Oxidative mechanisms of hemoglobin-based blood substitutes.

Chemically or genetically altered cell-free hemoglobin (Hb) has been developed as an oxygen carrying therapeutic. Site-directed modifications are introduced and serve to stabilize the protein molecules in a tetrameric and/or a polymeric functional form. Direct cytotoxic effects associated with cell-free Hb have been ascribed to redox reactions (involving either 1 or 2 electron steps) between the heme group and peroxides. These interactions are the basis of the pseudoperoxidase activity of Hb and can be cytotoxic when reactive species are formed at relatively high concentrations during inflammation and typically lead to cell death. Peroxides relevant to biological systems include hydrogen peroxide (H2O2), lipid hydroperoxides (LOOH), and peroxynitrite (ONOO-). Reactions between Hb and peroxides form the ferryl oxidation state of the protein, analogous to compounds I and II formed in the catalytic cycle of many peroxidase enzymes. This higher oxidation state of the protein is a potent oxidant capable of promoting oxidative damage to most classes of biological molecules. Further complications are thought to arise through the disruption of key signaling pathways resulting from alteration of or destruction of important physiological mediators.