Nitric oxide synthases: analogies to cytochrome P450 monooxygenases and characterization of recombinant rat neuronal nitric oxide synthase hemoprotein

Proteome of plant peroxisomes: new perspectives on the role of these organelles in cell biology

Peroxisomes are cell organelles bounded by a single membrane with a basically oxidative metabolism. Peroxisomes house catalase and H(2)O(2)-producing flavin-oxidases as the main protein constituents. However, since their discovery in early fifties, a number of new enzymes and metabolic pathways have been reported to be also confined to these organelles. Thus, the presence of exo- and endo-peptidases, superoxide dismutases, the enzymes of the plant ascorbate-glutathione cycle plus ascorbate and glutathione, several NADP-dehydrogenases, and also L-arginine-dependent nitric oxide synthase activity has evidenced the relevant role of these organelles in cell physiology. In recent years, the study of new functions of peroxisomes has become a field of intensive research in cell biology, and these organelles have been proposed to be a source of important signal molecules for different transduction pathways. In plants, peroxisomes participate in seed germination, leaf senescence, fruit maturation, response to abiotic and biotic stress, photomorphogenesis, biosynthesis of the plant hormones jasmonic acid and auxin, and in cell signaling by reactive oxygen and nitrogen species (ROS and RNS, respectively). In order to decipher the nature and specific role of the peroxisomal proteins in these processes, several approaches including in vivo and in vitro import assays and generation of mutants have been used. In the last decade, the development of genomics and the report of the first plant genomes provided plant biologists a powerful tool to assign to peroxisomes those proteins which harbored any of the two peroxisomal targeting signals (PTS, either PTS1 or PTS2) described so far. Unfortunately, those molecular approaches could not give any response to those proteins previously localized in plant peroxisomes by classical biochemical and cell biology methods that did not contain any PTS. However, more recently, proteomic studies of highly purified organelles have provided evidence of the presence in peroxisomes of new proteins not previously reported. Thus, the contribution of proteomic approaches to the biology of peroxisomes is essential, not only for elucidation of the mechanisms involved in the import of the PTS1- and PTS2-independent proteins, but also to the understanding of the role of these organelles in the cell physiology of plant growth and development.

Oxygen metabolism by endothelial nitric-oxide synthase

Nitric-oxide synthase (NOS) catalyzes both coupled and uncoupled reactions that generate nitric oxide and reactive oxygen species. Oxygen is often the overlooked substrate, and the oxygen metabolism catalyzed by NOS has been poorly defined. In this paper we focus on the oxygen stoichiometry and effects of substrate/cofactor binding on the endothelial NOS isoform (eNOS). In the presence of both L-arginine and tetrahydrobiopterin, eNOS is highly coupled (>90%), and the measured stoichiometry of O(2)/NADPH is very close to the theoretical value. We report for the first time that the presence of L-arginine stimulates oxygen uptake by eNOS. The fact that nonhydrolyzable L-arginine analogs are not stimulatory indicates that the occurrence of the coupled reaction, rather than the accelerated uncoupled reaction, is responsible for the L-arginine-dependent stimulation. The presence of 5,6,7,8-tetrahydrobiopterin quenched the uncoupled reactions and resulted in much less reactive oxygen species formation, whereas the presence of redox-incompetent 7,8-dihydrobiopterin demonstrates little quenching effect. These results reveal different mechanisms for oxygen metabolism for eNOS as opposed to nNOS and, perhaps, partially explain their functional differences.

