Evidence for the hepatic origin of a major azo dye metabolite-binding protein from rat bile

Small amounts of metabolite-binding protein (MBP) originally characterized from the bile were detected in rat serum and cytosol by an indirect enzyme-linked immunoabsorbant assay. The site of MBP synthesis was shown to be the liver based upon results of (1) the in vitro translation of liver poly(A)+ mRNA, followed by immunoprecipitation with anti-MBP sera and sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography of the immunoprecipitate, and (2) immunoprecipitation of bile collected from [3H]leucine perfused liver in situ and SDS-polyacrylamide gel electrophoresis and fluorography of the immunoprecipitate. To determine whether part of the MBP in bile is derived from the circulation, [125I]MBP was injected intravenously and bile was collected and subjected to SDS-polyacrylamide gel electrophoresis and fluorography. Intact [125]MBP was not detected in bile even though several other iodinated bile proteins were taken up by the liver from the circulation and secreted intact into the bile under similar experimental conditions. These data indicate that MBP is synthesized in the liver and secreted into the bile and circulation independently. In addition, MBP was not found in brain, spleen or kidneys.