Impact of oxidized plant phenolics on the nutritional quality of dietar protein to a noctuid herbivore, Spodoptera exigua

Treatment of selected dietary proteins (i.e. casein, soy protein, gluten, zein and tomato foliar protein) with chlorogenic acid and polyphenol oxidase significantly reduced protein quality to Spodoptera exigua larvae as measured by larval growth rate. The reduction in growth was negatively correlatable with the content of lysine, histidine, cystein and methionine for each protein. These amino acids contain nucleophilic centres (e.g. -SH, -NH₂) that are susceptible to alkylation by quinones formed from the enzymatic oxidation of chlorogenic acid. The treatment of the proteins with chlorogenic acid and polyphenol oxidase caused significant losses in alkylatable amino acids (i.e. cysteine, histidine, methionine and lysine). The amount of chlorogenic acid bound to each protein was significantly increased by polyphenol oxidase activity and was dependent upon the content of the alkylatable amino acids. In the absence of polyphenol oxidase, the influence of alkylatable amino acids content was different. The toxicity of chlorogenic acid was directly proportional to the content of alkylatable amines in a model protein, casein. Also, the ability of chlorogenic acid to bind to protein was inversely proportional to alkylatable amine content. The importance of phenolics and phenolic oxidizing enzymes in determining protein quality to insect herbivores is discussed.