Une des propriétés majeures de la thyroïde est de capter l’iode de la circulation sanguine grâce à la présence d’un transporteur d’iodure (NIS pour Natrium Iodide Symporter). Cette capacité d’accumulation d’iode par les thyrocytes joue un rôle clé dans la synthèse des hormones thyroïdiennes ainsi dans le diagnostic et le traitement des cancers de la thyroïde. Cependant, en raison d’une diminution ou de l’absence de l’expression du NIS dans certaines tumeurs et métastases, des patients deviennent réfractaires à la radiothérapie métabolique et présentent une radiorésistance, causant ainsi un problème de santé publique.L’oncogène BRAFV600E, un puissant activateur de La voie MAP kinase, est détecté dans 40 - 60% des cancers thyroïdiens de type papillaires (CPT) qui représentent 80% de la totalité des cancers thyroïdiens. La mutation BRAFV600E est associée aux tumeurs thyroïdiennes les plus agressives. Cependant l’inhibition pharmacologique de la voie MAP kinase induite constitutivement par l’oncogène BRAFV600E ne permet pas, à elle seule, de rétablir l’expression du NIS chez des patients atteints d’un cancer de la thyroïde muté BRAFV600E. Ceci suggère que d’autres mécanismes compensatoires peuvent contribuer à la radiorésistance. Une étude récente menée sur un modèle murin a montré que la régulation négative du NIS par l’oncogène BRAFV600E est médiée par la voie du TGF beta. Une autre a montré que l’expression du NIS serait dépendante de l’état redox de la cellule, suggérant un rôle des espèces réactives de l’oxygène (ROS). Dans les cellules les ROS peuvent être produites par les NADPH oxydases (NOX/DUOX). La thyroïde en exprime trois : DUOX2 nécessaire à la synthèse des hormones thyroïdiennes ainsi que DUOX1 et NOX4 dont le rôle physiologique reste inconnu. NOX4, surexprimé dans les CPTs, a été montré être un nouvel effecteur clé de la voie du TGF beta dans d’autres cancers.Dans mon projet de thèse, je me suis intéressée à l’étude du rôle de NOX4 dans la régulation négative du NIS dans les CPT mutés BRAFV600E. L’étude du mécanisme, réalisée à partir de deux lignées humaines issues de cancers papillaires mutés pour BRAF (BCPAP et 8505C), a permis d’établir que l’oncogène BRAFV600E contrôle l’expression de NOX4 au niveau transcriptionnel via la voie TGF-beta/Smad3. Ces résultats ont été validés sur une lignée de rat exprimant de manière conditionnelle BRAFV600E ainsi que sur des thyrocytes humains en culture primaire. De manière importante, l’utilisation d’antioxydants tels que le N-acetyl cystéine (NAC) ou l’inhibition spécifique de l’expression de NOX4 par ARN interférence permet de réinduire l’expression du NIS. Ces résultats qui montrent que les ROS produites par NOX4 inhibent l’expression du transporteur de l’iode (NIS) établissent un lien entre l’oncogène BRAFV600E et NOX4. Une analyse comparative de l'expression de NOX4 effectuée à partir de 500 cancers papillaires de la thyroïdes mutés ou non pour BRAF (données TCGA) confirme que NOX4 est significativement augmenté dans les cancers porteurs de la mutation BRAF et que ceci est corrélé à une diminution de l’ARNm du NIS. Par ailleurs, le niveau de NOX4 est inversement corrélé au score de différenciation thyroïdien, suggérant que NOX4 pourrait être impliqué dans le processus de dédifférenciation. Cette étude ouvre une nouvelle opportunité pour l’optimisation de l’utilisation de la radiothérapie métabolique dans le traitement des cancers thyroïdiens réfractaires à l’iode I131 et présente NOX4 comme une cible thérapeutique potentielle. / One of the major properties of the thyroid is iodine uptake from the bloodstream through an iodide transporter (NIS for Natrium Iodide Symporter). This capacity plays a key role in the thyroid hormones synthesis, but also in both diagnosis and treatment of thyroid cancer. However, due to a decrease or absence of the NIS expression in some tumors and metastases, patients become refractory to the metabolic radiotherapy and present a radioresistance, which