Nitrophorins (NPs) represent a group of nitric oxide (NO)-carrying heme proteins found in the saliva of Rhodnius prolixus. In its adult phase R. prolixus expresses at least 4 nitrophorins (designated NP1-4 in order of their increasing abundance in the saliva of adult insects). Two additional nitrophorins, NP5 and NP6, have been detected mainly in the five instar nymphal stages of insect development. NP7 has not been isolated from the insects but was instead recognized in a cDNA library. NPs belong to the lipocalin/cytosolic fatty-acid binding protein family which have a large beta-barrel ligand-binding cavity. Lipocalins are mainly low molecular weight extracellular proteins that bind principally small hydrophobic ligands, and form covalent or non-covalent complexes with soluble macromolecules, as well as membrane bound-receptors. They participate in processes such as ligand transport, modulation of cell growth and metabolism, regulation of immune response, smell reception, tissue development and animal behavior. Cytosolic fatty-acid binding proteins, also bind hydrophobic ligands in a non-covalent, reversible manner, and have been implicated in intracellular uptake, transport and storage of hydrophobic ligands, regulation of lipid metabolism and sequestration of excess toxic fatty acids, as well as in signaling, gene expression, inflammation, cell growth and proliferation, and cancer development.
Jiyao W et al.(2023). "The conserved domain database in 2023.",