5-Keto-D-fructose: formation and utilization in the course of D-fructose as similation by Gluconabacter cerinus

The accumulation of 5-keto-d-fructose (5KF) by Gluconobacter cerinus grown on d-fructose in unbuffered medium was shown to be optimal at pH 4.0 after cell growth ceased. During the exponential phase of growth or at neutral pH after the onset of the stationary phase, 5KF production continued but did not accumulate because of its rapid reutilization by reduction to d-fructose. The extent of isotope incorporation into C5 of ribonucleic acid ribose when cells were grown in the presence of specifically labeled d-glucose and d-fructose clearly indicated that (i) the hexose monophosphate oxidative pathway is the predominant metabolic route for carbohydrate assimilation and (ii) extensive randomization of label between C1 and C6 of d-fructose occurred prior to its conversion into pentose. It is suggested that the cyclic oxidation and reduction through the symmetrical 5KF molecule, which accounts for the observed randomization of isotope in d-fructose, provides the cells with an effective mechanism for the regeneration of nicotinamide adenine dinucleotide phosphate during the period of intensive growth.