cause a public health problem.The BRAFV600E oncogene, a potent activator of the MAP kinase pathway, is detected in 40-60% of papillary thyroid cancer (PTC), which represent 80% of total thyroid cancers. The BRAFV600E mutation is associated with the more aggressive thyroid tumors. However, the pharmacological inhibition of the MAP kinase pathway, constitutively induced by the BRAFV600E oncogene, is not able to restore alone the expression of NIS in patients with BRAFV600E mutated thyroid cancer. This suggests that other compensatory mechanisms may contribute to the radioresistance. A recent study in a mouse model demonstrated that downregulation of NIS by BRAFV600E oncogene is mediated through the TGF beta activation. An other showed that the expression of NIS is dependent on the redox status of the cell, suggesting a role for the reactive oxygen species (ROS). In cells, ROS can be produced by the NADPH oxidases (NOX/DUOX). The Thyroid gland expresses three of them: DUOX2, which is necessary for the thyroid hormones synthesis, but also DUOX1 and NOX4 whose the physiological role remains unknown. NOX4, which is overexpressed in the PTCs, has been shown to be a new key effector of the TGF beta pathway.In my thesis project, I was interested in studying the role of NOX4 in the negative regulation of NIS in BRAFV600E mutated CPT. The study of the mechanism, made from two human cell lines derived from BRAF-mutated papillary thyroid cancers (BCPAP and 8505C), has revealed that the oncogene BRAFV600E controls the expression of NOX4 at the transcriptional level via the TGF-beta/Smad3 pathway. These results were validated on both a rat thyroid cell line conditionnaly expressing BRAFV600E and on human thyrocytes in primary culture. Importantly, the use of antioxidants such as N-acetyl cysteine (NAC) or specific inhibition of NOX4 expression by RNA interference allow reinduction of NIS expression. These results, which show that ROS produced by NOX4 inhibit the expression of iodine transporter (NIS), establish a link between the oncogene BRAFV600E and NOX4. A comparative analysis of the NOX4 expression, made from 500 papillary thyroid cancers mutated or not for BRAF (TCGA data), confirms that NOX4 is significantly increased in BRAF-mutated cancers and that this is correlated with a decrease of NIS mRNA. Furthermore, the level of NOX4 is inversely related to thyroid differentiation score, suggesting that NOX4 might be involved in the dedifferentiation process. This study opens a new opportunity for optimizing the use of metabolic radiotherapy in the treatment of thyroid cancers refractory to radioiodine I131and makes NOX4 as a potential therapeutic target.

Local production of reactive oxygen species (ROS) and changes in the redox environment influence the metabolism and function of β cells of the Langerhans islets (LO). Changing the ratio between NAD(P)H / NAD(P)+ redox partners significantly affects sensitive proteins and ROS production. ROS are able to reversibly modify some amino acid residues (eg Cys, Met) of antioxidant enzymes and their interaction partners. Such a signaling cascade allows the transmission of a signal over longer distances and can also interfere with the influence of gene expression. The unique enzyme NADPH oxidase 4 (NOX4) is present on membranes within β cells and constitutively produces H2O2 depending on the presence of NAD(P)H. After glucose stimulation, both NAD(P)H and Nox4 mRNA levels increase. As previously observed in our laboratory, C57BL/6J mice with a specific Nox4 deletion in β cells have a disrupted biphasic insulin release and exhibit insulin resistance in fat and muscle tissue. We found that the absence of NOX4 in C57BL/6J mice affects LO architecture. Wildtype (WT) mice on a normal, predominantly carbohydrate diet (ND) have the majority of small LO with an area of up to 5 000 μm2 (measured on histological sections). High-fat diet (HFD) feeding of WT for 8 weeks leads to the development of diabetic phenotype and...

