Purification and identification of a lepidopteran fatty acid amide hydrolase and its role in larval metabolism

Several fatty-acid amide compounds (FACs) in caterpillar oral secretions enhance plant defense responses during herbivory. The most common FACs contain a fatty acid moiety either linolenic or linolenic acids that is obtained from the plant, while the amino acid moiety is almost exclusively endogenous glutamine. The levels of these FAC elicitors within the caterpillar appear to be controlled by two enzymes, one in the gut tissue membranes that synthesizes FACs and the second in the gut lumen that hydrolyzes them. Since FACs are persistent despite obvious fitness costs, it is believed that they are important to the performance and/or survival of the caterpillar.

In order to better understand the role of FACs within the caterpillar several approaches are being used. First, the FAC hydrolase has been purified by liquid chromatography and identified by N-terminal sequencing and tandem MS/MS as an aminoacylase-1 like protein. In mammals, orthologous enzymes are involved in nitrogen recycling and detoxification pathways. However, in caterpillars the FAC hydrolase is present in the gut lumen and hydrolyzes a specific substrate that is purposely synthesized by the caterpillar, thus supporting a novel digestive role.

A second approach to determine the role of FACs involves feeding the caterpillars augmented diets and measuring performance. When fed diets with increased levels of linolenic acid, Helithis virescens and Helicoverpa zea caterpillars appeared to have different requirements of linolenic acid for optimum performance. A difference in linolenic acid requirement may explain the difference between the rates of FAC hydrolysis in these species.