Biosynthetic alternatives for acetogenic production of sorbic acid in the potato aphid (Homoptera: Aphididae)

Following in vivo injection of [1‐¹⁴C]‐sodium acetate, triacylglycerols of the potato aphid, Macrosiphum euphorbiae (Thomas), were extracted and derivatized to p‐bromophenacyl fatty acid esters for two‐dimensional TLC and GLC‐MSD (mass selective detector) analysis. Radiolabeled sorbic (E, E‐2,4‐hexadienoic) acid ester was detected, demonstrating that this short chain fatty acid unique to aphids is biosynthesized via an acetogenic pathway. Crotonic (E‐2‐butenoic) or hexenoic acids were not detected in labeled or unlabeled potato aphid samples or unlabeled samples from the oleander aphid, Aphis nerii Fonscolombe. Crotonic or hexenoic acids might have been expected if an incomplete cycling by fatty acid synthetase or a novel desaturase acting on the prevalent hexanoate, respectively, were responsible for sorbic acid synthesis in aphids. A peroxisomal β‐oxidation route to sorbic acid from longer chain fatty acids was not indicated since injections of clofibrate, a peroxisomal proliferator, with or without C18 polyunsaturated lipids gave no substantial increase in C6 lipids. Also, some characteristic enzyme activities of peroxisomal β‐oxidation were not found in an ultracentrifugal “peroxisomal” fraction from the potato aphid. Although the individual biochemical steps from acetate to sorbate in aphids remain unclear, an unusual acetate‐malonate pathway is indicated. Clarification of the biosynthetic steps to sorbic acid should identify at least one novel enzyme for animals. © 1994 Wiley‐Liss, Inc.