Effluents of industrial wastewaters from oil refineries, paper mills, dyes, ceramic factories, resins, textiles and plastic contain high concentrations of aromatic compounds, which are toxic to organisms. Degradation of these compounds to tolerant limits before releasing them into the environment is an urgent requirement. Candida tropicalis yeast is an important representative of eucaryotic microorganisms that are able to utilize phenol. During the first phase of phenol biodegradation, cytoplasmatic NADPH-dependent phenol hydroxylase of C. tropicatis oxidizes phenol to catechol. Catechol is in the second phase of biodegradative process oxidized to cis,cis-muconic acid by the reaction catalyzed with catechol-1,2-dioxygenase. In this diploma thesis we investigated the effect of the heavy metal ions on NADPH-dependent phenol hydroxylase and catechol-1,2-dioxygenase of C. tropicalis. Phenol hydroxylase was inhibited by Cu2+ and Pb2+ ions. Catechol dioxygenase was inhibited by all substances containing heavy metal ions (Fe2+ , Mn2+ , Cd2+ , Cu2+ and Pb2+ ), which were tested in this work. The most effective inhibition was produced by Pb2+ followed by Mn2+ , Cd2+ Fe2+ and Cu2+ ions. The higher sensitivity of catechol-1,2-dioxygenase to heavy metal ions might follow from the presence of histidine residue...

Carotid artery stenosis (CAS) develops from atherosclerotic lesions and plaques. Plaque rupture or stenosis may result in occlusion of the carotid artery. Accordingly, the asymptomatic disease becomes symptomatic, characterized by ischemic stroke or transient ischemic attacks, indicating an urgent need for better understanding of the underlying molecular mechanisms and eventually prevent symptomatic CAS. NOX4, a member of the NADPH oxidase family, has anti-atherosclerotic and anti-inflammatory properties in animal models of early atherosclerosis. We hypothesized that NOX4 mRNA expression is linked to protective mechanisms in CAS patients with advanced atherosclerotic lesions as well. Indeed, NOX4 mRNA expression is lower in patients with symptomatic CAS. A low NOX4 mRNA expression is associated with an increased risk of the development of clinical symptoms. In fact, NOX4 appears to be linked to plaque stability, apoptosis and plaque hemorrhage. This is supported by cleaved caspase-3 and glycophorin C and correlates inversely with plaque NOX4 mRNA expression. Even healing of a ruptured plaque appears to be connected to NOX4, as NOX4 mRNA expression correlates to fibrous cap collagen and is reciprocally related to MMP9 activity. In conclusion, low intra-plaque NOX4 mRNA expression is associated with an increased risk for symptomatic outcome and with reduced plaque stabilizing mechanisms suggesting protective effects of NOX4 in human advanced atherosclerosis.

Aortic dissection is a detrimental disease with a high mortality. However, the mechanisms regulating the susceptibility to aortic dissection remain unknown. We hypothesize that endothelial oxidative stress due to the activation of the reactive oxygen species (ROS)-generating Nox2 enzyme may play an important role in the development of aortic dissection. To investigate this, we generated transgenic mice (C57BL/6J background) with endothelial specific over-expression of Nox2 (Nox2 Tg) under the control of a tie-2 promoter. Expression of the human Nox2 transgene was confirmed by qRT-PCR to be found only in endothelial cells (EC) isolated from transgenic mice, and not in Wt EC or vascular smooth muscle cells (VSMC) and macrophages isolated from either genotype. Wild-type (Wt) littermates and Nox2 Tg male mice (22-24 weeks old, n=11) were treated with saline or Ang II (1mg/kg/day) via subcutaneous mini-pump for 28 days. There was no significant difference in the pressor responses to Ang II between Wt and Nox2 Tg mice (Wt 121±7mmHg vs. Nox2-Tg 122±6mmHg). However, 5/11 Nox2 Tg mice developed aortic dissections compared to 0/11 Wt mice (P<0.05). Immunohistochemistry revealed significant increases in endothelial VCAM-1 expression, MMP activity and CD45+ inflammatory cell recruitment in the aortas of Nox2 Tg mice after 5 days of Ang II infusion. Inflammatory cell recruitment was confirmed by FACS analysis of cells from digested aortas (P<0.05). Explanted aortas from Nox2-Tg mice had significantly greater secreted pro-inflammatory cytokine, Cyclophilin A (CypA) both at baseline and after 5 days of Ang II infusion compared to Wt littermates. Compared to primary Wt EC and VSMC, Nox2-Tg primary EC, but not primary VSMC, had increased ROS production which was accompanied by increased endothelial CypA secretion and ERK1/2 activation. Furthermore, conditioned media from Nox2-Tg EC induced greater ERK1/2 phosphorylation compared to conditioned media from Wt controls. Knockdown of CypA from sEND.1 endothelial conditioned media by siRNA knockdown abolished VSMC Erk1/2 phosphorylation. In conclusion, we demonstrate for the first time that a specific increase in endothelial ROS through the over-expression of Nox2 was sufficient to induce aortic dissection in response to Ang II stimulation. Endothelial secreted CypA could be the signalling mechanism by which increased endothelial ROS regulates the inflammatory response and the susceptibility to aortic dissection.

