A new monoclonal antibody (FDO26G) is described which was raised against purified human 3β-hydroxysteroid dehydrogenase type I (3β-HSD type I). FDO26G reacted strongly with villous syncytiotrophoblast, weakly with some trophoblast cells in chorion laeve, and not at all with extravillous trophoblast in cytotrophoblast cell islands and decidual trophoblast. All these types of trophoblast reacted strongly with monoclonal antibody FDO161G, which has previously been shown to react with 3β-HSD type I and, like FDO26G, reacts strongly with adrenal cells.
Mapping experiments using a combination of lacZ fusion polypeptides and synthetic peptides located the FDO26G epitope to residues 354–366 at the C-terminal end of the molecule, a sequence that is identical 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 candidate phosphorylation site. This suggested that the lack of reactivity of FDO26G to 3β-HSD in extravillous trophoblast might be due to phosphorylation at serine 359. Peptide 354–366 was synthesized with phosphoserine at residue 359 and its binding to FDO26G was compared with that of the unphosphorylated peptide. FDO26G bound the phosphopeptide at least as strongly as the unphosphorylated peptide. It is concluded that 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 hitherto unidentified third isoform of human 3β-HSD is expressed in these cells.