Expression and characterization of European sea bass (Dicentrarchus labrax) somatolactin:: Assessment of in vivo metabolic effects

The complementary DNA coding for European sea bass somatolactin was expressed in the pET-3a bacteria expression vector. The recombinant somatolactin (rbSL) was purified by size exclusion chromatography, and 95% of the protein remained in the oxidized form with negligible aggregation over prolonged cold storage. The identity of the recombinant protein was demonstrated by Western blotting with a rabbit polyclonal antibody against gilthead sea bream somatolactin. The same antibody was utilized in a radioimmunoassay procedure, using rbSL as standard and radioiodinated tracer. Curve displacements of pituitary and plasma samples paralleled the rbSL standard, and the midrange of the assay (8 ng/ml) was low enough to measure in a consistent manner the circulating SL concentration. To assess biological activity a single dose of rbSL (0.1 mug/g of body mass) was administered to juvenile gilthead sea bream by intraperitioneal injection. In comparison with saline-treated fish, rbSL did not modify the circulating amount of insulin-like growth factor 1, whereas a 50% increase was found with the same dose of recombinant trout growth hormone (rtGH). Hormone treatment did not modify nitrogen-ammonia excretion, but both rbSL and rtGH increased carbon dioxide output and oxygen uptake, which in turn decreased the respiratory quotient (CO2 output per O-2 uptake). This pattern of gas exchange suggests the enhancement of lipid catabolism, which is consistent with the observation that both hormones were able to inhibit the hepatic activity of acetyl-coenzyme A carboxylase. These new insights provide direct evidence for the involvement of fish somatolactin in energy homeostasis, which may serve to maintain the lipolytic tonus in different physiologic states.