Origin and Nature of the Proteins of Bile

In the dog, as shown before in the human, the major plasma proteins in hepatic bile (albumin, orosomucoid, transferrin and immunoglobulin IgG), with the exception of immunoglobulin IgA, were found to be derived entirely from the plasma. Biliary IgA was shown to have two sources, the plasma and local biosynthesis. These conclusions were based on comparisons on specific radioactivities of different proteins in plasma and bile, after intravenous injection of canine plasma whose proteins had been labelled biosynthetically with tritium. – The transfer of plasma proteins from blood to bile was found to proceed along two parallel pathways, one involving bulk transfer of unmodified plasma, and one based on molecular sieving favouring the transfer of plasma proteins of low molecular weight. – From a comparative study of hepatic lymph and plasma, it was concluded that the molecular sieve process was completed by the time the plasma proteins reached the interstitial fluid. The sieving membrane was therefore assigned anatomically to the vascular walls, which were found to have the properties of an isoporous membrane with a pore radius of 102 Å. No further molecular sieving was found to take place during the crossing of the second barrier between blood and bile, viz. the epithelial sheet separating interstitial fluid from the bile.