Studies on the Isoleucine Fermentation

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Abstract
The isoleucine fermentation utilizing D-threonine was previously reported by the authors. To prove that D-threonine is a direct precursor of isoleucine in this fermentation, the experiments using D-threonine-2-C14 and uniformly labeled glucose-C14 were performed. In the case using D-threonine-2-C14, the specific radioactivities of isoleucine and threonine were quite identical throughout the culture period. This result clearly indicates that D-threonine is directly incorporated into isoleucine. In the carbon skeleton of isolcucine, the carbon atoms other than those derived from D-threonine were considered to be originated from glucose. To confirm this, the fermentation using uniformly labeled glucose-C14 was carried out. Although the radioactivity was scarcely observed in threonine, the considerable activities were detected in isoleucine and 2-ketogluconate. On the assumption that two carbon stoms are incorporated from glucose into isoleucine via 2-ketogluconate, the specific radioactivities per C14 atom of glucose, 2-ketogluconate and isoleucine were calculated and almost equal values were obtained throghout the culture period. Consequently, it is clear that the carbon skeleton of isoleucine is not originated from the carbons of L-threonine formed from glucose, but derives from the carbons of D-threonine, and of active acetaldehyde formed from glucose. Biosynthetic pathway of isoleucine in this fermentation is supposed to be identical with the known pathway in microorganisms after D-threonine is converted to α-ketobutyrate by the action of D-threonine dehydratase and this α-ketobutyrate condenses with active acetaldehyde formed from glucose via 2-keto-gluconate.

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