Nearly 30 years have passed since the discovery of xanthine oxidoreductase (XOR) as a critical source of reactive species in ischemia/reperfusion injury. Since then, numerous inflammatory disease processes have been associated with elevated XOR activity and allied reactive species formation solidifying the ideology that enhancement of XOR activity equates to negative clinical outcomes. However, recent evidence may shatter this paradigm by describing a nitrate/nitrite reductase capacity for XOR whereby XOR may be considered a crucial source of beneficial •NO under ischemic/hypoxic/acidic conditions; settings similar to those that limit the functional capacity of nitric oxide synthase. Herein, we review XOR-catalyzed reactive species generation and identify key microenvironmental factors whose interplay impacts the identity of the reactive species (oxidants vs. •NO) produced. In doing so, we redefine existing dogma and shed new light on an enzyme that has weathered the evolutionary process not as gadfly but a crucial component in the maintenance of homeostasis.

Numerous inflammatory disorders are associated with elevated levels of xanthine oxidoreductase (XOR) and allied enhancement of reactive species formation contributory to systemic pathology. Despite a long standing association between increased XOR activity and negative clinical outcomes, recent reports describe a paradigm shift where XOR mediates beneficial actions by catalyzing the reduction of NO2− to •NO. While provocative, these observations contradict reports of improved outcomes in similar models upon XOR inhibition as well as reports revealing strict anoxia as a requisite for XOR-mediated •NO formation. To garner a more clear understanding of conditions necessary for in vivo XOR-catalyzed •NO production, this review critically analyzes the impact of O2 tension, pH, substrate concentrations, glycoaminoglycan docking and inhibition strategies on the nitrite reductase activity of XOR and reveals a hypoxic milieu where this process may be operative. As such, information herein serves to link recent reports in which XOR activity has been identified as mediating the beneficial outcomes resulting from nitrite supplementation to a microenviromental setting where XOR can serve as substantial source of •NO.

Rationale Soluble guanylate cyclase (sGC) heme iron, in its oxidized state (Fe3+), is desensitized to nitric oxide (NO) and limits cyclic guanosine 3′, 5′-monophosphate (cGMP) production needed for downstream activation of PKG-dependent signaling and blood vessel dilation. Objective While reactive oxygen species are known to oxidize the sGC heme iron, the basic mechanism(s) governing sGC heme iron recycling to its NO-sensitive, reduced state, remain poorly understood. Methods and Results Oxidant challenge studies show vascular smooth muscle cells have an intrinsic ability to reduce oxidized sGC heme iron and form protein-protein complexes between cytochrome b5 reductase 3 (Cyb5R3), also known as methemoglobin reductase, and oxidized sGC. Genetic knockdown and pharmacological inhibition in VSMCs reveal Cyb5R3 expression and activity is critical for NO-stimulated cGMP production and vasodilation. Mechanistically, we show Cyb5R3 directly reduces oxidized sGC required for NO sensitization as assessed by biochemical, cellular, and ex vivo assays. Conclusions Together, these findings identify new insights into NO-sGC-cGMP signaling and reveal Cyb5R3 as the first identified physiological sGC heme iron reductase in VSMCs, serving as a critical regulator of cGMP production and PKG dependent signaling.

Accumulation of senescent cells over time contributes to aging and age-related diseases. However, what drives senescence in vivo is not clear. Here we used a genetic approach to determine if spontaneous nuclear DNA damage is sufficient to initiate senescence in mammals. Ercc1-/∆ mice with reduced expression of ERCC1-XPF endonuclease have impaired capacity to repair the nuclear genome. Ercc1-/∆ mice accumulated spontaneous, oxidative DNA damage more rapidly than wild-type (WT) mice. As a consequence, senescent cells accumulated more rapidly in Ercc1-/∆ mice compared to repair-competent animals. However, the levels of DNA damage and senescent cells in Ercc1-/∆ mice never exceeded that observed in old WT mice. Surprisingly, levels of reactive oxygen species (ROS) were increased in tissues of Ercc1-/∆ mice to an extent identical to naturally-aged WT mice. Increased enzymatic production of ROS and decreased antioxidants contributed to the elevation in oxidative stress in both Ercc1-/∆ and aged WT mice. Chronic treatment of Ercc1-/∆ mice with the mitochondrial-targeted radical scavenger XJB-5–131 attenuated oxidative DNA damage, senescence and age-related pathology. Our findings indicate that nuclear genotoxic stress arises, at least in part, due to mitochondrial-derived ROS, and this spontaneous DNA damage is sufficient to drive increased levels of ROS, cellular senescence, and the consequent age-related physiological decline.

These observations support that the pleiotropic signalling actions of electrophilic fatty acids represent a therapeutic strategy for limiting the complex pathogenic responses instigated by obesity.