Heme-based protein that delivers nitric oxide gas (NO) to the victim while feeding, resulting in vasodilation and inhibition of platelet aggregation. Also binds tightly to histamine, which is released by the host to induce wound healing. NO release is pH dependent and linked to loop dynamics. 2 H(+) + 3 nitrite = H2O + nitrate + 2 nitric oxide Binds 1 heme b (iron(II)-protoporphyrin IX) group per subunit. Expressed by the salivary glands. Superposition of the structures of NP4 in the presence of ammonia or nitrite shows a conformational change of the protein upon NO(2)(-) binding. Belongs to the calycin superfamily. Nitrophorin family.