Cyclooxygenase activity of hemoglobin.

When hemoglobin (Hb) diffuses into the extracellular space of living tissues, which may occur during hemolysis or when erythrocytes escape from vessels, its catalytic potential toward several endogenous or exogenous compounds can be activated. There has been much attention focused on the capacity of Hb to metabolize polyunsaturated fatty acids (PUFAs), giving rise to the formation of products that may be important in the pathogenesis of systemic (intravascular hemolysis) or local (micro- or macrohemorrhages, inflammation, hemorrhagic infarcts) conditions in which erythrocyte lysis occurs. At low concentrations, Hb catalyzes a quasi-lipoxygenase reaction on linoleic acid with remarkably high substrate specificity. The fatty acid cyclooxygenases, which may be regarded as a special type of lipoxygenase that initiate the biosynthesis of prostaglandins and thromboxanes, require heme for their activity. Hb is a good candidate for exerting cyclooxygenase activity owing to its peroxidative properties already described in the chapter. There is also some discussion about the cyclooxygenase activity of Hb, which has been made possible by the use of either physicochemical or biological analytical method