IDENTIFICATION OF B10, AN ALKALINE PHOSPHODIESTERASE OF THE APICAL PLASMA-MEMBRANE OF HEPATOCYTES AND BILIARY CELLS, IN RAT SERUM - INCREASED LEVELS FOLLOWING BILE-DUCT LIGATION AND DURING THE DEVELOPMENT OF CHOLANGIOCARCINOMA

Alkaline phosphodiesterase (APDE) is associated with the cellular plas ma membrane of many organs. Several isoforms are also detected in norm al human serum and their respective amounts vary in liver diseases but their significance is unknown. The aims of this study were: 1) to ide ntify a serum form of B10, an APDE exclusively localized at the apical pole of the plasma membrane of rat hepatocytes and biliary cells; 2) to gain insight into its origin; and 3) to investigate its behavior, i n two liver diseases in which an abnormal membrane expression of B10 h as been reported, namely cholestasis and cholangiocarcinoma. A soluble form of B10 was immunoprecipitated from normal rat serum, which amoun ted to 13% of total serum APDE activity. By sodium dodecyl sulfate-pol yacrylamide gel electrophoresis, the size of the serum enzyme was 125 kd, which is slightly lower than that found in the plasma membrane (13 0 kd). In bile, a 120-kd and a 130-kd form was found. A sixfold and fi vefold increase of B10 APDE activity was observed in the serum of bile duct-ligated rats and in the Long-Evans Cinnamon (LEC) rats which spo ntaneously develop cholangiocarcinoma. The molecular size of the form present in serum was unchanged. A threefold increase was also observed in LEC rats which had not yet developed a cholangiocarcinoma. In conc lusion, we identified a soluble form of B10 in normal rat serum. The i ncrease in serum B10 in the experimental and pathological conditions i nvestigated does not seem to result from passage of the biliary form t o the serum but seems to be caused by increased cleavage of the membra ne form. Its rise early during the onset of cholangiocarcinoma suggest s that B10 in the serum might be a marker of carcinogenesis and/or be involved in the development of cholangiocarcinoma.