The occurrence of two types of hemopexin-like protein in medaka and differences in their affinity to heme

Full-length cDNA clones encoding two types of hemopexin-like protein, mWap65-1 and mWap65-2, were isolated from the HNI inbred line of medaka Oryzias latipes. The deduced amino acid sequence of mWap65-2 resembled mammalian hemopexins more closely than that of mWap65-1. Histidine residues required for the high affinity of hemopexins for hemes were conserved in mWap65-2, but not in mWap65-1. Surprisingly, mWap65-1, but not mWap65-2, showed heme-binding ability as revealed by hemin-agarose affinity chromatography, even though mWap65-1 lacked the essential histidine residues. Furthermore, RT-PCR analysis of different tissues demonstrated that the transcripts of mWap65-2 were restricted to liver, whereas those of mWap65-1 were found in various tissues including liver, eye, heart and brain. Quantitative RT-PCR revealed that transcripts of mWap65-2 were expressed earlier than those of mWap65-1 during ontogeny. However, the accumulated mRNA levels of both mWap65-1 and mWap65-2 did not differ significantly in fish acclimated to either 10 degrees C or 30 degrees C for 5 weeks. These characteristics suggest that the two proteins have different physiological functions and that mWap65-2 is not a hemopexin.