Metabolic Pathway for Palatinose of Oral Actinomyces Species

Objective: Palatinose (isomaltulose; 6-O-alpha-D-glucopyranosyl-D-fructofuranose), one of the isomers of sucrose, is widely used as a sugar substitute since it is not fermented and not used as a substrate of glucan formation by mutans streptococci. Recently, it has been reported that palationse is fermented into acids by oral bacteria other than mutans streptococci, especially oral Actinomyces species. Thus, we investigated the acid productivity from palatinose by oral Actinomyces and its metabolic pathway for palationse. Methods: Actinomyces naeslundii ATCC12104 and two clinically isolates of A. naeslundii were grown in trypticase-based complex media containing 1% palatinose under anaerobic conditions. The cells were harvested at a logarithmic phase of growth, washed, suspended and used for the measurement of acid production by a pH-stat at pH 7.0 under anaerobic conditions. A part of washed cells were also used for enzyme assays. Results: The washed cells of all the A. naeslundii strains produced acids from palatinose (3.36 ± 0.9 nmol/min/mg cells), though the acid production rates were lower than those from glucose (15.9 ± 3.5 nmol/min/mg cells) and sucrose (27.9 ± 8.8 nmol/min/mg cells). The cell-free extracts prepared from the washed cells showed palatinase activity (36.5 ± 8.8 U/g protein), which hydrolyzes palatinose into glucose and fructose, while the membrane fraction had no such activity. Unlike other palatinose-fermenting bacteria such as Klebsiella and Fusobacterium species, A. naeslundii had no activity of phosphoenolpyruvate-dependent palatinose phosphotransferase, which incorporates palatinose and phosphorylates it to palatinose 6-phosphate. Conclusion: This is the first report to show that oral Actinomyces can incorporate palatinose and hydrolyze it to glucose and fructose by palatinase, then degrade these monosaccharides into acids. Partly supported by Grants-in-Aid (#13470446, #14571944 and #14370687) from the JSPS, and by a Grant H12-RHTA-005 from the MHLW.