The evolutionarily conserved, cofactor-dependent, enzyme xanthine oxidoreductase exists in both cell-associated and circulatory forms. The exact role of the circulating form is not known; however, several putative physiological and pathological functions have been suggested that range from purine catabolism to a mediator of acute respiratory distress syndrome. Regulation of gene expression, cofactor synthesis and insertion, post-translational conversion, entry into the circulation, and putative physiological and pathological roles for human circulating xanthine oxidoreductase are discussed.
Hewinson, J., Stevens, C. R., & Millar, T. M. (2004). Vascular physiology and pathology of circulating xanthine oxidoreductase: from nucleotide sequence to functional enzyme. Redox Report, 9(2), 71-79. https://doi.org/10.1179/135100004225004797