe stability as fluorescence levels are restored to those expected for each monomer. These adenosine analog probes are capable of providing information on DNA structure as it responds to binding or catalysis through interaction with other molecules.

Pterin interactions with distinct reductase activities of NO synthase

Besides oxidizing L-arginine, neuronal NO synthase (NOS) NADPH-dependently reduces various electron acceptors, including cytochrome c and tetrazolium salts. The latter NADPH diaphorase reaction is used as a NOS-specific histochemical stain. Both reductase activities have been utilized to analyse electron transfer mechanisms within NOS. Basal L-arginine turnover by homodimeric NOS is enhanced by exogenous tetrahydrobiopterin, and the intra-subunit electron flow may include intermediate trihydrobiopterin. In the present work we have investigated the possible role of the tetrahydrobiopterin binding site of NOS in its reductase activities by examining the effects of anti-pterin type (PHS) NOS inhibitors. Although the type I anti-pterin, PHS-32, which does not affect basal dimeric NOS activity, also had no effect on either reductase activity, the type II anti-pterin, PHS-72, which inhibits basal NOS activity, inhibited both reductase activities and the NADPH diaphorase histochemical stain. Pterin-free NOS monomers catalysed both cytochrome c and tetrazolium salt reduction. Our data suggest that both NOS reductase activities are independent of tetrahydrobiopterin. However, occupation of an exosite near the pterin site in NOS by type II anti-pterins may interfere with the electron flow within the active centre, suggesting that steric perturbation of the pterin binding pocket or reductase interaction contribute to the mechanism of inhibition by this class of NOS inhibitors.

A kinetic and conformational study on the interaction of tetrahydropteridines with tyrosine hydroxylase

Tetrahydropterins are obligatory cofactors for tyrosine hydroxylase (TH), the rate-limiting enzyme of catecholamine biosynthesis. A series of synthetic analogues of 6(R)-L-erythro-5,6,7, 8-tetrahydrobiopterin (BH(4)) with different substituents in positions C2, N3, C4, N5, C6, C7, and N8 on the ring were used as active site probes for recombinant human TH. The enzyme tolerates rather bulky substituents at C6, as seen by the catalytic efficiency (V(max)/K(m)) and the coupling efficiency (mol of L-DOPA produced/mol of tetrahydropterin oxidized) of the cofactors. Substitutions at C2, C4, N5, and N8 abolish the cofactor activity of the pterin analogues. Molecular docking of BH(4) into the crystal structure of the catalytic domain of ligand-free rat TH results in complexes in which the pteridine ring pi-stacks with Phe300 and the N3 and the amino group at C2 hydrogen bonds with Glu332. The pteridine ring also establishes interactions with Leu294 and Gln310. The distance between C4a in the pteridines and the active site iron was 4.2 +/- 0.5 A for the ensemble of docked conformers. Docking of BH(4) analogues into TH also shows that the most bulky substituents at C6 can be well-accommodated within the large hydrophobic pocket surrounded by Ala297, Ser368, Tyr371, and Trp372, without altering the positioning of the ring. The pterin ring of 7-BH(4) shows proper stacking with Phe300, but the distance between the C4a and the active site iron is 0.6 A longer than for bound BH(4), a finding that may be related to the high degree of uncoupling observed for 7-BH(4).