Possible function of ecdysteroid-22-O-acyltransferase in the larval gut of tobacco budworm,Heliothis virescens.

In the larval gut of the tobacco budworm,Heliothis virescens, most ingested ecdysteroids are transformed to ecdysteroid-22-acyl esters by a novel enzyme, ecdysteroid-22-O-acyltransferase. This conversion is different from the normal metabolic pathway in which the compounds are usually transformed to more water-soluble products. Most ecdysteroid-22-O-acyltransferase activity was found in the midgut epithelial-cell membrane and was active only during feeding stages. The activity decreased as the larvae became committed to pupation, and it was not enhanced by feeding ecdysteroids to the fifth-instar larvae. Our preliminary results showed that the esterification was very specific to 22-OH in ecdysteroids. We suggest that detoxification is the major function of ecdysteroid-22-O-acyltransferase.