Mechanisms of oxidant-mediated microvascular injury following reperfusion of the ischemic intestine.

Based on work from our laboratory and studies by others, we propose the following hypothesis to explain the interaction among xanthine oxidase, PMNs, and tissue injury in the postischemic small intestine (Figure 2). During the ischemic period, ATP is catabolized to yield hypoxanthine. The hypoxic stress also triggers the conversion of NAD-reducing xanthine dehydrogenase to the oxygen radical-producing xanthine oxidase via a protease. When the intestine is reperfused, molecular oxygen is reintroduced into the tissue where it reacts with hypoxanthine and xanthine oxidase to produce a burst of superoxide anion and hydrogen peroxide. In the presence of ferric iron, superoxide anion and hydrogen peroxide react via the Haber-Weiss reaction to form hydroxyl radicals. This highly reactive and cytoxic free radical then initiates lipid peroxidation of cell membrane components and the subsequent release of substances that activate, attract, and promote the adherence of PMN to microvascular endothelium. The adherent PMN then causes further endothelial cell injury via the release of superoxide and various proteases.