, Volume 201, Issue 2, pp 173-178

First online:

Characterization of mannitol metabolism in the mangrove red algaCaloglossa leprieurii (Montagne) J.Agardh

  • Ulf KarstenAffiliated withSchool of Biological Science, University of New South Wales Email author 
  • , Kevin D. BarrowAffiliated withSchool of Biochemistry and Molecular Genetics, University of New South Wales
  • , Oliver NixdorfAffiliated withDepartment of Biology, Marine Botany, University of Bremen
  • , John A. WestAffiliated withSchool of Botany, University of Melbourne
  • , Robert J. KingAffiliated withSchool of Biological Science, University of New South Wales

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A metabolic pathway, known as the mannitol cycle in fungi, has been identified as a new entity in the eulittoral mangrove red algaCaloglossa leprieurii (Montagne) J. Agardh. Three specific enzymes, mannitol-1-phosphate dehydrogenase (Mt1PDH; EC, mannitol-1-phosphatase (MtlPase; EC, mannitol dehydrogenase (MtDH; EC and one nonspecific hexokinase (HK; EC were determined and biochemically characterized in cell-free extracts. Mannitol-1-phosphate dehydrogenase showed activity maxima at pH 7.0 [fructose-6-phosphate (F6P) reduction] and pH 8.5 [oxidation of mannitol-1-phosphate (Mt1P)], and a very high specificity for both carbohydrate substrates. TheK m values were 1.4 mM for F6P, 0.09 mM for MOP, 0.020 mM for NADH and 0.023 mM for NAD+. For the dephosphorylation of MOP, MtlPase exhibited a pH optimum at 7.2, aK m value of 1.2 mM and a high requirement of Mg2+ for activation. Mannitol dehydrogenase had activity maxima at pH 7.0 (fructose reduction) and pH 9.8 (mannitol oxidation), and was less substrate-specific than Mt1PDH and MtlPase, i.e. it also catalyzed reactions in the oxidative direction with arabitol (64.9%), sorbitol (31%) and xylitol (24.8%). This enzyme showedK m values of 39 mM for fructose, 7.9 mM for mannitol, 0.14 mM for NADH and 0.075 mM for NAD+. For the non-specific HK, only theK m values for fructose (0.19 mM) and glucose (7.5 mM) were determined. The activities of the anabolic enzymes Mt1PDH and MtlPase were always at least two orders of magnitude higher than those of the degradative enzymes, indicating a net carbon flow towards a high intracellular mannitol pool. The function of mannitol metabolism inC. leprieurii as a biochemical adaptation to the environmental extremes in the mangrove habitat is discussed.

Key words

Caloglossa Hexokinase Mannitol-1-phosphate dehydrogenase Mannitol-1-phosphatase Mannitol dehydrogenase Osmolyte metabolism