Kinetics of binding and hydrolysis of juvenile hormone II in the hemolymph of Trichoplusia ni (Hübner)

Juvenile hormone II (JH II) was studied for the kinetic behavior in its reaction with JH-esterase and the binding protein from the hemolymph of the last instar larvae of the cabbage looper, Trichoplusia ni. Steady state kinetics revealed 70.6 nM and 65 nmol substrate hydrolyzed min^-1 ml, respectively, for the K_m and V_max for JH hydrolysis and 175 nM and 8.1 μM, respectively, for the equilibrium dissociation constant (K_d) and the total concentration of the binding sites for the interaction with the binding protein. The rate constant for the dissociation of bound JH II was experimentally evaluated to be 0.095 min^-1. This in turn enabled the second order rate constant of association to be calculated as 5.41 × 10⁵ M^-1 min^-1. The slow tight binding inhibitor of JH-esterase, 3-octylthio 1,1,1-trifluoropropan-2-one, was applied to the enzyme for the Ackermann-Potter kinetic analysis. The data from this treatment indicated the compound acts as pseudo-irreversible and stoichiometric inhibitor and allowed determination of k_cat (31.8 min^-1) and the enzyme molar equivalency (1.5 μM) in whole hemolymph. The above determination helped compare the interaction of JH II with the two major components that regulated JH titer following biosynthesis. We conclude that degradation of JH in the hemolymph of prewandering last stadium larvae is described by the kinetic dissociation constant of the hemolymph binding protein. A model based on mass action is presented which may help to describe the distribution of JH in the last larval stadium.