Malic acid accumulation by Aspergillus flavus - I. Biochemical aspects of acid biosynthesis

The accumulation and excretion of l-malic acid, and to a lesser extent succinic and fumaric acids, by Aspergillus flavus occurs under aerobic conditions in a medium containing a high glucose concentration, a limiting amount of nitrogen and a neutralizing agent (CaCO₃). An overall malic, succinic, and fumaric acid molar yield of up to 68.6% (moles of acid produced per mole of glucose utilized) is obtained after incubation for 6 to 8 days, although a transient molar yield of 76% was measured. Glucose consumption and formation of acids were faster in a fermentor than in shake flasks. During the acid production stage, the activity of malate dehydrogenase increased 6 to 10-fold while that of fumarase changed only slightly. Cycloheximide greatly inhibited both l-malic acid production and the increase in malate dehydrogenase activity, without affecting fumarase activity. The results suggest that de novo protein synthesis is involved in the increase of malate dehydrogenase and that this enzyme is essential for l-malic acid production and accumulation.