Enzymes involved in the last steps of the biosynthesis of mannitol in brown algae

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Mannitol-1-phosphate dehydrogenase (EC 1. 1. 1. 17) and mannitol-1-phosphatase (EC number yet unassigned) were detected in the brown algae, Spatoglossum pacificum and Dictyota dichotoma. The enzymes were extracted from algal fronds and their properties were investigated using partially purified preparations. Mannitol-1-phosphatase shows maximum activity at pH 7. The enzyme had a narrow substrate specificity. The Km value for mannitol-1-phosphate is 8.3 × 10^<-4> M (30℃, pH 7.0). The enzyme is activated by Mg^<++> and Mn^<++> and is strongly inhibited by PCMB, Hg^<++> and NaF. Mannitol-1-phosphate dehydrogenase showed maximum activities at pH values 6.5 and 10.2 in reductive and oxidative reactions, respectively. The dehydrogenase also showed narrow substrate specificity ; mannitol-1-phosphate and NAD or fructose-6-phosphate and NADH_2 are utilized, respectively, in oxidative and reductive reactions by the enzyme. Km values for these substrates and the coenzymes are 2.5 × 10^<-4> M and 7.1 × 10^<-5> M for the first pair and 2.8 × 10^<-4> M and 1.3 × 10^<-5> M for the latter pair. This enzyme was strongly inhibited by PCMB and Hg^<++>, but was only slightly affected by adenosine phosphates. Possible roles of these enzymes in the biosynthesis of mannitol in brown algae are discussed.
日本植物生理学会の論文