Recruitment of governing elements for electron transfer in the nitric oxide synthase family

At least three building blocks are responsible for the molecular basis of the modulation of electron transfer in nitric oxide synthase (NOS) isoforms: the calmodulin-binding sequence, the C-terminal extension, and the autoregulatory loop in the reductase domain. We have attempted to impart the control conferred by the C termini of NOS to cytochrome P450 oxidoreductase (CYPOR), which contains none of these regulatory elements. The effect of these C termini on the properties of CYPOR sheds light on the possible evolutionary origin of NOS and addresses the recruitment of new peptides on the development of new functions for CYPOR. The C termini of NOSs modulate flavoprotein-mediated electron transfer to various electron acceptors. The reduction of the artificial electron acceptors cytochrome c, 2,6-dichlorophenolindophenol, and ferricyanide was inhibited by the addition of any of these C termini to CYPOR, whereas the reduction of molecular O(2) was increased. This suggests a shift in the rate-limiting step, indicating that the NOS C termini interrupt electron flux between flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) and/or the electron acceptors. The modulation of CYPOR by the addition of the NOS C termini is also supported by flavin reoxidation and fluorescence-quenching studies and antibody recognition of the C-terminal extension. These experiments support the origin of the NOS enzymes from modules consisting of a heme domain and CYPOR or ferredoxin-NADP(+) reductase- and flavodoxin-like subdomains that constitute CYPOR, followed by further recruitment of smaller modulating elements into the flavin-binding domains.

Distribution of nitroreductive activity toward nilutamide in rat

Nilutamide is a pneumotoxic and hepatotoxic nitroaromatic (R-NO2) antiandrogen used in the treatment of prostate carcinoma in man. Previously, we established that in the rat lung, the drug is metabolized into the corresponding hydroxylamine (R-NHOH) and amine (R-NH2) derivatives. These results evidenced a cytosolic oxygen-sensitive (type II) nitroreductase activity in lung. In the present studies, we extended the characterization of nilutamide metabolism in liver, brain, kidney, heart, blood, intestine (small, cecum, and large, and their respective luminal contents) of male Sprague-Dawley rats. Subcellular fractions for all tissues (except blood) examined (postmitochondrial, cytosolic, and microsomal) were prepared by differential ultracentrifugation. Blood and intestinal contents were sonicated before investigation. Incubations were run in the presence or absence of O2 to assess type I and II nitroreductase activities. Organic extracts were analyzed by HPLC methods and results were expressed as pmoles of R-NH2 formed per milligram protein per minute. Four distinct nitroreductive activities were evidenced. Cytosolic and microsomal type II nitroreductase activities were detected in all tissue samples studied. Type I NR activity was not observed in any of the cytosols, but was detected in the small intestine, lung, kidney, and liver microsomes. Nilutamide was also reduced in the intestinal lumen, possibly by a bacterial type I nitroreductase. Highest activities were observed in cytosols and were oxygen sensitive. These results evidence and characterize previously unknown nitroreductive activities toward nilutamide in rat tissues that might provide some explanation to the side effects of nilutamide and other nitroaromatic compounds observed in human therapeutics.

Reactive oxygen species, antioxidant systems and nitric oxide in peroxisomes

Peroxisomes are subcellular organelles with an essentially oxidative type of metabolism. Like chloroplasts and mitochondria, plant peroxisomes also produce superoxide radicals (O2*(-)) and there are, at least, two sites of superoxide generation: one in the organelle matrix, the generating system being xanthine oxidase, and another site in the peroxisomal membranes dependent on NAD(P)H. In peroxisomal membranes, three integral polypeptides (PMPs) with molecular masses of 18, 29 and 32 kDa have been shown to generate radicals O2*(-). Besides catalase, several antioxidative systems have been demonstrated in plant peroxisomes, including different superoxide dismutases, the ascorbate-glutathione cycle, and three NADP-dependent dehydrogenases. A CuZn-SOD and two Mn-SODs have been purified and characterized from different types of peroxisomes. The four enzymes of the ascorbate-glutathione cycle (ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase) as well as the antioxidants glutathione and ascorbate have been found in plant peroxisomes. The recycling of NADPH from NADP(+) can be carried out in peroxisomes by three dehydrogenases: glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and isocitrate dehydrogenase. In the last decade, different experimental evidence has suggested the existence of cellular functions for peroxisomes related to reactive oxygen species (ROS), but the recent demonstration of the presence of nitric oxide synthase (NOS) in plant peroxisomes implies that these organelles could also have a function in plant cells as a source of signal molecules like nitric oxide (NO*), superoxide radicals, hydrogen peroxide, and possibly S-nitrosoglutathione (GSNO).