NADPH-cytochrome P450 reductase is the electron transfer partner for the cytochromes P450, heme oxygenase, and squalene monooxygenase, and is a component of the nitric oxide synthases and methionine synthase reductase. P450 reductase shows very high selectivity for NADPH and uses NADH only poorly. Substitution of tryptophan 677 with alanine (W677A) has been shown by others to yield a 3-fold increase in turnover with NADH, but profound inhibition by NADP+ makes the enzyme unsuitable for in vivo applications. In the present study site-directed mutagenesis of amino acids in the 2'-phosphate-binding site of the NADPH domain, coupled with the W677A substitution, was used to generate a reductase that was able to use NADH efficiently in vivo without inhibition by NADP+. Of 11 single, double, and triple mutant proteins, two (R597M/W677A and R597M/K602W/W677A) showed up to a 500-fold increase in catalytic efficiency (kcat/Km) with NADH. Inhibition by NADP+ was reduced by up to four orders of magnitude relative to the W677A protein and was equal to or less than that of the wild-type reductase. Both proteins were 2- to 3-fold more active than wild-type reductase with NADH in reconstitution assays with cytochrome P450 1A2 and with squalene monooxygenase. In a recombinant cytochrome P450 2E1 Ames bacterial mutagenicity assay the R597M/W677A protein increased the sensitivity to dimethylnitrosamine by approximately 2-fold, suggesting that the ability to use NADH afforded a significant advantage in this in vivo assay. In addition to providing a valuable tool for understanding the determinants of nucleotide cofactor specificity in this and related enzymes, these mutants might also lend themselves to creation of bioremediation schemes with increased enzymatic activity and robustness in situ, as well as cost-effective reconstitution of enzyme systems in vitro that do not require the use of expensive reducing equivalents from NADPH.

The Dorsal Raphe Nucleus (DRN) is a complex brain region that has been implicated in disorders such as anxiety and depression. The DRN is divided into subregions through its rostrocaudal and mediolateral axis. It has been reported that after a single restraint session there is differential spatial activation of nitric oxide synthase (NOS) across the DRN. The temporal profile of NOS activity during acute stress is not known but it is important because duration of acute stress is associated with different general responses. In this report rats were restrained for 1, 3, or 6 hours and nicotinamide adenine phosphate diaphorase (NADPH-d) was stained as an index to NOS activity to determine the spatio-temporal profile of NOS throughout a 6 hour restraint. Astrocyte reactivity was also measured to determine whether NOS activation correlated with GFAP expression since astrocytes react to neural activity and store and release l-arginine, the precursor for nitric oxide production. The results showed that the DRN had a dynamic response to acute restraint stress, most notably in the caudal lateral wings where activation increased after 3 hours of restraint (p = > 0.001) but neuron count decreased after 6 hours (p = 0.040). Astrocytes did not correlate with NOS activation but they showed spatio-temporal differences as well whereas they were more active in the rostral half of the DRN. In conclusion, the present study suggests that NOS produced in the DRN may have a role in prolonged exposure to acute stress and that subregions show differential NOS activation.

