Salivary thiol oxidase activity of Rhodnius prolixus

Cysteine and other thiol compounds can accelerate the unloading of nitric oxide (NO) from salivary nitrosyl-nitrophorins of the blood sucking bug Rhodnius prolixus. The dependence of NO unloading on cysteine concentration is biphasic, showing a maximum between 0.5 and 1 mM cysteine. The proposed mechanism of action for the unloading is a series of reactions where cysteine (at low concentrations) reacts with the heme group of nitrophorins to form cystine and superoxide. The superoxide then reacts with NO to form peroxynitrite, which decays to a mixture of nitrite and nitrate anions. At high cysteine concentrations, cysteine is converted to cystine and H2O and thus no removal of NO from nitrophorins is observed. The thiol oxidase activity of Rhodnius nitrophorins is similar to that observed before in plant peroxidases [Pichorner et al., Phytochemistry 31, 3371 (1992)]. The possible physiological significance of this reaction to probing and feeding by R. prolixus is discussed.