[Role of some intermediates of galactosylglycerol metabolism in the osmotic balance in Ochromonas malhamensis].

The osmotic balance in Ochromonas malhamensis Pringsheim is achieved mainly by formation or degradation of α-galactosylglycerol (isofloridoside). The time course of the incorporation of radioactivity from (14)CO2 under conditions of high isofloridoside production suggests (Fig. 1) that isofloridoside-phosphate is indeed a precursor in the formation of isofloridoside. To study changes in relative pool sizes of intermediates cell constituents were prelabelled by assimilation of (14)CO2. Upon an increase of osmotic pressure the concentration of isofloridoside-phosphate increases rather rapidly sustaining the above conclusion. The final rate of isofloridoside formation, however, is reached long before the final concentration of the precursor is established (Fig. 2). On a decrease of osmotic pressure isofloridoside-phosphate concentration first increases and than decreases (Fig. 3). The data indicate that control of isofloridoside formation might occur at least at two steps: at the transfer of galactose to glycero-3-phosphate as well as at the dephosphorylation of isofloridoside-phosphate. Under conditions resulting in increased isofloridoside degradation the pool size of free glycerol is rapidly increased about ten fold, whereas the pool of free galactose responds to a far smaller extent (Fig. 3). This might indicate the operation of a transgalactosidase or an α-galactosidase in cooperation with an unusually potent galactose kinase.