Activity of phospholipid-synthesizing enzymes in rat liver plasma membranes and the source of biliary lecithin.

The potential for the synthesis of phosphatidylcholine by the bile canalicular membrane of the liver cell was assessed by measuring the activity of a number of phospholipid synthesizing enzymes in isolated bile canalicular membrane fractions from rat liver. The activity of these various enzymes was compared to that present in noncanalicular liver cell plasma membranes and in microsomes. The CDP-choline:1,2-diacyl-sn-glycerol-cholinephosphotransferase was virtually absent from the bile canalicular membranes but the specific activities of S-adenosyl-L-methionine:phosphatidyl-ethanolamine N-methyltransferase and acyl-CoA:1-acyl-sn-glycero-3-phosphorylcholine acyltransferase were 11-15% of those found in the microsomes. The bile canalicular membranes also contained detectable acyl-CoA:sn-glycero-3-phosphate acyltransferase activity and the ability to potentiate the Ca++-stimulated exchange of bases between different phospholipids. These findings indicate that the bile canalicular membranes have a very limited capacity for the formation of phosphatidylcholine under the assay conditions employed.