Bioinformatic identification of cytochrome b5 homologues from the parasitic nematode Ascaris suum and the free-living nematode Caenorhabditis elegans highlights the crucial role of A. suum adult-specific secretory cytochrome b₅ in parasitic adaptation.

We previously reported that adult Ascaris suum possesses NADH-metmyoglobin and NADH-methaemoglobin reductase systems that are located in the cells of the body wall and in the extracellular perienteric fluid, respectively, which helps them adapt to environmental hypoxia by recovering the differential functions of myoglobin and haemoglobin. A. suum cytochrome b5, an adult-specific secretory protein and an essential component of the NADH-metmyo (haemo) globin reductase system, has been extensively studied, and its unique nature has been determined. However, the relationship between A. suum cytochrome b5 and the canonical cytochrome b5 proteins, from the free-living nematode Caenorhabditis elegans is unclear. Here, we have characterised four cytochrome b5-like proteins from C. elegans (accession numbers: CAB01732, CCD68984, CAJ58492, and CAA98498) and three from A. suum (accession numbers: ADY48796, ADY46277, and ADY48338) and compared them with A. suum cytochrome b5 in silico. Bioinformatic and molecular analyses showed that CAA98498 from C. elegans is equivalent of A. suum cytochrome b5, which was not expressed as a mature mRNA. Further, the CAA98498 possessed no secretory signal peptide, which occurs in A. suum cytochrome b5 precursor. These results suggest that this free-living nematode does not need a haemoprotein such as the A. suum cytochrome b5 and highlight the crucial function of this A. suum adult-specific secretory cytochrome b5 in parasitic adaptation.