Characterization of the human plasma binding protein for vitamin D and its metabolites synthesized by the human hepatoma-derived cell line, Hep 3B.

Screening of three human hepatoma-derived cell lines revealed the presence of an immunologically similar plasma binding protein for vitamin D and its metabolites in media from Hep 3B cells. Approximately 3% of protein synthesized and secreted by these cells was immunoprecipitated by specific antiserum to the D-binding protein. Medium content of the protein increased over 11 days following cell seeding, and negligible amounts of 125I-labeled binding protein added to cultures were degraded over 48 h. The hepatoma-derived binding protein was indistinguishable from plasma binding protein or reference pure protein in gel filtration, sucrose gradient ultracentrifugation, and polyacrylamide gel electrophoresis systems. The Hep 3B cell product was found to bind mole/mol with monomeric actin, and bind vitamin D sterols with an affinity and specificity characteristic of the human plasma binding protein. The findings argue strongly for the identity of the Hep 3B cell product and the human plasma protein. The continuous availability of the Hep 3B cell line provides a reasonable model for investigations of biosynthesis and release of this important plasma protein.