HIGH-AFFINITY BINDING OF PUTATIVE MOLT-INHIBITING HORMONE (MIH) AND CRUSTACEAN HYPERGLYCEMIC HORMONE (CHH) TO MEMBRANE-BOUND RECEPTORS ON THE Y-ORGAN OF THE SHORE CRAB CARCINUS-MAENUS

Receptor-binding assays for putative moult-inhibiting hormone (MIH) and crustacean hyperglycaemic hormone (CHH) were developed to determine the distribution and characteristics of their receptors on crude membrane preparations of Carcinus maenus tissues. High-affinity, specific, displaceable and saturable binding of [I-125]MIH indicative of receptor-ligand interaction was observed on Y-organ preparations. All tissues examined including the Y-organ specifically bound [I-125]CHH, and, for this tissue, high-affinity binding characteristics were determined by Scatchard analysis. As previous studies had shown that CHH is active in repressing ecdysteroid biosynthesis by Y-organs in vitro, it is argued that this neuropeptide has a physiological role in moult control, further strengthening the emerging scenario of a complex, multihormonal control of moulting. Receptor binding experiments using heterologous MIHs from Necora and Cancer revealed that all these MIHs were similarly effective at displacing homologous (Carcinus) radioligand, a feature reflected in their somewhat similar biological activities. Nevertheless, all three MIHs could be distinguished in terms of chromatographic and immunochemical behaviour and in terms of amino acid analysis in a manner correlated with evolutionary relationships within the Brachyura. It is therefore suggested that binding domains of MIHs have been highly conserved.