The initial steps of sorbitol catabolism were studied in 4 strains ofActinomyces naeslundiiandActinomyces viscosusthat had been isolated from human dental plaque. Cell‐free extracts were prepared from cells grown in the presence of either sorbitol, xylitol or glucose. The extracts from all strains grown on sorbitol had nicotinamide adenine dinucleotide‐linked dehydrogenase activities for sorbitol and xylitol and reduced nicotinamide adenine dinucleotide‐linked reductase activities for fructose and xylulose. Two of the strains also exhibited these activities when grown in the presence of xylitol, and all glucose‐grown cells lacked them. The results indicate that sorbitol metabolism in oral actinomyces involve oxidation of sorbitol to fructose by an inducible enzyme, nicotinamide adenine dinucleotide‐linked sorbitol dehydrogenase. This step is followed by the phosphorylation of fructose with guanosine triphosphate as a main phosphoryl donor. Thus, the initial catabolic pathway of sorbitol inA. naeslundiiandA. viscosusis different from those described for other oral
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