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Journal of Lipid Research, Vol. 7, 38-45, January 1966
Copyright © 1966 by Lipid Research, Inc.

In vivo studies on pathways for the biosynthesis of lecithin in the rat

Pål Bjørnstad and Jon Bremer

Institute of Clinical Biochemistry, University of Oslo, Rikshospitalet, Oslo, Norway

The in vivo biosynthesis of lecithin in rats has been studied with the precursors choline-1,2-¹⁴C, ethanolamine-1,2-¹⁴C and methionine-CH₃-¹⁴C or -CH₃-³H. Lecithin synthesis from choline is rapid in all organs. No sex difference was observed in this pathway. The biosynthesis of lecithin by methylation of phosphatidyl ethanolamine is of quantitative significance in the liver, but not in extrahepatic tissues. More lecithin is synthesized by this pathway in female rats. In liver the lecithin synthesized via both pathways enters a common pool which is in rapid equilibrium with lecithin of blood plasma. A sex difference in the utilization of radioactive ethanolamine for the formation of phosphatidyl ethanolamine was observed (greater utilization in the female). Incorporation of ethanolamine into phospholipids of extrahepatic tissues was slow in both sexes. With labeled methionine as precursor the liver cytidine diphosphate (CDP) choline had a specific activity identical with that of liver lecithin after 20 min, while the specific activity of phosphoryl choline remained low. With labeled choline as precursor the phosphoryl choline reached a specific activity 50 times that of lecithin after 20 min, while the specific activity of CDP choline was only four times that of lecithin. These findings indicate that the reaction:

CDP choline + diglyceride phosphatidyl choline + CMP is freely reversible in vivo.

Supplementary key words lecithin • biosynthesis • alternative pathways • liver • extrahepatic tissues • sex differences • rat • transmethylation • phosphatidyl ethanolamine • intermediates • cytidine diphosphate choline • phosphoryl choline • methyl-³H methionine • choline-1,2-¹⁴C • ethanolamine-1,2-¹⁴C

Submitted on June 1, 1965
Accepted on August 13, 1965

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