A pheromone-degrading aldehyde oxidase in the antennae of the moth Manduca sexta

Antennae of the tobacco hornworm moths Manduca sexta contain an aldehyde oxidase (AOX) that oxidizes aldehydes to carboxylic acids. The enzyme, which is distinguishable from aldehyde-oxidizing activities in other tissues, is secreted into the receptor lymph that bathes the primary olfactory dendrites. First detectable about 3 d before eclosion, AOX levels increase through the first day after eclosion. This parallels the development of the antennal responsiveness to bombykal (a male attractant aldehydic pheromone produced by female M. sexta) and trans-2-hexenal (an aldehyde commonly found in leaves). The AOX is about 60% more abundant in antennae of males than in antennae of females. The antennal AOX is a dimer with Mr of 295 kDa and is capable of oxidizing a variety of aldehydes. Of all aldehydes examined, the pheromone bombykal was the best substrate with an apparent Km of 5 microM, whereas the next best substrate, benzaldehyde, had an apparent Km of 255 microM. Using kinetic parameters estimated in vitro and the assumption of first-order kinetics, the half-life of bombykal in sensilla was estimated to be about 0.6 msec. The affinity of the antennal AOX for bombykal, its location in the receptor lymph, and its pattern of developmental expression all suggest that it plays a role in modulating the sensitivity of adult M. sexta to aldehyde odors and, in particular, the sensitivity of males to the pheromone bombykal.