Watanabe K, et al. - Given that xanthine oxidoreductase (XOR) plays a key role in making reactive oxygen species (ROS), which promotes development of endothelial dysfunction, which is associated with coronary artery spasm (CAS), researchers investigated the association between plasma XOR activity and CAS. They recruited 104 patients suspected for CAS presenting with no significant coronary artery stenosis and who underwent intracoronary acetylcholine provocation tests. Three patient groups were formed based on the XOR activity. The incidence rate of CAS increased with increasing XOR activity. This association was assessed for the first time via this study, with findings revealing a significant association of higher plasma XOR activity with CAS.