Diacylglycerol Metabolism in the Green Alga Dunaliella salina under Osmotic Stress

The sn-1,2-diacylglycerol (DAG) content and molecular species composition of Dunaliella salina whole cells and cell fractions were measured by complementary high performance liquid chromatography and gas chromatography techniques. At 4.2 nanomoles per 100 nanomoles lipid phosphorus, the whole cell DAG level was high in comparison with most animal tissues. The DAG concentration was highest in the microsome-enriched fraction, followed by that in the chloroplast and in the plasma membrane fractions. The predominant DAG molecular species in all cell fractions contained oleic (18:1), linoleic (18:2), or linolenic (18:3) acid in the sn-1 position and palmitate (16:0) in the sn-2 position. Recent studies have raised the possibility of DAG serving a signal transducing function in osmotically stressed D. salina cells. During the first 30 seconds following hypoosmotic shock, there was a 40% increase in the plasma membrane DAG content, whereas the DAG content of the microsome-enriched fraction was unchanged. On a nanomole per 100 nanomoles phospholipid basis, the rise in plasma membrane DAG nearly matched the previously reported (KJ Einspahr, TC Peeler, GA Thompson Jr [1988] J Biol Chem 263: 5775-5779) transient fall in phosphatidylinositol 4,5-bisphosphate. Furthermore, 18:1/16:0 DAG, one of the major plasma membrane DAG molecular species increasing in amount after hypoosmotic shock, was the characteristic molecular species of plasma membrane phosphatidylinositol, phosphatidylinositol 4-phosphate, and phosphatidylinositol 4,5-bisphosphate, but no other lipid of that membrane. Evidence was found for a rise in 16:0/18:2 and 16:0/18:3 DAG as well following hypoosmotic shock. This pattern suggested that phosphatidylcholine hydrolysis also contributed to the stress-induced production of DAG in the D. salina plasma membrane. The extent of the sudden DAG increase was sufficient to consider it a potential second messenger in phospholipase C-mediated signal transduction.