Fractionation and analysis of rat liver14CH3-lecithins labeled in vivo

The¹⁴CH₃-lecithins were biosynthesized by normal adult rats injected with¹⁴CH₃-methionine. About 20% of the dose was incorporated into liver lecithins. The¹⁴CH₃-lecithins were isolated by thin-layer chromatography. Separation of lecithins on AgNO₃-treated silica gel yielded lecithins containing a saturated fatty acid in combination with mainly one unsaturated fatty acid, namely, oleic, linoleic, eicosatrienoic, or arachidonic acid. These fractions were eluted with methanolic choline chloride, which prevented elution of AgNO₃. The lecithins, after extraction into petroleum ether, were analyzed for radioactivity and for fatty acid composition. Yields were about 75%, based upon fatty acids or radioactivity applied to the plate. Specific activities differed sharply between the fractions, and arachidonoyllecithins had the highest specific activity. The sum of the activities contributed by each of the fractions agreed well with the specific activity of total lecithins, indicating the recovery of intact lecithin molecules. The recovery of intact molecules allows this procedure to be used with lecithins containing any isotopic labels. The high specific activity of arachidonoyl-lecithins relative to the other fractions indicates a high degree of specificity in the metabolic reactions which lead to the formation of rat liver lecithins.