Mannitol Metabolism in Agaricus bisporus: Purification and Properties of Mannitol Dehydrogenase

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Abstract
Summary: Mannitol dehydrogenase (EC 1.1.1.138) has been purified to homogeneity from fruit bodies of Agaricus bisporus. Mr values of 115000 were determined by gel filtration and 130000 by rate zonal ultracentrifugation. The sedimentation coefficient is 6.5S. The native protein is composed of four subunits of Mr 29000. The enzyme is specific for NADP, and shows low activity with sorbitol. Normal Michaelis-Menten kinetics are exhibited for both mannitol and NADP, giving Km values of 16.2 mM and 36μM respectively at pH 7.0. The Km value for NADPH is 38.5μM and that for fructose approximately 1.2 M. The Vmax is 591 μM min1 (mg protein)1 for mannitol synthesis and 5μmol min1 (mg protein)1 for fructose synthesis at pH 7.0. Inhibition of fructose synthesis by NADPH is stronger than inhibition of mannitol synthesis by NADP. The results are discussed with respect to the control of enzyme activity under physiological conditions.

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