Effects of the cytolytic seed protein enterolobin on coleopteran and lepidopteran insect larvae

Enterolobin, a novel 55 KDa cytolytic and inflammatory protein fromEnterolobium contortisiliquum seeds, was tested for its toxic effects on larvae of the coleopteranCallosobruchus maculatus and the lepidopteranSpodoptera littoralis. Bioassays performed with enterolobin incorporated into artificial seeds showed that the phytocytolysin was toxic to larvae ofC. maculatus, causing 70% mortality at a concentration of 0.01% (w/w) and 100% mortality at 0.025%. The protein proved to be innocuous to larvae ofS. littoralis.In vitro proteolysis studies using larval gut enzymes, analysed on SDS-PAGE, showed that onlyS. littoralis proteases could digest enterolobin, suggesting that the insect's digestive proteases were able to inactivate the cytolysin before it could exert any toxic effect;C. maculatus proteases, on the other hand, were unable to hydrolyse enterolobin. The mechanism of toxicity of enterolobin did not appear to involve any damage to the microvilli of the epithelial gut cells ofC. maculatus as shown by electron microscopy. Some tentative hypotheses are considered in order to explain the toxic mechanism of action of enterolobin towardsC. maculatus.