Vasoactive intestinal peptide, a singlet oxygen quencher

The neuropeptide vasoactive intestinal peptide (VIP), a highly basic 28-amino acid peptide, has a widespread distribution in the body. The functional specificity of this peptide not only includes its potent vasodilatory activity, but also its role in protecting lungs against acute injury, in preventing T-lymphocyte proliferation and in modulating immune function. We have investigated the possible antioxidant properties of VIP and found that VIP does not have significant O2-, OH., or H2O2 scavenging ability. However, VIP was found to inhibit, in a dose-dependent manner, the 1O2-dependent 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) formation. 1O2 was produced in photosensitizing systems using rose bengal or methylene blue as sensitizers and was detected as TEMP-1O2 product (TEMPO) by electron paramagnetic resonance (EPR) spectroscopic techniques. The formation of TEMPO signal was strongly inhibited by known singlet quenchers, e.g. beta-carotene, histidine as well as azide, but not by catalase (20 micrograms/ml) which removes H2O2 and mannitol (6 mM) or ethanol (5.9 mM) which remove OH.. Superoxide dismutase (2.5 micrograms/ml) inhibited the photoreaction up to 20% by removing O2- and most probably by blocking the secondary charge transfer pathway of 1O2 formation. These results suggest that the formation of nitroxide radical by 1O2 attack on TEMP may be used as a simple and specific assay for 1O2, and VIP can serve as an effective 1O2 scavenger/quencher, thus it may modulate the oxidative tissue injury caused by this reactive species of oxygen.