Ascaris suum cytochrome b5, an adult-specific secretory protein reducing oxygen-avid ferric hemoglobin

Abstract

The anaerobic parasitic nematode Ascaris suum has an oxygen-avid hemoglobin in the perienteric fluid, the biological function of which remains elusive. Here, we report that Ascaris cytochrome b 5 is expressed specifically in the intestinal parasitic stage and is secreted into the perienteric fluid, thus co-localizing with Ascaris hemoglobin. We also found that cytochrome b 5 reduces Ascaris non-functioning ferric methemoglobin more efficiently than mammalian methemoglobin. Furthermore, a computer graphics model of the electron transfer complex between Ascaris cytochrome b 5 and Ascaris hemoglobin strongly suggested that these two proteins are physiological redox partners. Nitric oxide has been reported to react easily with oxygen captured in hemoglobin to form nitrate, but not toxic free radicals, which may result in production of methemoglobin for the cytochrome b 5 to regenerate functional ferrous hemoglobin. Therefore, our findings suggest that Ascaris cytochrome b 5 is a key redox partner of Ascaris hemoglobin, which acts as an antioxidant.