NEUROPEPTIDE REGULATION OF BIOSYNTHESIS OF THE JUVENOID, METHYL FARNESOATE, IN THE EDIBLE CRAB, CANCER-PAGURUS

The neuropeptide mandibular organ (MO)-inhibiting hormone (MO-IH), syn thesized and secreted from the X-organ-sinus-gland complex of the eyes talk, regulates the biosynthesis of the putative crustacean juvenile h ormone, methyl farnesoate (MF). Using radiolabelled acetate as a precu rsor for isoprenoid biosynthesis, farnesoic acid (FA), farnesol, farne sal, MF and geranyl geraniol were detected in MOs cultured for 24 h. T reatment of MOs with extract of sinus gland inhibited the final step o f biosynthesis of MF, catalysed by FA O-methyltransferase, Additionall y, treatment of MOs with purified MO-IH exhibited a dose-dependent inh ibition of this final step of MF synthesis. The extent of this inhibit ion was dependent on the ovary stage of the MO-donor animal, being max imal in MOs from animals in the early stages of ovarian development. A ssay of FA O-methyltransferase activity, using [H-3]FA in the presence of S-adenosyl-L-methionine, demonstrated that the enzyme was located in the cytosolic fraction of MOs and was inhibited by incubation of MO s with MO-IH prior to preparation of subcellular fractions. For cytoso lic preparations taken from vitellogenic animals, both V-max and K-m w ere appreciably lower than for those taken from non-vitellogenic anima ls. Conversely, eyestalk ablation of early-vitellogenic animals, which removes the source of MO-IH in vivo, resulted in enhancement of the c ytosolic FA O-methyltransferase activity. Although both V-max and K-m show an appreciable increase upon eyestalk ablation, the increased enz yme activity is probably reflected by the fact that V-max/K-m (an appr oximate indication of k(cat)) has increased 5-fold. The combined evide nce demonstrates that MO-IH inhibits FA O-methyltransferase, the enzym e which catalyses the final step of MF biosynthesis in MOs.