Pregnancy-associated endometrial alpha 2-globulin, the major secretory protein of the luteal phase and first trimester pregnancy endometrium, is not glycosylated prolactin but related to beta-lactoglobulins

Abstract

We previously reported that the major secretory protein of the endometrium during the first trimester of pregnancy, pregnancy-associated endometrial alpha 2-globulin (alpha 2-PEG), also represented the major secretory protein during the mid- to late luteal phase. Recently, the major secretory endometrial protein during the luteal phase of the menstrual cycle was identified as glycosylated PRL (G-PRL). Although certain properties of alpha 2-PEG resemble G-PRL, in this study G-PRL was demonstrated to be immunochemically distinct from alpha 2-PEG, and deglycosylation of alpha 2-PEG produced a protein unrelated to PRL. The sequence of the 38 N-terminal amnio acid residues of alpha 2-PEG was determined by a gas phase sequenator. A sequence of Met-Asp-Ileu-Pro-Gln-Thr-Lys-Gln-Asp-Leu-Glu-Leu-Pro-Lys-Leu-Ala-G ly-Lys-Trp- His-Ser-Met-Ala-Met-Ala-Thr-Asn-?-Ileu-Ser-Leu-Met-Ala-Thr-Leu-Lys -Ala-Pro was obtained which was unique and unrelated to that of PRL. However, sequence homology between alpha 2-PEG and the major milk whey protein beta-lactoglobulin of the horse was demonstrated. The data indicate that alpha 2-PEG is a unique protein and is a human homolog of the beta-lactoglobulin family.