ENZYMES OF ECDYSTEROID TRANSFORMATION AND INACTIVATION IN THE MIDGUT OF THE COTTON LEAFWORM, SPODOPTERA-LITTORALIS - PROPERTIES AND DEVELOPMENTAL PROFILES

In the midgut cytosol of Lepidoptera, ecdysteroids undergo inactivatio n by transformation via the 3-dehydro derivative to the corresponding 3-epiecdysteroid (3 alpha-hydroxy) and by phosphate conjugation. The o xygen-dependent oxidase catalyses formation of 3-dehydroecdysteroid, w hich can be reduced either irreversibly by 3-dehydroecdysone 3 alpha-r eductase to 3-epiecdysteroid, or by 3-dehydroecdysone 3 beta-reductase back to the initial ecdysteroid. Furthermore, these ecdysteroids unde rgo further inactivation by phosphorylation. These ecdysteroid transfo rmations have been investigated in last instar larvae of the cotton le afworm, Spodoptera littoralis. The products of the phosphorylation hav e been characterized as predominantly ecdysteroid 2-phosphate accompan ied by smaller amounts of the corresponding 22-phosphate. The phosphot ransferases require Mg2+ and ATP. Whereas the 3-dehydroecdysone 3 alph a-reductase has a clear preference for NADPH rather than NADH, the cor responding 3 beta-reductase markedly favours NADH. The physiological s ignificance of the latter enzyme is unclear. The profiles of the vario us enzymic activities in dialysed midgut cytosol supplemented with app ropriate cofactors were determined throughout the last larval instar. All activities were detectable throughout the instar, but the respecti ve enzymes exhibited maxima at different times. Ecdysone oxidase showe d a peak early in the instar, with 3-dehydroecdysone 3 alpha-reductase increasing to a peak as the former activity declined. The 3-dehydroec dysone 3 beta-reductase exhibited peak activity late in the instar, a profile similar to that observed for the corresponding haemolymph enzy me involved in reduction of the 3-dehydroecdysone product of the proth oracic glands to ecdysone. Thus, the significance of the midgut 3 beta -reductase may be related to production of active hormone. Both ecdyst eroid 22-, and 2-phosphotransferases showed high activities early in t he instar and then declined. The physiological significance of the pro files for the ecdysone oxidase, the 3-dehydroecdysone 3 alpha-reductas e and phosphotransferases is unclear.