Role of juvenile hormone binding protein in modulating function of JH epoxide hydrolase in eggs of Manduca sexta

Abstract

Juvenile hormone (JH) is catabolized primarily by two enzymes, JH esterase (JHE) and JH epoxide hydrolase (JHEH). A JH binding protein (JHBP) may modulate this degradation by ligand sequestration. In this study, the effects of JHBP on the metabolism of JH by JHEH were examined. 1. (1) Purified JHEH from Manduca sexta (Lepidoptera) eggs converted JH to JH-diol four to five times faster than JH-acid was converted to JH-acid-diol in the absence of JHBP. 2. (2) The presence of JHBP dramatically decreased the degradation of JH, but not that of JH-acid. Similar results were obtained by using crude unsolubilized microsomes. 3. (3) The inhibition of JHEH activity by JHBP was due to the formation of a JH-JHBP complex that could be detected at the origin of a silica gel TLC plate. 4. (4) A crude homogenate of newly oviposited M. sexta eggs was incubated with a physiological concentration of JH. JH-acid-diol and JH-acid were found as the major metabolites, and the rate of hydration of JH-acid by JHEH was faster than that of JH. These results suggested that JH was protected from JHEH by cytosolic JHBP as a JH-JHBP complex; in contrast, JH-acid, a poor ligand for JHBP, was efficiently metabolized into JH-acid-diol by JHEH in vivo. Thus, while JH was not efficiently degraded by JHEH in the presence of JHBP, JHEH could nevertheless act as the ultimate scavenger for JH by hydrating JH-acid generated by JHE during embryogenesis in vivo.