The Intracellular Distribution of Enzymes of the Glycerol Cycle in the Unicellular Alga Dunaliella parva

Glycerol is the main osmoticum of the unicellular green alga Dunaliella [1-4]. During the process of osmoregulation this solute has to be synthesized or degraded efficiently depending on the extent and direction of changes in the salinity of the external medium. Synthesis of glycerol starts from DHAP [5]. A glycerol-3-phosphate dehydrogenase (EC 1.1.1.8) reduces DHAP to glycerol-3-phosphate and this compound is dephosphorylated to glycerol by a specific glycerol-3-phosphate phosphatase (EC 3.1.3.21) [6]. The degradation of glycerol is cata­ lyzed by a DHA oxidoreductase (EC 1.1.1.156) which oxidizes glycerol to DHA [7], DHA is then phosphorylated to DHAP by a DHA kinase [8, 9]. Since DHAP is the first intermediate of glycerol synthesis, but also the end product of glycerol degradation, this pathway was called „glycerol cycle” [5]. Data about the turnover of this cycle under different osmotic conditions and its regula­ tion are rare [10]. One cycling of DHAP requires one molecule ATP, whereas consumption and pro­ duction of reduction equivalents are balanced. Such a futile cycle may have the function of a valve [11].