Biosynthesis of melanotropins and endorphins by the lead-haematoxylin positive cells in pars intermedia of the cichlid teleost Sarotherodon mossambicus

Abstract

Abstract-1. Biosynthesis in the lead-haematoxylin positive pars intermedia cells of Sarotherodon mossambicus was analysed by SDS and acid-urea electrophoresis and high pressure liquid chromatography.2. Pulse and pulse-chase experiments revealed that a non-glycosylated 30K-precursor was processed into a number of smaller products.3. Melanotropic and endorphin-like peptides appeared to be the final products of this precursor-product processing.4. Our results indicate that the biosynthetic pathways in the lead-haematoxylin positive cells in the teleost pars intermedia are similar to those described for pars intermedia cells in other vertebrate groups. IN T R O D U C T IO NPhysiological and immunohistological investigations in a number of teleosts have indicated that the predominant, lead-haematoxylin positive, cell type in the teleostean pars intermedia is similar to the pars intermedia cells in other vertebrate groups, whereas the second, periodic acid-SchifT (PAS) positive, cell type is unique to teleosts (Baker, 1972;Olivereau, 1971Olivereau, , 1972 Thornton and Howe, 1974;Follenius and Dubois, 1980;Van Eys and Van den Oetelaar, 1981;Van Eys and Peters, 1981). Biochemical assays on the teleostean pituitary have shown the presence of sub stances with melanotropic, corticotropic and opiate like activity (Baker, 1972; Baker and Ball, 1975;Hunter and Baker, 1979; Carter and Baker, 1980). Furthermore in salmon the primary structure of a number of peptides with such an activity was eluci dated and shown to be similar to those found in other vertebrates (Kawauchi and Muramoto, 1979a; Kawauchi et al., 1980a, b, c, d). In addition, McLean and Lowry (1981) showed that the primary structure of MSH-like peptides in the more primitive dogfish is similar to that in higher vertebrates.In vitro studies on amphibian and mammalian pars intermedia tissue revealed that biosynthesis in this lobe is of a complicated nature. Evidence has accrued to suggest that in the pars intermedia cells a precursor, pro-opiomelanocortin, is processed via a number of intermediates into a-MSH and /i-endorphin Eipper, 1979, 1980;Gianoulakis et al., 1979;Crine et al., 1979 Crine et al., , 1980Loh, 1979;Loh et al., 1981;Jenks et al., 1979). Recently Martens et al. (1983) Studies on the biosynthetic processes in the teleost pars intermedia have been precluded thus far. due to the complex intermingling of PAS positive and leadhaematoxylin positive cells. The recent identification of the products of the PAS-positive cells in Sarolherodon mossambicus (Van Eys et al., 1983), offered the possibility to investigate the nature of the biochemical processes in the most predominant, leadhaematoxylin positive, (further in this paper referred to as M SH cells) and to compare the results with findings in other vertebrate groups. MATERIALS A N D M ETHODS AnimalsSexually mature females, 10-12 cm long with a body weight of 9-12 g were used. Fish were kept on a black background in fresh water of 25 C under a 12 hr light/12 hr dark regimen. Fish were kept under the...