The exposure of plants to high salt concentrations causes accumulation of sodium ions. This leads to the inability of the plants to uptake water, a disturbance of ion homeostasis, a decrease in photosynthesis and oxidative stress. As a result of the salt stress, the availability of NADPH decreases. The adaptation to the concentrations of salt depends on plant's ability to compensate for the decreased availability of NADPH, which can be further used in antioxidative cycles and the synthesis of antioxidative compounds and osmoprotectants. In this work, the reduction of relative water content, a decrease in the Rubisco enzyme activity, an increase of Hsp70 in the leaves and an increase in the accumulation of sodium ions was shown in cucumber plants (Cucumis sativa L. convar. Jogger F1) exposed to salt stress (100 mM NaCl). As a consequence of salt stress, an increase in the activity of NADPH providing enzymes was found. Particularly on the second and third day of salt stress, an increase in the activity (up to 270 %) of: NADP-isocitrate dehydrogenase, glucose-6-phosphate dehydrogenase, NADP-malic enzyme, non-phosphorylating glyceraldehyde-3-phosphate dehydrogenase in leaves was detected. The activity of less abundant NADP-dehydrogenases (glucose 1-dehydrogenase, gluconate 2-dehydrogenase, galactose...

Die NADPHd-Färbung stellt bekanntermaßen Neurone dar, die die neuronale NOS exprimieren. Die Anfärbung der Neurone ist in ihrer Qualität dabei mit Golgi-basierten Versilberungstechniken vergleichbar. Aufgrund dieser Eigenschaften ermöglicht diese Methode morphologische und funktionelle Untersuchungen. Somit ist sie geradezu zur Bearbeitung neuropathologischer Fragestellungen prädestiniert. Im Putamen werden durch diese Technik vorwiegend Interneurone angefärbt. Anhand morphologischer Kriterien wurden die nitrinergen Neurone klassifiziert. Im menschlichen Putamen konnten dabei 12 Neuronentypen (NADPHd I bis XII) unterschieden werden, die nur zum Teil in bereits bestehende Klassifikationssysteme eingeordnet werden konnten. Ausgehend von dieser Klassifikation ist es möglich, in vergleichenden Studien Veränderungen NADPHd-positiver Neurone im Rahmen neurodegenerativer Erkrankungen festzustellen. Im Falle der vorliegenden Arbeit wurde dabei das Putamen schizophrener Patienten untersucht. Aufgrund der geringen Anzahl von drei untersuchten schizophrenen Gehirnen ließen sich nur vorläufige Aussagen in Bezug auf Unterschiede NADPHd-positiver Neurone im Putamen Gesunder und Schizophrener treffen. Solche Unterschiede wurden in der Morphologie dieser Neurone gefunden, aber auch in deren Dichte: Im Putamen Schizophrener lag die Dichte NADPHd-positiver Neurone signifikant unter der bei der gesunden Kontrollgruppe ermittelten Dichte. Neben diesem numerischen Unterschied konnten auch morphologisch auffällige Neurone gefunden werden, die in der gesunden Kontrollgruppe nicht vorhanden waren. Sowohl im Claustrum als auch in der das Claustrum umgebenden weißen Substanz der Capsulae externa et extrema konnten NADPHd-positive Neurone nachgewiesen werden. Die NADPHd-positiven Neurone des Claustrums ließen sich zum Teil nach bereits bestehenden Einteilungen klassifizieren. In den äußeren Kapseln lagen sie zumeist parallel zur Richtung der Fasermassen angeordnet und zählten zu den interstitiellen Zellen der weißen Substanz. / The NADPHd-staining is known to stain selectively neurons expressing the neuronal NOS. The staining results are comparable to Golgi impregnation techniques because not only the cell soma is stained but also the dendrites. Thus, morphological and functional aspects can be examined using that techniqe. This method was used to stain, characterize and classify nNOS-positive neurons of the human putamen. Predominantly, interneurons were stained. They displayed a homogenous staining of the cell soma and the dendrites showing clear morphological differences. The interneurons could be classified into 12 different types (NADPHd I to XII) which only partially corresponded to previously described neuron types. Based on this classification system of a healthy brain, it is possible to find abnormalities of NADPHd-positive interneurons in neurodegenerative diseases. In this study, the putamen of three schizophrenic subjects was examined. Differences could not only be found for the morphology of NADPHd-positive interneurons but also for their frequency: The number of NADPHd-positive interneurons was significantly reduced in the putamen of schizophrenics. However, since only three brains of schizophrenics were examined these results can only be judged preleminary. In the claustrum and in the white matter surrounding the claustrum NADPHd-positive neurons were found as well. Regarding the claustrum, the NADPHd-positive neurons fit partially in previous classification systems. The NADPHd-positive neurons of the external capsules were part of the interstitial cells of the white matter.

