Purification and some properties of ovine placental lactogen

A method has been described for the purification of ovine placental lactogen (oPL) involving the use of freshly obtained sheep foetal cotyledons. Tissue was extracted with 0.1 M-ammonium bicarbonate and the supernatant fraction, adjusted to pH 7, was brought to 60% saturation with ammonium sulphate. The resulting precipitate was then subjected to a sequence of chromatographic steps using columns of Sephadex G-100 and carboxymethylecellulose. During each stage of the purification, the lactogenic activity was monitored with a pregnant rabbit mammary gland radioreceptor assay. The yield of oPL corresponded to 8 mg/kg wet foetal tissue and the oPL possessed lactogenic activity equivalent to 1 mg ovine prolactin/mg protein and GH-like activity equivalent to 0.8 mg human GH/mg protein. The biological activity of oPL was confirmed using a rabbit intraductal mammary gland assay in vivo. After polyacrylamide gel electrophoresis at pH 8.9, oPL was resolved into one major band (isoelectric point 8.2--8.4) and four minor components, which were thought to be deamidation products of oPL. Microimmunoelectrophoresis and immunodiffusion studies confirmed that the preparation of oPL was free from serum protein contaminants.