Regulation of mannitol biosynthesis and degradation by Cryptococcus neoformans

Cryptococcus neoformans, an encapsulated yeast that is an opportunistic pathogen of AIDS patients, produced and secreted mannitol when incubated with an appropriate carbon source. Glucose, fructose, and mannose were good growth substrates and were converted to mannitol. Maltose and xylose were good growth substrates but were not converted to mannitol. Cells of C. neoformans that were grown on a non-mannitol-generating carbon source, such as peptone or xylose, were able to convert glucose to mannitol only after a prolonged lag period in the presence of glucose. It was concluded that the enzymes of the mannitol biosynthetic pathway were not constitutively expressed but were induced in response to glucose or to a glucose metabolite. Enzymes required to catabolize mannitol, however, were constitutively expressed. The production of mannitol was inhibited by anaerobiosis, by the respiratory poison rotenone, and by polyethylenesulfonate, a specific inhibitor of fungal NADP-dependent dehydrogenases. When cells were incubated with deuterated glucose, the deuterium content of the mannitol produced was much lower than that of the glucose precursor, indicating that the glucose was diluted by an intracellular pool of an intermediate. We had previously shown that C. neoformans contains a large intracellular pool of glucose 6-phosphate, and we now conclude that this pool of glucose 6-phosphate is metabolically active.