γ-Carboxyglutamic acid content of hepatocellular carcinoma-associated des-γ-carboxy prothrombin

Abstract Serum des-γ-carboxy prothrombin (DCP) is a useful marker for the diagnosis of hepatocellular carcinoma (HCC), but the exact mechanism of its synthesis and its structural properties in liver diseases are unknown. DCP is measured by the monoclonal antibody MU-3. The purpose of this study was to examine the epitope of MU-3 and to characterize the differences in DCP between HCC and benign liver diseases. The epitope of MU-3 was examined by ELISA using prothrombin Gla domain polypeptides and was determined to be amino acid residues 17–27 of the prothrombin Gla domain, which has four γ-carboxyglutamic acid residues (Gla) at positions 19, 20, 25 and 26. Peptides having a glutamic acid residue (Glu) at these positions reacted strongly to MU-3 but lost reactivity when Glu 19 or 20 was changed to Gla. In the order of γ-carboxylation, MU-3 reacted strongly to DCP containing 0–1 Gla, weakly to 2–4 Gla and not at all to DCP containing more than five Gla. After adsorbing normal prothrombin with barium carbonate, DCP reaction to MU-3 was measured by determining the amount of DCP that was adsorbed by MU-3-coated beads. The proportion of DCP reacting to MU-3 in HCC was 41.0–76.8%, whereas in patients with benign liver diseases, only 0–42.1% reacted to MU-3. These results indicate that DCP variants preferentially synthesized in HCC have less than four Gla, which are restricted to positions 16, 25, 26 and 29, whereas DCP variants in benign liver diseases have more than five Gla.