D-Mannitol dehydrogenase and D-mannitol-1-phosphate dehydrogenase in Platymonas subcordiformis: some characteristics and their role in osmotic adaptation.

D-Mannitol-1-phosphate dehydrogenase (EC 1.1.1.17) and D-mannitol dehydrogenase (EC 1.1.1.67) were estimated in a cell-free extract of the unicellular alga Platymonas subcordiformis Hazen (Prasinophyceae), D-Mannitol dehydrogenase had two activity maxima at pH 7.0 and 9.5, and a substrate specifity for D-fructose and NADH or for D-mannitol and NAD(+). The K m values were 43 mM for D-fructose and 10 mM for D-mannitol. D-Mannitol-1-phosphate dehydrogenase had a maximum activity at pH 7.5 and was specific for D-fructose 6-phosphate and NADH. The K m value for D-fructose 6-phosphate was 5.5 mM. The reverse reaction with D-mannitol 1-phosphate as substrate could not be detected in the extract. After the addition of NaCl (up to 800 mM) to the enzyme assay, the activity of D-mannitol dehydrogenase was strongly inhibited while the activity of D-mannitol-1-phosphate dehydrogenase was enhanced. Under salt stress the K m values of the D-mannitol dehydrogenase were shifted to higher values. The K m value for D-fructose 6-phosphate as substrate for D-mannitol-1-phosphate dehydrogenase remained constant. Hence, it is concluded that in Platymonas the D-mannitol pool is derectly regulated via alternative pathways with different activities dependent on the osmotic pressure.