After initial purification of ovine placental lactogen (oPL) using the procedures described previously [(1976) Endocrinology 98, 65-75], the oPL preparation was further purified by high-performance liquid chromatography (HPLC) using an anionic exchange column (Bio-Sil TSK DEAE-2-SW). Two forms of oPL with different relative mobilities on HPLC were isolated and designated oPL-I and oPL-II. Subsequent analysis by polyacrylamide gel electrophoresis containing SDS revealed that oPL-I and oPL-II are nearly homogeneous (greater than 90% pure) and are identical in apparent M r (approx. 22 000–23 000). Like human growth hormone (hGH), oPL-I and oPL-II are equally active in the radioreceptor assays for growth hormone-like activity (RRA-GH) and for prolactin-like activity (RRA-RRL). In the radioimmunoassay of oPL, both oPL-I and oPL-II are immunologically similar. Analysis of amino acid composition revealed that oPL-I and oPL-II consist of 199 and 196 residues, respectively, and have almost identical residues except that oPL-I has a higher content of glycine. Furthermore, both oPLs have a general similarity in amino acid composition to oGH and oPRL except for a lower content of methionine and leucine but with a higher content of lysine. Our studies demonstrated the presence of two similar forms of oPL. Whether these two similar forms of oPL share identical primary structure remains to be determined.
