The neuropeptide mandibular organ (MO)-inhibiting hormone (MO-IH), synthesized and secreted from the X-organ-sinus-gland complex of the eyestalk, regulates the biosynthesis of the putative crustacean juvenile hormone, methyl farnesoate (MF). Using radiolabelled acetate as a precursor for isoprenoid biosynthesis, farnesoic acid (FA), farnesol, farnesal, MF and geranyl geraniol were detected in MOs cultured for 24 h. Treatment of MOs with extract of sinus gland inhibited the final step of biosynthesis of MF, catalysed by FA O-methyltransferase. Additionally, treatment of MOs with purified MO-IH exhibited a dose-dependent inhibition of this final step of MF synthesis. The extent of this inhibition was dependent on the ovary stage of the MO-donor animal, being maximal in MOs from animals in the early stages of ovarian development. Assay of FA O-methyltransferase activity, using [3H]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 MOs with MO-IH prior to preparation of subcellular fractions. For cytosolic preparations taken from vitellogenic animals, both Vmax and Km were appreciably lower than for those taken from non-vitellogenic animals. Conversely, eyestalk ablation of early-vitellogenic animals, which removes the source of MO-IH in vivo, resulted in enhancement of the cytosolic FA O-methyltransferase activity. Although both Vmax and Km show an appreciable increase upon eyestalk ablation, the increased enzyme activity is probably reflected by the fact that Vmax/Km (an approximate indication of kcat) has increased 5-fold. The combined evidence demonstrates that MO-IH inhibits FA O-methyltransferase, the enzyme which catalyses the final step of MF biosynthesis in MOs.

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