XDH and congestive heart failure: Landmesser et al.[38] discovered that the activity of extracellular SOD bound to the endotheliumwas significantly decreased, whereas the activity of extracellular XO wassignificantly augmented in HF patients in comparison with the control group.These findings imply that the increase in XO activity is intimately associatedwith oxidative stress in the vasculature of chronic heart failure (CHF) patients [38] (Fig. 2).