Regulation of Glycerol Synthesis in Response to Osmotic Changes in Dunaliella

Abstract

Changes in phosphometabolites, following osmotic shock, were analyzed by two-dimensional thin layer chromatography, in extracts of the halotolerant alga Dunaliella salina in order to clarify the regulation of glycerol synthesis from starch. The experiments were carried out in wild-type and in osmotically defective mutant cells. It is demonstrated that hyperosmotic shock induces a decrease in fructose 6-phosphate and an increase in fructose-1,6-bisphosphate indicating the activation of phosphofructokinase. Two mutants, which are specifically defective in their response to hyperosmotic shock, accumulate glucose 6-phosphate or phosphogluconate following shock, and have remarkably reduced activities of glucose-6-phosphate dehydrogenase and of phosphogluconate dehydrogenase, respectively. These results indicate that the pentose-phosphate oxidative pathway has a major role in glycerol synthesis. Hyperosmotic shock leads to a transient accumulation of phosphorylcholine and to a decrease of inositolbisphosphate in D. salina extracts. Accumulation of phosphorylcholine is not detected in osmotically defective mutants. Hypoosmotic shock induces an increase in inositolbisphosphate but not in phosphorylcholine. These results are consistent with previous indications for differential activations of phospholipases by hyper or hypoosmotic shock in Dunaliella. Based on these results we suggest that (a) phosphofructokinase is an important checkpoint enzyme in the regulation of glycerol production, and (b) that the pentose-phosphate pathway has a major role in keeping oxidation-reduction balance during glycerol synthesis. The possible role of lipid breakdown products as second messengers in regulating glycerol production in Dunaliella is discussed.