RATE OF ISOLEUCINE METABOLISM IN LEPIDOPTERAN CORPORA ALLATA - REGULATION OF THE PROPORTION OF JUVENILE-HORMONE HOMOLOGS RELEASED

We tested the hypothesis that shifts in the proportions of the various juvenile hormones (JHs) and precursor acids released by lepidopteran corpora allata (CA) are associated with changes in the activity of enz ymes involved in the production of propionate from isoleucine, Measure ments of enzyme activity were made on CA of adult Pseudaletia unipunct a, using the assay developed by Brindle st al, [Arch, Insect Biochem, Physiol,, 19, 1 (1992)], Female and male CA of P. unipuncta release th e three principal forms of JH and JH acid (JHA), respectively, in prop ortions that vary according to age and total hormonal output, In addit ion, female CA incorporate isoleucine-derived propionate into JH I and JH II in a stoichiometric fashion, The branched-chain amino acid (BCA A) transaminase, an enzyme that catalyzes the first step in the conver sion of isoleucine into propionate, was 5-25 times more active in homo genates of male CA than in those of females, and its activity increase d as a function of age in males but not in females, Similar trends wer e observed for the accumulation of metabolites produced in later steps of this conversion, These observations support the hypothesis that th e age-related increase in the biosynthesis of JHA I by male CA is asso ciated with a rise in the production of propionate, whereas limited av ailability of propionate in female glands leads to an age-dependent de crease in the proportion of JH I released, These results also imply th e existence of a mechanism regulating propionate production in lepidop teran CA, The regulatory factor, however, is not Manduca allatostatin as it had no detectable effect on isoleucine metabolism by either inta ct male or female CA, Lastly, changing the concentration of acetate an d propionate in the CA incubation medium had a greater impact on the p roportions of JH I and JH II released than on the proportion of JH III .