Abstract
Phospholipase A2 (PLA2) hydrolyses phospholipids at sn-2 position to release free fatty acids and lysophospholipids. Secretory type of PLA2 (sPLA2) has been found in many different animals including insects. Insect sPLA2s have been divided into venomous and nonvenomous PLA2s. A non-venomous sPLA2 (Se-sPLA2) has been identified in beet armyworm, Spodoptera exigua. Its high enzyme activity is detected in hemolymph of naïve larvae. However, the physiological role of high sPLA2 activity in hemolymph remains unclear. To determine the physiological role of sPLA2 in hemolymph, a recombinant Se-sPLA2 (rSe-sPLA2) was expressed in a bacterial expression system and purified to test antimicrobial activity against various microbes. Purified rSe-sPLA2 exhibited typical enzyme kinetic properties, including becoming saturated at high substrate concentrations, exhibiting optimal activity at pH 7–9, and being inactivated at high temperatures. However, a reducing agent (dithiothreitol) or calcium chelator treatment inhibited the catalytic activity. A specific inhibitor to sPLA2 also inhibited the enzyme activity of rSe-sPLA2 while other type PLA2 inhibitors did not. Furthermore, eight bacterial metabolites of Xenorhabdus and Photorhabdus known to be inhibitory against insect PLA2 significantly inhibited the enzyme activity of rSe-sPLA2. High concentrations of rSe-sPLA2 (above 0.5 mM) showed significant cytotoxicity to hemocytes of S. exigua. At concentrations without showing cytotoxicity, rSe-sPLA2 possessed significant antimicrobial activities against entomopathogenic bacteria (Serratia marscens and Entercoccus mondtii) and fungi (Beauveria bassiana and Metarhyzium rileyi). Hemolymph obtained from larvae treated with RNA interference specific to Se-sPLA2 significantly lost such antimicrobial activities. However, the addition of rSe-sPLA2 to the hemolymph significantly rescued such antimicrobial activities. These results indicate that Se-sPLA2 possesses antimicrobial activity, suggesting that it might act as a prophylactic agent against microbial pathogens in the hemolymph of S. exigua.
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