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Mechanism of Fumaric Acid Accumulation in Rhizopus nigricans

^a Department of Biological Sciences and Graduate Division of Microbiology, University of Cincinnati, Cincinnati, Ohio

ABSTRACT

It is doubtful that the glyoxylate bypass plays a significant role in the accumulation of fumaric acid by fungi, as has been postulated. In high glucose media, which favor fumarate production, isocitrate lyase (threo-D_s isocitrate glyoxylate lyase), which is the key enzyme of the glyoxylate bypass, is strongly repressed. The specific activity of this enzyme remains low as long as glucose is present in the medium, even though fumarate formation proceeds at a high level. In addition, the activity of isocitrate lyase is inhibited by phosphoenolpyruvate, which would be formed from glucose. Alternatively, evidence is presented that bulk accumulation of fumaric acid under aerobic conditions in high glucose media takes place through a C₃ plus C₁ carbon dioxide fixation. CO₂ fixation was measured by the direct incorporation of NaHC¹⁴O₃ into fumaric acid, and by demonstrating that the specific radioactivity of fumaric acid formed from uniformly labeled C¹⁴-glucose was decreased in the presence of nonradioactive carbonate. The extent of decrease in specific radioactivity is in accord with a C₃ plus C₁ CO₂ fixation mechanism.

¹ Present address: Institute of Marine Science, University of Miami, Miami, Fla.