Diacylglycerol Metabolism in the Green Alga Dunaliella salina under Osmotic Stress

Abstract

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 chro- matography and gas chromatography techniques. At 4.2 nano- moles 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. Dur- ing 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 un- changed. 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 spe- cies of plasma membrane phosphatidylinositol, phosphatidylino- sitol 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 pattem suggested that phosphatidylcholine hydroly- sis 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 messen- ger in phospholipase C-mediated signal transduction.