Mechanism ofFumaric AcidAccumulation in Rhizopus nigricans

Itisdoubtful that theglyoxylate bypass plays a significant role intheaccumula- tionoffumaric acidbyfungi, ashasbeenpostulated. Inhighglucose media, which favor fumarate production, isocitrate lyase (threo-D8 isocitrate glyoxylate lyase), whichisthekeyenzymeoftheglyoxylate bypass, isstrongly repressed. Thespecific activity ofthis enzymeremains lowaslong asglucose ispresent inthemedium, eventhough fumarate formation proceeds ata highlevel. Inaddition, theactivity ofisocitrate lyase isinhibited byphosphoenolpyruvate, whichwouldbeformed fromglucose. Alternatively, evidence ispresented thatbulkaccumulation offumaric acidunder aerobic conditions inhighglucose mediatakes place through a C3 plus C1carbon dioxide fixation. CO2fixation was measured bythedirect incorporation ofNaHC'403 intofumaric acid, andbydemonstrating thatthespecific radioac- tivity offumaric acidformedfromuniformly labeled C(4-glucose was decreased in thepresenceofnonradioactive carbonate. Theextent ofdecrease inspecific radio- activity isinaccord witha C3 plus ClC02fixation mechanism.