Spatial and electrogenic properties of superoxide-producing cytochrome b-559 incorporated into liposomes

NADPH Oxidases (NOX): An Overview from Discovery, Molecular Mechanisms to Physiology and Pathology

The reactive oxygen species (ROS)-producing enzyme NADPH oxidase (NOX) was first identified in the membrane of phagocytic cells. For many years, its only known role was in immune defense, where its ROS production leads to the destruction of pathogens by the immune cells. NOX from phagocytes catalyzes, via one-electron trans-membrane transfer to molecular oxygen, the production of the superoxide anion. Over the years, six human homologs of the catalytic subunit of the phagocyte NADPH oxidase were found: NOX1, NOX3, NOX4, NOX5, DUOX1, and DUOX2. Together with the NOX2/gp91phox component present in the phagocyte NADPH oxidase assembly itself, the homologs are now referred to as the NOX family of NADPH oxidases. NOX are complex multidomain proteins with varying requirements for assembly with combinations of other proteins for activity. The recent structural insights acquired on both prokaryotic and eukaryotic NOX open new perspectives for the understanding of the molecular mechanisms inherent to NOX regulation and ROS production (superoxide or hydrogen peroxide). This new structural information will certainly inform new investigations of human disease. As specialized ROS producers, NOX enzymes participate in numerous crucial physiological processes, including host defense, the post-translational processing of proteins, cellular signaling, regulation of gene expression, and cell differentiation. These diversities of physiological context will be discussed in this review. We also discuss NOX misregulation, which can contribute to a wide range of severe pathologies, such as atherosclerosis, hypertension, diabetic nephropathy, lung fibrosis, cancer, or neurodegenerative diseases, giving this family of membrane proteins a strong therapeutic interest.

Papain proteolysis releases a soluble NADPH dependent diaphorase activity from bovine neutrophil membranes

An NADPH dependent cytochrome c reductase has been purified from resting bovine neutrophil membranes. A high degree of purification, approaching homogeneity, is indicated by the presence of a single 75 kDa protein band on silver stained SDS-PAGE (10%). The purified protein catalyzes as well an NADPH dependent reduction of iodonitrotetrazolium violet (INT). Limited papain digestion of the purified preparation produces a 65 kDa product which retains both enzymatic activities. In a similar fashion papain digestion of the plasma membrane bound protein generates a fully active soluble NADPH dependent INT and cytochrome c reductase preparation (65 kDa). Proteolytic cleavage would appear to occur at a protein-membrane anchor remote from the proteins catalytic site. The cytochrome c reductase acts independently of the O2-generating cytochrome b558, a leukocyte plasma membrane protein which also catalyzes an NADPH dependent INT reduction.

The cytosolic component p47(phox) is not a sine qua non participant in the activation of NADPH oxidase but is required for optimal superoxide production

The superoxide (O-2)-generating NADPH oxidase of phagocytes is a multicomponent complex consisting of a membrane-associated flavocytochrome (cytochrome b559), bearing the NADPH binding site and two redox centers (FAD and heme) and three cytosolic activating components: p47(phox), p67(phox), and the small GTPase Rac (1 or 2). The canonical view is that the induction of O-2 generation involves the stimulus-dependent assembly of all three cytosolic components with cytochrome b559, a process mimicked in vitro by a cell-free system activated by anionic amphiphiles. We studied the requirement for individual cytosolic components in the activation of NADPH oxidase in a cell-free system consisting of purified and relipidated cytochrome b559, recombinant p47(phox), p67(phox), and Rac1, and the amphiphile, lithium dodecyl sulfate. We found that pronounced activation of NADPH oxidase can be achieved by exposing cytochrome b559 to p67(phox) and Rac1, in the total absence of p47(phox) (turnover = 60 mol O-2/s/mol cytochrome b559). However, maximal activation (turnover = 153 mol O-2/s/mol cytochrome b559) could only be obtained in the presence of p47(phox). O-2 production, in the absence of p47(phox), was dependent on: high molar ratios of p67(phox) and Rac1 to cytochrome b559, Rac1 being in the GTP-bound form, cytochrome b559 being saturated with FAD, and an optimal concentration of amphiphile. Single cytosolic components or combinations of two cytosolic components, other than p67(phox) and Rac1, were incapable of activation. We conclude that p67(phox) and Rac1 are the only cytosolic components directly involved in the induction of electron transport in cytochrome b559. p47(phox) appears to facilitate or stabilize the interaction of p67(phox) and, possibly, Rac1 with cytochrome b559, and is required for optimal generation of O-2 under physiological conditions.

Aerobic and anaerobic functioning of superoxide-producing cytochrome b-559 reconstituted with phospholipids

Cytochrome b-559 reconstituted with phospholipids and FAD represents the simplest model of the respiratory burst NADPH oxidase and reproduces the main catalytic features of this system (Koshkin, V. and Pick, E. (1993) FEBS Lett. 327, 57-62; (1994) FEBS Lett. 338, 285-289). In the present report it is shown that activation by oxygen, characteristic of the NADPH oxidase complex, is an intrinsic property of flavocytochrome b-559, in principle independent of its complexation with the other components of NADPH oxidase. Facilitation of electron transfer from NADPH to FAD is found to be the reason for this phenomenon. Kinetic studies of anaerobic operation of flavocytochrome b-559 revealed the functional heterogeneity of two hemes, manifested as a dramatic difference in their reducibility under these conditions.