Saliva of the blood-sucking bug Rhodnius prolixus contains at least seven homologous nitrophorins, designated NP1 to NP7 in order of their relative abundance in the glands.
The Rhodnius prolixus nitrophorin structures reveal lipocalin-like eight-stranded β-barrel, three α-helices and two disulfide bonds, with heme inserted into one end of the barrel.
Members of the lipocalin family are known to bind a variety of small hydrophobic ligands, including biliverdin, in a similar fashion.
Asp-70 forms an unusual hydrogen bond with one of the heme propionates, suggesting the residue has an altered pKa.
In NP1-histamine structure, the planes of His-59 and histamine imidazole rings lie in an arrangement almost identical to that found in oxidized cytochrome b5.