MECHANISM OF CITRIC ACID FORMATION FROM GLUCOSE BY ASPERGILLUS NIGER

A medium containing glucose-1-C14 as the sole carbon source was fermented by Aspergillus niger under conditions giving a steady rate of oxygen consumption and a good yield of citric acid (63%). The citric acid was isolated and degraded by chemical methods to determine the carbon-14 concentration of the methylene carbons, the tertiary carbon, the tertiary carboxyl carbon, and the primary carboxyl carbons. These were found to contain, respectively, 35.6, 21.2, 7.25, and 5.99% of the C14 concentration of carbon-1 of the glucose. A mathematical analysis of these data in the light of current theories on citric acid formation suggested following conclusions: (a) 37–40% of the total citric acid was formed from recycled C4-dicarboxylic acid, (b) 40% of the dicarboxylic acid was formed through C2,C2 condensation and 60% through C1,C3 condensation, (c) 78% of the glucose was dissimilated through the Embden–Meyerhof scheme, the remainder being dissimilated through a mechanism involving carboxyl labeled pyruvic acid.