Extracts of Chaetomium globosum contained an unstable, NAD-dependent mannitol dehydrogenase, oxidizing mannitol to fructose. The enzyme had a low affinity for its substrates but showed high activity in vitro. Compared to fructose, exogenous mannitol was poorly utilized by the fungus. Raising the pH of the medium resulted in improved growth of the fungus on mannitol, in increased formation of ketose in the medium, and in stimulation of mannitol dehydrogenase formation. By contrast, glucokinase and fructokinase activities were depressed by growth at the higher pH.
The possible involvement of a mannitol dehydrogenase in the utilization of exogenous mannitol is discussed, together with the factors probably contributing to the poor growth of the fungus on mannitol and the control of cellular mannitol formation.