The impact of xanthine oxidase (XO) on hemolytic diseases

Abstract

Hemolytic diseases are associated with elevated levels of circulating free heme that can mediate endothelial dysfunction directly via redox reactions with biomolecules or indirectly by upregulating enzymatic sources of reactive species. A key enzymatic source of these reactive species is the purine catabolizing enzyme, xanthine oxidase (XO) as the oxidation of hypoxanthine to xanthine and subsequent oxidation of xanthine to uric acid generates superoxide (O₂^•-) and hydrogen peroxide (H₂O₂). While XO has been studied for over 120 years, much remains unknown regarding specific mechanistic roles for this enzyme in pathologic processes. This gap in knowledge stems from several interrelated issues including: 1) lethality of global XO deletion and the absence of tissue-specific XO knockout models have coalesced to relegate proof-of-principle experimentation to pharmacology; 2) XO is mobile and thus when upregulated locally can be secreted into the circulation and impact distal vascular beds by high-affinity association to the glycocalyx on the endothelium; and 3) endothelial-bound XO is significantly resistant (> 50%) to inhibition by allopurinol, the principle compound used for XO inhibition in the clinic as well as the laboratory. While it is known that circulating XO is elevated in hemolytic diseases including sickle cell, malaria and sepsis, little is understood regarding its role in these pathologies. As such, the aim of this review is to define our current understanding regarding the effect of hemolysis (free heme) on circulating XO levels as well as the subsequent impact of XO-derived oxidants in hemolytic disease processes.

Keywords: Heme toxicity; Hemolysis; Reactive oxygen species; Therapeutics; Xanthine oxidase.

Fig. 1

A red blood cell (RBC) undergoing hemolysis releases hemoglobin (Hb) and heme. Hb is bound by haptoglobin (Hp) and targeted to macrophages for degradation. Heme is either 1) bound by hemopexin (Hx) and targeted to the liver for iron storage, redistribution, or degradation by cellular heme oxygenase-1 (HO-1); or 2) degraded directly in the plasma by HO-1 into iron, carbon monoxide (CO) and biliverdin (gray panel). Hemolytic diseases cause elevated levels of hemolysis which saturate Hx and HO-1. When these pathways are saturated, heme activates toll-like receptor-4 (TLR4) triggering an immune response. Separately, heme can also generate reactive oxygen species (ROS) directly and indirectly through iron (green panel).

Fig. 2

Xanthine oxidase (XO) released from the liver can bind to the surface of endothelial cells via interactions with glycosaminoglycans (GAGs). As hypoxanthine is converted to xanthine, and xanthine to uric acid by XO, reactive oxygen species (ROS) are generated at the surface of endothelial cells. XO produces either O₂^•- or H₂O₂ which can lead to production of ONOO^- or ^•HO (top). Treatment with an XO inhibitor such as febuxostat inhibits XO and blocks the conversion of hypoxanthine to xanthine and xanthine to uric acid preventing ROS production and damage at the endothelial cell surface (bottom).