ANALYSIS OF ENERGETIC AMINO-ACID-METABOLISM IN ACYRTHOSIPHON-PISUM - A MULTIDIMENSIONAL APPROACH TO AMINO-ACID-METABOLISM IN APHIDS

Aphids are highly specialized insects that feed on the phloem-sap of p lants, the amino acid composition of which is very unbalanced. Amino a cid metabolism is thus crucial in aphids, and we describe a novel inve stigation method based on the use of C-14-labeled amino acids added in an artificial diet. A metabolism cage for aphids was constructed, all owing for the collection and analysis of the radioactivity incorporate d into the aphid body, expired as CO2, and rejected in the honeydew an d exuviae. This method was applied to the study of the metabolism of e ight energetic amino acids (aspartate, glutamate, glutamine, glycine, serine, alanine, proline, and threonine) in the pea aphid, Acyrthosiph on pisum. All these amino acids except threonine were subject to subst antial catabolism as measured by high (CO2)-C-14 production. The highe st turnover was displayed by aspartate, with 60% of its carbons expire d as CO2. For the first time in an aphid, we directly demonstrated the synthesis of three essential amino acids (threonine, isoleucine, and lysine) from carbons of common amino acids. The synthesis of these thr ee compounds was only observed from amino acids that were previously c onverted into glutamate. This conversion was important for aspartate, and lower for alanine and proline. To explain the quantitative results of interconversion between amino acids, we propose a compartmentation model with the intervention of bacterial endosymbiotes for the synthe sis of essential amino acids and with glutamate as the only amino acid supplied by the insect to the symbiotes. Moreover, proline exhibited partial conversion into arginine, and it is suggested that proline is probably indirectly involved in excretory nitrogen metabolism. (C) 199 5 Wiley-Liss, Inc.