Enhanced nitrite reductase activity associated with the haptoglobin complexed hemoglobin dimer: Functional and antioxidative implications

► Ligand binding for the haptoglobin-hemoglobin complex is similar to the Hb dimer. ► Nitric oxide production by the haptoglobin-hemoglobin complex is significantly enhanced. ► Enhanced NO production should limit vascular toxicity from acellular Hb.