Characterization of a member of the NnrR regulon in Rhodobacter sphaeroides 2.4.3 encoding a haem-copper protein

Upstream of the nor and nnrR cluster in Rhodobacter sphaeroides 2.4.3 is a previously uncharacterized gene that has been designated nnrS. nnrS is only expressed when 2.4.3 is grown under denitrifying conditions. Expression of nnrS is dependent on the transcriptional regulator NnrR, which also regulates expression of genes required for the reduction of nitrite to nitrous oxide, including nirK and nor. Deletion analysis indicated the sequence 5'-TTGCG(N4)CACAA-3', which is similar to sequences found in nirK and nor, is required for nnrS expression. Mutation of this sequence to the consensus Fnr-binding sequence by changing two bases in each half site caused nnrS expression to become nitrate independent. Inactivation of nnrS did not affect nitric oxide metabolism, nor did it affect expression of any of the genes involved in nitric oxide metabolism. However, taxis towards nitrate and nitrite was affected by nnrS inactivation. Purification of a histidine-tagged NnrS demonstrated that NnrS is a haem- and copper-containing membrane protein. Genes encoding putative orthologues of NnrS are sometimes but not always found in bacteria encoding nitrite and/or nitric oxide reductase.

Localization of nitric-oxide synthase in plant peroxisomes

The presence of nitric-oxide synthase (NOS) in peroxisomes from leaves of pea plants (Pisum sativum L.) was studied. Plant organelles were purified by differential and sucrose density gradient centrifugation. In purified intact peroxisomes a Ca(2+)-dependent NOS activity of 5.61 nmol of L-[(3)H]citrulline mg(-1) protein min(-1) was measured while no activity was detected in mitochondria. The peroxisomal NOS activity was clearly inhibited (60-90%) by different well characterized inhibitors of mammalian NO synthases. The immunoblot analysis of peroxisomes with a polyclonal antibody against the C terminus region of murine iNOS revealed an immunoreactive protein of 130 kDa. Electron microscopy immunogold-labeling confirmed the subcellular localization of NOS in the matrix of peroxisomes as well as in chloroplasts. The presence of NOS in peroxisomes suggests that these oxidative organelles are a cellular source of nitric oxide (NO) and implies new roles for peroxisomes in the cellular signal transduction mechanisms.

Anti-pterins as tools to characterize the function of tetrahydrobiopterin in NO synthase

Nitric oxide synthases (NOS) are homodimeric enzymes that NADPH-dependently convert L-arginine to nitric oxide and L-citrulline. Interestingly, all NOS also require (6R)-5,6,7, 8-tetrahydro-L-biopterin (H4Bip) for maximal activity although the mechanism is not fully understood. Basal NOS activity, i.e. that in the absence of exogenous H4Bip, has been attributed to enzyme-associated H4Bip. To elucidate further H4Bip function in purified NOS, we developed two types of pterin-based NOS inhibitors, termed anti-pterins. In contrast to type II anti-pterins, type I anti-pterins specifically displaced enzyme-associated H4Bip and inhibited H4Bip-stimulated NOS activity in a fully competitive manner but, surprisingly, had no effect on basal NOS activity. Moreover, for a number of different NOS preparations basal activity (percent of Vmax) was frequently higher than the percentage of pterin saturation and was not affected by preincubation of enzyme with H4Bip. Thus, basal NOS activity appeared to be independent of enzyme-associated H4Bip. The lack of intrinsic 4a-pterincarbinolamine dehydratase activity argued against classical H4Bip redox cycling in NOS. Rather, H4Bip was required for both maximal activity and stability of NOS by binding to the oxygenase/dimerization domain and preventing monomerization and inactivation during L-arginine turnover. Since anti-pterins were also effective in intact cells, they may become useful in modulating states of pathologically high nitric oxide formation.