Absence of detectable xanthine oxidase in human myocardium.

The enzyme xanthine oxidase has been implicated as a generator of toxic oxygen metabolites that contribute to ischemic injury. Because substantial species variability has been demonstrated and because there are minimal human data available, the relevance of xanthine oxidase to human heart damage has been in doubt. We report the absence of xanthine oxidase activity in nine human heart biopsy specimens obtained during cardiac surgery, and in two larger samples obtained during heart transplantation. A sensitive radiochemical assay was used to assess enzyme activity. Our findings imply that oxygen free radicals generated by xanthine oxidase are not relevant in terms of human myocardial injury.