Incorporation In Vivo and In Vitro of Radiolabeled Sphingolipid Precursors into Paramecium tetraurelia Lipids

Paramecium tetraurelia contains high concentrations of ethanolamine sphingolipids, especially in its ciliary membrane. Three ethanolamine sphingophospholipids with different long chain bases (dihydrosphingosine, sphingosine and phytosphingosine), and their phosphonyl analogs, were previously identified and characterized. In the present study, radiolabeling experiments on lag‐ and log‐phase cells were performed to investigate the extent of sphingolipid biosynthetic capacities of the ciliate. Long chain bases of sphingolipids are formed by an initial condensation reaction of serine with a fatty‐coenzyme A. Thus, radiolabeled palmitic acid, stearic acid and serine were used as precursor compounds in these experiments. The results indicated that (1) sphingolipid precursors were incorporated into every major lipid fraction. (2) ethanolamine sphingophosphonolipids accumulated faster than the ethanolamine sphingophospholipids, (3) in contrast to these sphingolipids, the glycerolipid, phosphatidyethanolamine. accumulated faster than its phosphono analog, and (4) palmitic acid, but not stearic acid, was incorporated into the long chain bases of ethanolamine sphingophospho‐ and sphingophosphonolipids. consistent with an earlier report demonstrating that these lipids contain only C,g long chain bases. Since P. tetraurelia takes up serine and other water‐soluble substrates very slowly, and catabolizes fatty acids rapidly, label is randomized in intact cells. Thus, cell‐free protocols provide useful experimental systems for studies of sphingolipid biosynthesis than do intact organisms, when the uptake of precursor substrates are slow.