Hemoglobin (Hb) is released from red blood cells during hemolysis. Extracellular Hb causes vascular instability due to depletion of small signal transduction molecules such as nitric oxide. Free Hb also leads to oxidative tissue damage especially on the endothelium: However, the mechanism of this damage was poorly characterized. In the present work, we identified lipid oxidation products as the ultimate toxic reaction species. In free Hb, especially in its ferric redox state (Fe3+), heme is only loosely bound in the heme pockets and thus can easily be released. Free heme accumulates in an apolar milieu such as in liposomes or cell membranes. Inside liposomes, heme is able to exponentially accelerate lipid peroxidation and thus formation of lipid oxidation products, finally exerting endothelial toxicity. We have identified haptoglobin (Hp) as a potent inhibitor of Hb toxicity. Hp binds Hb irreversibly and thus forms a stabilizing complex that completely inhibits heme release. We have further found Hemopexin (Hx) as a second potent inhibitor of Hb oxidative toxicity. Hx, in contrast to Hp, does not bind to Hb but encapsulates free heme at very high affinity and thus prevents translocation of Hx into liposomes. The specific target-specificities of Hp and Hx, respectively, provide a biochemical explanation for the enigmatic description of Hp
as the primary and Hx as the backup protection system against Hb toxicity. Subsequent invivo and later clinical studies are necessary to confirm the protective potency of Hp and Hx with further data based on the fundamental biochemical and cell biological mechanics identified within this thesis.