We reported previously that Ascaris suum
cytochrome b5, 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 b5 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 b5, 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 Ascaris perienteric
methemoglobin and body wall
metmyoglobin toward the nematode
cytochrome b5 were comparable to that for mammalian
hemoglobin and
cytochrome b5; 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.