Mannitol metabolism in the intertidal mangrove red alga Caloglossa leprieurii: salinity effects on enzymatic activity

Abstract

The effect of salinity on the activity and Km values of the enzymes mannitol-1-phosphate dehydrogenase (Mt1PDH) (E.C. 1.1.1.17), mannitol-1-phosphatase (Mt1Pase) (E.C. 3.1.3.22), and mannitol dehydrogenase (MtDH) (E.C. 1.1.1.67) extracted from the red alga Caloglossa leprieurii (Montagne) J. Agardh was investigated. These enzymes participate in the metabolic pathways responsible for the biosynthesis and degradation of mannitol. Addition of NaCl (up to 600 mM) to the enzyme assays demonstrated that Mt1PDH, Mt1Pase, and MtDH have different sensitivities to salt. Fructose-6-phosphate reduction mediated by Mt1PDH was strongly inhibited by increasing salinity, but the reverse reaction (mannitol-1-phosphate oxidation) was only slightly affected. Mannitol (up to 600 mM) was completely non-inhibitory to the enzyme activity. The Km value changed from 0.68 mM under control conditions to 1.20 mM at 600 mM NaCl with fructose-6-phosphate as substrate and from 0.09 mM under control conditions to 1.34 mM at 600 mM NaCl with mannitol-1-phosphate as substrate. Mt1Pase was completely inhibited in the presence of both 600 mM NaCl and 600 mM mannitol, indicating feedback regulation of the enzyme by this polyol. Both the reductive and oxidative reaction of MtDH were partly inhibited by increasing salinity. Although mannitol oxidation was slightly stimulated with rising mannitol concentrations, the reverse reaction was negatively affected. The data generally are consistent with the proposed function of mannitol as an organic osmolyte and compatible solute. The role of mannitol metabolism as a means of biochemical adaptation by C. leprieurii to the environmental extremes of the mangrove habitat is discussed.