**2. An overview of hepatic ischemia reperfusion injury**

As noted above, the patholophysiology of IRI is multifactorial and involves a multitude of oxidative and cellular mechanisms. Briefly, hepatic IRI can be described as a two phase process with early (acute) and late (sub-acute) injury (Fan, Zwacka, & Engelhardt, 1999; Zwacka, Zhang, Zhou, Halldorson, & Engelhardt, 1998b). The distinction is particularly important because potential theraputic targets (i.e. methods of increasing NO in the hepatic micro-envirnoment) may have different effects on these two phases. Early injury is mediated by a rapid change in the biochemical redox state of the tissue to a more oxidative one. It occurs within 5 minutes, and is not associated with leukocyte infiltration. Following the acute state is an increase in endothelial cell adhesion molecules, chemokines and cytokines. These molecules then herald the late phase characterized by a significant infiltration of polymorphonuclear neutrophils, further release of a reactive oxygen species (ROS) and extensive inflammation and tissue injury.

NO plays a significant role in the acute phase of IRI, as this phase is associated with a rapid decrease in available NO. This decrease occurs either by depressed production by eNOS in sinusoidal endothelial cells (SECs), increased degradation by ROS, or both. The ROS implicated are chiefly O2•- (superoxide, see next paragraph), but also include hydrogen peroxide (H2O2). In the last few years, the implicated enzyme responsible for production of ROS has shifted from hepatoctye xanthine oxidase to NADPH oxidase in Kuppfer cells or mitochondrial sources of ROS (Hines & Grisham, 2011).

The term "reactive oxygen species" in the context of hepatic IRI primarily refers to superoxide. Two studies that incorporated manganese superoxide dismutase (MnSOD) – an enzyme which degrades superoxide – into liver tissue showed attenuation of IRI (He et al., 2006; Zwacka et al., 1998a). Therefore superoxide itself seems important in IRI. The mechanism by which superoxide imparts its damage is somewhat unclear, but it is known that membrane lipid peroxidation is associated with oxidative damage. Perhaps more importantly, damage by superoxide to mitochondrial membrane proteins and therefore ATP generating capacity and may a more important mechanism in IRI(Madesh & Hajnóczky, 2001; Moon et al., 2008).
