Abstract : NADPH oxidase, Nox4, belongs to the Nox family which could generate reactive oxygen species by transferring an electron to molecular oxygen. Despite its wide distribution in tissues, Nox4 is still poorly understood. Unlike the other Noxes, Nox4 shows some unique characters: the constitutive activity, H2O2 formation. Nox4 involved ROS has been proposed to be implicated in several pathologies. Thus, to study the structure/function and the regulation of the activity of Nox4 will provide new ideas and new drug targets for the effective prevention and treatment of clinical diseases related with ROS. To know more about Nox4, in this study, 5 novel monoclonal antibodies were raised against a truncated recombinant protein (AA: 206-578) of Nox4. The specificity of 3 mAbs (8E9, 5F9, 6B11) was confirmed by western blot analysis in HEK293 transfected cells and human kidney cortex. In FACS studies, only mAb 8E9 could react with intact tet-induced T-RExTM Nox4 cells. Immunofluorescence confocal microscopy showed that Nox4 localized not only in the perinuclear and endoplasmic reticulum regions but also at the plasma membrane of the cells which was further confirmed by TIRF-microscopy. An interesting phenomena is that mAb 5F9 failed to detect Nox4 at the plasma membrane. Epitope determination showed that mAb 8E9 recognizes a region on the last extracellular loop of Nox4 (222H-E241), while mAb 6B11 (389S-P416) and 5F9 (392D-F398) are directed to its cytosolic tail. Cell-free oxidase assays showed a moderate but significant inhibition of constitutive Nox4 activity by mAb 5F9 and 6B11. To study the protein region which is responsible for the unique ability of Nox4 of releasing H2O2 rather than O2-, chimeric proteins and mutants were used. E-loop of Nox4 is 28 amino acid longer than that of Nox1 or Nox2. Deletion of E-loop amino acids only present in Nox4 or change of the two cysteines in the E-loop switch Nox4 from H2O2 to O2- generation. In the presence of a NO donor, the O2--producing Nox4 mutants, but not widetype Nox4, generated peroxynitrite, excluding artifacts of the detection systems as the apparent origin of O2-. A second approach was used to confirm the responsibility of E-loop for the H2O2 formation. In Cos7 cells, which exhibit some plasma membrane expression of Nox4, addition of the mAb 8E9 decreased H2O2 production but increased O2- formation. Unlike Nox1 or Nox2, the E-loop of Nox4 contains a highly conserved histidine H222. Mutation of H222 also switched Nox4 from H2O2 to O2- formation. The structure of the E-loop might hinder O2- egress and/or provide a source for protons to accelerate dismutation to form H2O2. Two bacterial protein expression approaches (in vitro RTS and bacterial induction) were used to produce Nox4 cytosolic tail for characterizing the electronic transfer property of Nox4. The presence of rare codons (1363AGA AGA CUA1371) and high level of hydrophobicity affects the production of soluble and active recombinant Nox4Aqc and Nox4Bqc. After optimization of the conditions, soluble and active recombinant proteins were obtained by RTS or by bacteria induction. The soluble proteins were produced in large scale, purified onto affinity chromatography and were tested for the diaphorase activity (INT and cytochrome c). Results showed that electronic acceptor cytochrome c gives a higher rate than INT. Nox4Aqc produced a lower specific activity by a cell-based system compared to the protein synthesized in cell-free technology. This activity is not stimulated by the addition of cytosolic factors. A new method, topological determination by ubiquitin fusion assay (TDUFA), was used to investigate the topology of Nox4 and p22phox. ubGFP fusion proteins are used as tools to obtain details of membrane protein topology. This method was first validated by using two membrane proteins with known topology and then should get more topology information of Nox4 and p22phox further.