Somatolactin, a novel pituitary protein: isolation and characterization from Sparus aurata

The complete amino acid sequence of somatolactin, a new pituitary protein belonging to the growth hormone/prolactin family, from the gilthead sea bream Sparus aurata has been determined. Somatolactin was isolated from the pituitary by alkaline extraction, gel filtration on a Sephadex G-100 column, and reversed-phase high-performance liquid chromatography (rpHPLC) on a TSK gel ODS-120T column. The purified protein was confirmed to be somatolactin by immunoblotting using chum salmon somatolactin antisera. It was found that Sparus aurata somatolactin consists of two forms; one form (28 kD) is probably a glycosylated form, while the other (25 kD) is a simple protein form, as was found also in Atlantic cod. The somatolactin consists of 207 amino acids that show remarkable conservation among fish somatolactins.