EPITOPIC HETEROGENEITY OF HUMAN 3-BETA-HYDROXYSTEROID DEHYDROGENASE IN VILLOUS AND EXTRAVILLOUS HUMAN TROPHOBLAST

A new monoclonal antibody (FDO26G) is desdribed which was raised again st purified human 3 beta-hydroxysteroid dehydrogenase type I (3 beta-H SD type I). FDO26G reacted strongly with villous syncytiotrophoblast, weakly with some trophoblast cells in chorion laeve, and not at all wi th extravillous trophoblast in cytotrophoblast cell islands and decidu al trophoblast. All these types of trophoblast reacted strongly with m onoclonal antibody FDO161G, which has previously been shown to react w ith 3 beta-HSD type I and, like FDO26G, reacts strongly with adrenal c ells. Mapping experiments using a combination of lacZ fusion polypepti des and synthetic peptides located the FDO26G epitope to residues 354- 366 at the C-terminal end of the molecule, a sequence that is identica l in the type I and type II forms of the enzyme. The epitope contains a consensus for a casein kinase-II site with serine 359 as the candida te phosphorylation site. This suggested that the lack of reactivity of FDO26G to 3 beta-HSD in extravillous trophoblast might be due to phos phorylation at serine 359. Peptide 354-366 was synthesized with phosph oserine at residue 359 and its binding to FDO26G was compared with tha t of the unphosphorylated peptide. FDO26G bound the phosphopeptide at least as strongly as the unphosphorylated peptide. It is concluded tha t the lack of staining of extravillous trophoblast by FDO26G is due to the presence of a different sequence at residues 354-366 and that a h itherto unidentified third isoform of human 3 beta-HSD is expressed in these cells.