Characterization of a juvenile hormone binding lipophorin from the blowfly Lucilia cuprina

The larval haemolymph of the sheep blowfly Lucilia cuprina (Weidemann) contains a juvenile hormone binding protein with a K_d for racemic JH III of 33 ± 6 nM. The density of the binding sites is 212 ± 33 pmol/mg haemolymph protein. The binding protein is equally specific for JH III and methyl farnesoate. Some natural juvenoids were ranked for their ability to displace [³H]JH III with JH III > JH II > JH I > JH III > JH III diol > JHB₃ = no detectable displacement. These data, together with displacement studies for 14 synthetic juvenoids, indicate some characteristics of the JH binding cleft. The binding protein is a high density lipophorin (density = 1.15 g/ml) and has subunit molecular weights of 228 kDa (apolipophorin I) and 70 kDa (apolipophorin II). The N-terminal amino acid sequences of the subunits have no discernible homology to any previously sequenced protein. Lipophorin-specific immunocytochemical staining occurs in a subset of fat body cells.