INCORPORATION IN-VIVO AND IN-VITRO OF RADIOLABELED SPHINGOLIPID PRECURSORS INTO PARAMECIUM-TETRAURELIA LIPIDS

Paramecium tetraurelia contains high concentrations of ethanolamine sp hingolipids, especially in its ciliary membrane. Three ethanolamine sp hingophospholipids with different long chain bases (dihydrosphingosine , sphingosine and phytosphingosine), and their phosphonyl analogs, wer e previously identified and characterized. In the present study, radio labeling experiments on lag- and log-phase cells were performed to inv estigate the extent of sphingolipid biosynthetic capacities of the cil iate. Long chain bases of sphingolipids are formed by an initial conde nsation reaction of serine with a fatty-coenzyme A. Thus, radiolabeled palmitic acid, stearic acid and serine were used as precursor compoun ds in these experiments. The results indicated that (I) sphingolipid p recursors were incorporated into every major lipid fraction, (2) ethan olamine sphingophosphonolipids accumulated faster than the ethanolamin e sphingophospholipids, (3) in contrast to these sphingolipids, the gl ycerolipid, phosphatidyethanolamine, accumulated faster than its phosp hono analog, and (4) palmitic acid, but not stearic acid, was incorpor ated into the long chain bases of ethanolamine sphingophospho- and sph ingophosphonolipids, consistent with an earlier report demonstrating t hat these lipids contain only C-18 long chain bases. Since P. tetraure lia takes up serine and other water-soluble substrates very slowly, an d catabolizes fatty acids rapidly, label is randomized in intact cells . Thus, cell-free protocols provide useful experimental systems for st udies of sphingolipid biosynthesis than do intact organisms, when the uptake of precursor substrates are slow.