Prolixin-S, an anticoagulant from the salivary gland of the blood-sucking insect Rhodnius prolixus is also one of the members of salivary gland hemoproteins. We produced recombinant protein using a baculovirus-insect cell expression system, reconstituted the hemoprotein and made some characterization of it as a nitric oxide carrier. The reconstituted protein exhibited the absorption spectrum of a high-spin ferric hemoprotein with a Soret absorption peak at 400 nm. By binding nitric oxide (NO-prolixin-S), the Soret band shifted from 400 nm to 420 nm and two sharp bands (Q bands, at 535 nm and 565 nm) also appeared in the visible region. In a bioassay with aortic smooth muscle, NO-prolixin-S showed strong relaxation activity in a dose-dependent manner, which demonstrated that prolixin-S really acts as an NO carrier. A Soret absorption change also indicated that nitric oxide was gradually released under these conditions (pH 7.4 and 37 degrees C). However, at low temperature (20 degrees C) and/or low pH (pH 6), which mimic those in the insect's salivary glands, the releasing became very slow. These different NO-binding properties would enable prolixin-S to reserve nitric oxide in the salivary glands and release it in the host's blood vessels.