The 18-kDa mouse epididymal protein (MEP 10) binds retinoic acid

Mouse epididymal protein (MEP) 10 has recently been characterized in our laboratory. Amino acid sequence analysis of the N-terminal of MEP 10 revealed an 86% similarity in sequence with rat proteins B and C, characterized by Brooks and Higgins [J Reprod Fertil 1980; 59:363-375]. Proteins B and C, have been recently shown to possess retinoic acid-binding ability [Newcomer ME, Ong DE. J Biol Chem 1990; 265:12876-12879; Ong DE, Chytil F. Arch Biochem Biophys 1988; 267:474-478]. Therefore, it was of interest to determine whether MEP 10 possessed the same ability to bind retinoic acid. Mouse caudal fluid was trace-labeled with 3H-retinoic acid and applied to a DEAE ion-exchange column. Analysis of the fractions for both the presence of radioactivity by scintillation counting and MEP 10 by ELISA revealed that the peak of radioactivity corresponded to the peak of MEP 10 immunoreactivity. These peak fractions were pooled and used for subsequent binding analysis and SDS-PAGE and Western blot analysis. SDS-PAGE and Western blot analysis revealed that the peak fractions were enriched for a protein of 18 kDa and that this protein was MEP 10. Competitive binding assays revealed that all-trans-retinoic acid was effective in inhibiting binding of labeled retinoic acid, but that the 13-cis isomer of retinoic acid was only moderately effective in inhibiting binding of the labeled ligand. All-trans-retinol was ineffective in the binding inhibition assay. Similar ligand specificity has also been described for the rat proteins B and C by Ong and Chytil [Arch Biochem Biophys' 1988; 267:474-478].(ABSTRACT TRUNCATED AT 250 WORDS)