Interrelationship between Bile Lecithin and Liver Lecithin Newly Synthesized through Methylation and Direct Acylation Pathways

Abstract

1) Specific activities of lecithin and its subfractions with different degrees of unsaturation in liver, plasma and bile of fed male Wistar rats were determined at various time periods from 10 to 240min following the intraportal injection of (methyl-3H)-methionine, lysolecithin labeled with (2-³H)-glyceriol and/or (methyl-¹⁴C)-choline and lysophosphatidyl-ethanolamine labeled with (2-³H)-glycerol. Among subfractions, the highest values were found in hexaenoic or tetraenoic or both subfractions with three respective precursors. 2) The mean value of specific activity ratio (bile/liver) was 1.36 or 0.94 on total lecithin in respect of radioactive methionine or lysolecithin. The values with methionine were 2.3, 1.5 and 2.0 in mono-dienoic, tetraenoic and hexaenoic subfractions, respectively. The secretion rate constant of bile lecithin calculated from isotopic data was 0.012 or 0.009 with methionine or lysolecithin. The highest values (0.037 and 0.021) were found in mono-dienoic subfractions among three different subfractions. 3) The turnover time of plasma lecithin was 70 or 125min with methionine or lysolecithin. 4) Certain aspects regarding liver lecithin metabolism as well as the secretion mechanism of bile and plasma lecithins were discussed using the data reported by other investigators as well as our previous and present data. 5) It seems to be quite probable that the marked differences on molecular species between bile and liver lecithins could be ascribed to the preferential uptake of α-palmitoyl-β-oligoenoyl species by a bile forming site from a minor dynamic pool which contained rather higher proportions of polyenoic species. This pool could not be regarded to be a particular fixed compartment but a variable dynamic one presumed to be mainly composed of lecithin being carried by a specific lecithin transport protein. In contrast, plasma lecithin, in which the secretion rate or turnover rate did not show marked differences among molecular species, may be brought about from a major static lecithin pool comprised mainly of membrane systems but not directly from the minor dynamic pool. The data also suggested that interconversion between molecular species might proceed at a fairly rapid rate and the rate could be different for different species.