In Säugetieren existieren im wesentlichen zwei Abwehrsysteme gegen oxidativen Streß, in welchen die Glutathionreduktase (GR) und Thioredoxinreduktase (TrxR) Schlüsselenzyme sind. Ein einzelnes Gen der Taufliege, genannt dmtrxr-1, kodiert sowohl für die durch alternatives Splicing entstehende cytoplasmatische und mitochondriale Form der DmTrxR-1. Zum Teil innerhalb des dmtrxr-1-Gens findet sich auf dem Komplementärstrang ein weiteres Gen, welches sniffer genannt wurde. In Kooperation wurde nachgewiesen, daß dieses Gen essentiell zur Verhinderung alterungsbedingter Neurodegeneration ist. Durch biochemische Charakterisierung konnte das rekombinant hergestellte Produkt dieses Gens in der vorliegenden Arbeit als Carbonylreduktase, ein zu den Kurzketten-Dehydrogenasen (short-chain dehydrogenases) gehörendes Enzym, identifiziert werden. Sniffer weist das für Carbonylreduktasen typische Substratspektrum mit Phenanthrenequinone als bestem Substrat auf und wird von Flavonoiden wie Quercetin und Rutin sowie Hydroxymercuribenzoat gehemmt. In verschiedenen Ansätzen konnten Kristalle des rekombinanten Proteins gewonnen werden, die inzwischen in Kooperation vermessen wurden und so zu einer Kristallstruktur mit einer Auflösung von 1,7 Angström führten. Durch diese Arbeiten konnte zum ersten Mal eine Verbindung zwischen einem charakterisierten Gen (snifffer), oxidativem Streß und neurodegenerativen Effekten auf molekularer Ebene nachgewiesen werden. Parasiten haben während ihres Lebenszyklus einen hohen Bedarf an Energie und sind abhängig von einer starken Syntheseleistung. Zur Bewältigung dieses Stresses benötigen sie hohe Aktivitäten an Adenylatkinase (AK; ATP + AMP &#61683; 2 ADP) und GTP-AMP-Phosphotransferase (GAK; GTP + AMP &#61683; GDP + ADP). Beide Enzyme wurden in Blutstadien des Malariaparasiten Plasmodium falciparum identifiziert und die entsprechenden Gene der PfAK und PfGAK auf den Chromosomen 10 und 4 respektive lokalisiert. Klonierung und heterologe Expression in E. coli ergab enzymatisch aktive Proteine mit einer Größe von 28,9 (PfAK), bzw. 28,0 kDa (PfGAK). Das rekombinante Protein der PfAK entspricht in seinen biochemischen Charakteristika denen der authentischen PfAK. Dies gilt auch für eine mögliche Assoziation mit einem stabilisierenden Protein mit einem Molekulargewicht von ca. 70 kDa und der hohen Substratspezifität für das Monophosphat-Nukleotid AMP. Die Spezifität für das Triphosphat-Substrat ist weniger stringent. Das beste Triphosphat-Substrat ist ATP mit einem Vmax-Wert von 75 U/mg und einem kcat von 2800 min-1. Die Sequenz der PfAK enthält eine amphiphatische Helix, welche als notwendig für die Translokation zytosolischer Adenylatkinasen in den Intermembranraum der Mitochondrien beschrieben wurde. Die PfGAK bevorzugt GTP und AMP als Substrat (100 U/mg; kcat = 2800 min-1 bei 25°C) und zeigt als Besonderheit keine messbare Aktivität mit ATP. Im Gegensatz zu ihrem Ortholog im Menschen (AK3) enthält die Sequenz der PfGAK ein Zinkfinger-Motiv und bindet Eisenionen. Erste Immunfluoreszenz-Analysen lokalisieren die PfGAK in den Mitochondrien. PfAK und PfGAK werden von den Dinukleosid-Pentaphosphat-Verbindungen AP5A beziehungsweise GP5A gehemmt. Die Ki-Werte liegen mit ca. 0.2 µM ungefähr 250-fach niedriger als die KM-Werte der entsprechenden Nukleotidsubstrate. Zur