Abstract
Juvenile hormone esterase activity has been found in the haemolymph as well as fat body and midgut cytosolic subcellular fractions of last larval instar Trichoplusia ni. Isoelectric focusing and inhibitor studies were unable to distinguish the esterase activities in the haemolymph and fat body, while the midgut activity had a different isoelectric point and was more susceptible to inhibition. In addition, the haemolymph and fat body displayed similar patterns of juvenile hormone esterase activity during development in the last larval instar which were completely unlike that seen in the midgut. R-20458 epoxide hydrolase activity in the fat body and midgut peaks during the late feeding/early wandering stage of the last instar, as does haemolymph JH I binding activity. Since the rate of epoxide hydration is much lower than maximal rates of juvenile hormone esterase activity and the fluctuations in JH binding activity during development are relatively small, their influence on juvenile hormone turnover may be less important than metabolism by juvenile hormone esterase. Juvenile hormone esterase activity can be induced to appear synchronously in the haemolymph and fat body of newly-moulted pupae by topical application of the juvenoid epofenonane. The accumulated evidence is consistent with the fat body serving as a source of the haemolymph juvenile hormone esterase.
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