Juvenile hormone binding proteins in the haemolymph of third instar larvae of Drosophila hydei

Abstract

Juvenile hormone I (JH I) is bound in vitro by two protein fractions in the haemolymph of mid-third instar larvae of Drosophila hydei. These binding proteins were characterized by gel filtration, density gradient centrifugation, isoelectric focusing, polyacrylamide gel electrophoresis and charcoal adsorption assay. The larger binding protein has a molecular weight of 1.08 × 10 5 , a sedimentation coefficient of approximately 7 S and an isoelectric point at pH 5.1. The equilibrium dissociation constant for JH I is about > 10 −5 M. This protein binds JH I after storage for five days at 0 C, but binding capacity is destroyed by heating to 60°C for 10 min. Proteolytic peptides of this binding protein still possess the capacity to bind JH. The smaller JH-binding protein has a molecular weight of 4.4 × 10 4 , a sedimentation coefficient in the range of 3–4 S, an isoelectric point at approximately pH 8.9 and an equilibrium dissociation constant of approximately 1 × 10 −7 M. Its binding capacity is rapidly lost in total haemolymph and after partial purification at 4°C.