The gradual decline i n juvenile hormone (JH) titer followed by its complete clearance early in the last larval instar is required for the onset of the metamorphosis of lepidopterous larvae. J H titer is regulated by both biosynthesis and degradation. Two major pathways for JH metabolism, ester hydrolysis and epoxide hydration, are due to JH esterase (JHE) and JH epoxide hydrolase (JHEH), respectively. In vitro experiments designed to elucidate the molecular mechanism of JH metabolism are described.
First, microsomal JHEH in Manduca sexta eggs was identified by using photoaffinity analogs of JH, and purified to homogeneity with ion exchange and hydroxylapatite columns. Purified JHEH from M. sexta eggs was kinetically characterized. The effects of p H and various reagents imply that JHEH in M. sexfa eggs and mammalian rnicrosomal EH strongly resemble each other. JH binding protein (JHBP) appears to protect JH from JHEH; however, the hydration of JH-acid is not affected by JHBP because JH-acid is not bound by JHBP, but is a good substrate for JHEH. Thus, the major function of JHEH in M. sexta eggs, which has a cytosolic JHBP, is likely to act as the ultimate scavenger for JH by hydrating JH-acid. Second, although virtually all JH exists as the JHBP.JH complex in hemolymph, ]HE nonetheless effectively hydrolyzes JH in a JHBPIH complex. Circular dichroism experiments suggest the possibility of a direct interaction between JHBP and JHE. These experimental approaches using reconstituted in vitro model systems may elucidate some of the complex interactions in the JH signaling and metabolic pathways. o 1996Wiley-l.