THE BIOSYNTHESIS OF MELANIN-CONCENTRATING HORMONE IN A FISH

This work investigated the biosynthesis of a neurohypophysial hormone, melanin‐concentrating hormone (MCH), in the trout. Sephadex G‐75 chromatography showed the presence of several large MCH‐immunoreactive molecules in hypothalamic and pituitary gland extracts, with different retention times on high‐performance liquid chromatography from the mature MCH1–17. About 10% of the total MCH‐immunoreactivity in the hypothalamus was attributable to large molecular weight forms but these contributed less than 1% to the immunoreactivity in the neurointermediate lobe. Both [35 S]methionine and [3 H]leucine were injected into the hypothalamus near the MCH perikarya (nucleus lateralis tuberis region) of anaesthetized fish, after which the fish were killed at intervals of up to 8 h post‐injection and the basal hypothalami, pituitary pars distales and neurointermediate lobes were extracted in acid. MCH‐related immunoprecipitates from these extracts were fractionated by sodium dodecyl sulphate polyacrylamide gel electrophoresis or by Sephadex G‐50 chromatography. The results show the incorporation of radiolabel into 15.3 K and 11.3 K precursors within 0.75 h, and their conversion, via several smaller intermediates, to a molecule resembling MCH1–17. The results are discussed in relation to the known cDNA sequence of salmon MCH. Labelled MCH first appeared in the neurointermediate lobe 4 h after injection, giving an estimated transit rate of 0.4 mm/h. Copyright © 1990, Wiley Blackwell. All rights reserved