The NADPH oxidases are a family of multimeric enzymes specifically designed to translocate electrons (from NADPH) across a membrane in a manner that results in the formation of ROS. Central to this function is a catalytic subunit (officially known as the Nox or Duox gene products) with a conserved general domain structure () that consists of six transmembrane helices, binding sites for NADPH and FAD, and heme-coordinating histidine residues. Five isoforms (Nox 1 to 5) function strictly as oxidases, and two isoforms (Duox 1 and 2) contain peroxidase homology domains, although the functional capacity of these domains is not clear.
FIG. 1. NADPH oxidase family members translocate electrons (from NADPH) across a membrane, which results in the formation of ROS (predominantly ). The seven NADPH oxidase family members (Nox 1 to 5 and Duox 1 and 2) share conserved features, including six transmembrane (more ...)
For the last several years, multiple Nox genes have been described, with the gene products demonstrating wide-ranging tissue distribution, suggesting that ROS production may have broad implications for the cellular phenotype (23
) (). The literature regarding the Nox
genes and the NADPH oxidase enzyme family is ambiguous. Many published reports refer to “NADPH oxidase” without regard to the specific gene product(s) involved. Moreover, the precise requirements for the catalytic activity of different Nox
gene products () is not yet complete. In this review, therefore, we identify specific NADPH oxidases by their Nox
gene product. The term “NADPH oxidase” will be reserved for instances in which the specific gene product is not known.
Table 1. NADPH Oxidase Family Members Exhibit a Wide Spectrum of Diversity Including Differences in Terms of Amino Acid Sequence Identity (here Compared to the Prototypical Nox2), Required Subunits for Activation, Intracellular Localization, Tissue Distribution, (more ...)
With regard to gene structure, the Nox/Duox family arose in concert with multicellular organization (60
), suggesting that they contribute to the distinction among the functions of different cell types. Moreover, observations in primitive organisms suggest that NADPH oxidases may be important for stress responses. In the slime mold, Dictyostelium discoideum
, nutrient stress induces individual amoebae to aggregate into a slug that can produce a spore-bearing fruiting body. Deletions that disable NADPH oxidase function interrupts the formation of a fruiting body (64
). Thus, the NADPH oxidase family is evolutionary conserved and affects stress-induced behavior.
The first-identified and best-studied Nox isoform is known as Nox2 (originally termed gp91phox
), which was identified in phagocytes for its role in antimicrobial ROS production (3
). Nox 2 requires another integral membrane protein, p22phox
for protein stabilization, but the activation of Nox2 is dependent on the translocation of several other cytosolic regulatory subunits, specifically p47phox
, and Rac. As indicated in , NADPH oxidase family members have distinct regulatory patterns that do not strictly mirror those of Nox2. For example, Nox1 is found in cells that do not express p47 or p67. In such cells, Nox1 catalytic activity is supported by Nox organizer 1 (NoxO1), and Nox activator 1 (NoxA1). With regard to Nox4, constitutive activity and no requirement of cytosolic factors for activity seem to exist. Thus, it is not surprising that NADPH oxidase activity has been implicated in a host of varied cellular functions.
Understanding how NADPH oxidase–derived ROS are able to achieve target selectivity and specificity is paramount (21
). Conceptually, the target selectivity of these may exist at the level of the chemical characteristics of the oxidants or, perhaps, intracellular antioxidants. Alternatively, selectivity of NADPH oxidase–derived ROS could also involve their specific reactivity with putative targets in the cytosol or proteins. Finally, one could consider the intracellular localization or compartmentalization or both of NADPH oxidase catalytic activity as a means of specifying ROS signaling. In the remainder of this review, we focus our attention on mechanisms related to specificity of cell signaling.