Free heme and hemopexin in acute kidney injury after cardiopulmonary bypass and transient renal ischemia

Free heme is released from hemoproteins during hemolysis or ischemia reperfusion injury and can be pro‐inflammatory. Most studies on nephrotoxicity of hemolysis‐derived proteins focus on free hemoglobin (fHb) with heme as a prosthetic group. Measurement of heme in its free, non‐protein bound, form is challenging and not commonly used in clinical routine diagnostics. In contrast to fHb, the role of free heme in acute kidney injury (AKI) after cardiopulmonary bypass (CPB) surgery is unknown. Using an apo‐horseradish peroxidase‐based assay, we identified free heme during CPB surgery as predictor of AKI in patients undergoing cardiac valve replacement (n = 37). Free heme levels during CPB surgery correlated with depletion of hemopexin (Hx), a heme scavenger‐protein. In mice, the impact of high levels of circulating free heme on the development of AKI following transient renal ischemia and the therapeutic potential of Hx were investigated. C57BL/6 mice were subjected to bilateral renal ischemia/reperfusion injury for 15 min which did not cause AKI. However, additional administration of free heme in this model promoted overt AKI with reduced renal function, increased renal inflammation, and reduced renal perfusion on functional magnetic resonance imaging. Hx treatment attenuated AKI. Free heme administration to sham operated control mice did not cause AKI. In conclusion, free heme is a predictor of AKI in CPB surgery patients and promotes AKI in transient renal ischemia. Depletion of Hx in CPB surgery patients and attenuation of AKI by Hx in the in vivo model encourage further research on Hx therapy in patients with increased free heme levels during CPB surgery. In n = 37 patients undergoing cardiopulmonary bypass surgery (CPB) circulatory levels of free heme were detected by an apo‐horseradish peroxidase‐based assay. Sources of free heme such as ischemic tissue and free hemoglobin are indicated by the black arrows. The heme‐scavenging protein hemopexin was depleted during CPB (red arrow, top left panel). Circulatory free heme levels during CPB were found to be predictive of postoperative acute kidney injury (AKI) (top right panel). In an experimental study, the effect of exogenous free heme administration on the development of AKI and the therapeutic potential of hemopexin were investigated in mouse models of sham surgery or transient renal ischemia for 15min (bottom left panel). Free heme caused overt AKI with reduced renal perfusion, enhanced inflammation and