![]() ![]() The continued, high-level expression of Hp in cartilaginous fishes in which Hb binding is not evident signals that Hp has (an)other, yet unstudied, role(s) in these species. ![]() We show that the Hb-binding ability of cartilaginous fish (nurse shark, Ginglymostoma cirratum small-spotted catshark, Scyliorhinus canicula and thornback ray, Raja clavata) and teleost fish (rainbow trout, Oncorhynchus mykiss) Hp is species specific, and where binding does occur it is likely mediated through a different structural mechanism to mammalian Hp. We demonstrate that Hp has been independently lost from multiple vertebrate lineages, that characterized Hb-interacting residues of mammals are poorly conserved in nonmammalian species maintaining Hp, and that the extended loop 3 region of Hp, which mediates CD163 binding, is present only in mammals. In this study, we show that Hp is a divergent member of the complement-initiating MASP family of proteins, which emerged in the ancestor of jawed vertebrates. The resultant Hp-Hb complexes are bound and cleared by the scavenger receptor CD163, limiting Hb-induced oxidative damage. In mammals, haptoglobin (Hp) is an acute-phase plasma protein that binds with high affinity to hemoglobin (Hb) released by intravascular hemolysis. ![]()
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