We reported previously that Ascaris suum cytochrome b(5), specifically expressed in this nematode at the adult stage and dually localized in extracellular perienteric fluid and hypodermis, is involved in both perienteric NADH-methemoglobin and cytosolic NADH-metmyoglobin reduction, where cytochrome b5 functions as an electron carrier between NADH-mediated cytochrome b(5) reductase and substrates, methemo(myo)globins to reduce the nonfunctional globins back to functional ferrous hemo(myo)globins. To further characterize NADH-methemo(myo)globin reductase systems, the midpoint potentials of A. suum perienteric hemoglobin and body wall myoglobin, as well as the affinities of Ascaris methemoglobin and metmyoglobin toward cytochrome b(5), were evaluated using potentiometric titration and surface plasmon resonance techniques, respectively. Midpoint potentials of +7.2 mV and +19.5 mV were obtained for Ascaris perienteric hemoglobin and body wall myoglobin, respectively. The affinities of Ascarls perienteric methemoglobin and body wall metmyoglobin toward the nematode cytochrome b(5) were comparable to that for mammalian hemoglobin and cytochrome b(5); association constants were 0.585 x 10(3) M(-1) and 2.32 x 10(3) M(-1), respectively, with rapid equilibration kinetics. These observations highlight the physiological importance of A. suum perienteric NADH-methemoglobin and cytosolic metmyoglobin reductase systems. Differential roles of A. suum perienteric hemoglobin and body wall myoglobin are also discussed from the viewpoint of oxygen homeostasis under hypoxic conditions. (C) 2009 Elsevier Ireland Ltd. All rights reserved.