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

Abstract

Summary: Mannitol dehydrogenase (EC 1.1.1.138) has been purified to homogeneity from fruit bodies of Agaricus bisporus. M r 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 M r 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 K m values of 16.2 mm and 36μm respectively at pH 7.0. The K m value for NADPH is 38.5μm and that for fructose approximately 1.2 m. The V max is 591 μm min−1 (mg protein)−1 for mannitol synthesis and 5μmol min−1 (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.