Metabolism of individual molecular species of phosphatidylcholine in the liver subcellular membranes and bile. Origin of bile phosphatidylcholine.

Membrane fractions, particularly microsomes and bile canalicular membranes, isolated from rat liver were examined, in order to elucidate the site of a subpool of phosphatidylcholine destined for bile in the liver cell which was demonstrated in a previous paper (Kawamoto, T., Okano, G. and Akino, T. (1980) Biochim. Biophys. Acta 619, 20--34). 1. The bile canalicular membranes isolated from rat liver were not contaminated by other membranes when examined by membrane marker enzymes, and showed typical membrane structure, when examined by electron microscopy. The activity of DCPcholine : diacylglycerol cholinephosphotransferase was almost absent from the bile canalicular membranes and acyl CoA : 1-acyl-sn-glycero-3-phosphocholine acyltransferase activity was 17% of that found in the microsomes. 2. The molecular class composition of phosphatidylcholine was similar in the bile canalicular membranes and microsomes. The proportion of palmitoyl and stearoyl species in the major molecular classes was also similar in both membrane types. 3. In the in vivo experiments with [2-(3H)]glycerol, the specific radioactivity of phosphatidylcholine and its species such as palmitoyl-linoleoyl species in the bile canalicular membranes were significantly higher than that in other membranes. When compared with the specific radioactivity of phosphatidylcholine species in bile that in the bile canalicular membranes was also distinctly higher.