XDH and pulmonary emphysema: The role of XOR in the pathophysiology of disease has been widely investigated and the enzyme’s importance has been substantiated by studies in a multitude of pathologies including cardiovascular disease [18, 19], ischemia-reperfusion injury [20], acute lung injury [6], and emphysema [14, 21–23], all of which are syndromes characterized by tissue hypoxia, a known major activator of XOR.