Action of juvenile hormone (JH) esterase on the JH-JH binding protein complex. an in vitro model of JH metabolism in a caterpillar

A decline in juvenile hormone (JH) titer early in the last larval instar leads to the metamorphosis of lepidopterous larvae into pupae. Clearance of JH is associated with an increase in the level of JH-specific esterase (JHE) that hydrolyzes JH into the biologically inactive JH-acid. Experiments with purified recombinant JHBP (rJHBP) and with both natural and recombinant JHE (rJHE) were designed to test the hypothesis that JHBP "protects" JH from JHE degradation in insect hemolymph. First, at low JHE concentrations, the rate of hydrolysis of JHBP-bound JH by JHE was determined by the rate of dissociation of JH from JHBP. Second, the rate of hydrolysis of JH increased with increasing JHE concentrations even in the presence of excess JHBP. In contrast, JH was not hydrolyzed by a general carboxylesterase in the presence of JHBP. Third, the rate of JH hydrolysis when the JHBP-JH complex was added to JHBP-depleted hemolymph was essentially identical to that in undiluted hemolymph. Taken together, these results suggest that JHBP protects JH from nonspecific hydrolytic processes, while still allowing highly specific JHE to hydrolyze JH by recognizing the JH-JHBP complex. Finally, JHBP efficiently extracted JH from lipophilic depots (the so-called "refractory pool", resulting in more rapid degradation of JH by JHE. Thus, JHBP is crucial for maintaining and distributing JH in hemolymph in the absence of JHE, and may assist in the rapid clearance of JH in the presence of JHE.