Lösung der vor allem im Rahmen einer rationalen Medikamentenentwicklung notwendigen Kristallstruktur des Zielmoleküls konnten bereits Kristalle der PfGAK erhalten werden. / In mammalia, two major systems with glutathione reductase (GR) and thioredoxin reductase (TrxR) as key enzymes defend the organism against oxidative stress. The single copy gene dmtrxr-1 codes for both the cytoplasmic and mitochondrial form of DmTrxR-1, generated by alternative splicing. Another gene, located on the complementary strand partially within the dmtrxr-1 gene, could be identified and was named sniffer. This gene is essential for prevention of age-related neuro-degeneration, as could be shown in a cooperation with the group of Prof. Schneuwly. In this thesis, biochemical characterization of the recombinant protein identified sniffer as a carbonyl reductase, an enzyme belonging to the short-chain-dehydrogenases. Sniffer shows the typical substrate spectrum of carbonyl reductases with phenthrenequinone as best substrate and is inhibited by the flavonoids quercetin and rutin and also by hydroxymercurybenzoate (HMB). Protein crystals could be obtained under different conditions. In a cooperation with the group of Prof. Klebe, these already lead to a crystal structure with a resolution of 1.7 angstrom. The work on sniffer is the first that directly links a characterized gene (sniffer), oxidative stress and neurodegeneration on the molecular level. For coping with energetic and synthetic challenges, parasites require high activities of adenylate kinase (AK; ATP + AMP &#61683; 2 ADP) and GTP:AMP phosphotransferase (GAK; GTP + AMP &#61683; 2 ADP). These enzymes were identified in bloodstream stages of Plasmodium falciparum. The genes encoding PfAK and PfGAK are located on chromosomes 10 and 4, respectively. Molecular cloning and heterologous expression in E. coli yielded enzymatically active proteins of 28.9 (PfAK) and 28.0 kDa (PfGAK). Recombinant PfAK resembles authentic PfAK in its biochemical characteristics including the possible association with a stabilizing protein and the high specificity for AMP as the mononucleotide substrate. Specificity is less stringent for the triphosphate, with ATP as the best substrate (75 U/mg; kcat = 2160 min-1). PfAK contains the sequence of the amphiphatic helix that is known to mediate translocation of the cytosolic protein into the mitochondrial intermembrane space. PfGAK exhibits substrate preference for GTP and AMP (100 U/mg; kcat = 2800 min-1); notably, there is no detectable activity with ATP. In contrast to its human orthologue (AK3), PfGAK contains a zinc finger motif and binds ionic iron. The dinucleoside pentaphosphate compounds AP5A and GP5A inhibited PfAK and PfGAK, respectively, with Ki values of appr. 0.2 µM which is more than 250-fold lower than the KM values determined for the nucleotide substrates. The disubstrate inhibitors are useful for studying the enzymatic mechanism of PfAK and PfGAK as well as their function in adenine nucleotide homeostasis; in addition, the chimeric inhibitors represent interesting lead compounds for developing nucleosides to be used as antiparasitic agents. To elucidate the structure which is necessary for the use as a drug target, crystallization studies have been performed and the first crystals could be obtained.