Purification and characterization of a cysteine dioxygenase from the yeast phase of Histoplasma capsulatum.

A cysteine dioxygenase, cysteine oxidase (EC 1.13.11.20), has been purified from the cytosolic fraction of yeast phase cells of the dimorphic fungus Histoplasma capsulatum. The cysteine oxidase is an iron-containing dioxygenase with a molecular weight of 10500 (+/- 1500) and is present only in the yeast phase of the fungus. The enzyme is highly specific for L-cysteine, with a Km of 2 X 10(-5) M in vitro. The product of cysteine oxidation is cysteinesulfinic acid, as analyzed by thin-layer chromatography and mass spectroscopy. To our knowledge, this is the first cysteine oxidase isolated from a fungus, and it probably plays an important role in the mycelial to yeast phase transition of H. capsulatum during which redox potential and cysteine levels are crucial factors.