Peptostreptococcus micros has a uniquely high capacity to form hydrogen sulfide from glutathione

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Abstract
There are high amounts of hydrogen sulfide in deep periodontal pockets. This volatile sulfur compound may be formed from L-cysteine, but only low levels of this amino acid can be expected to be present in periodontal pockets. Glutathione, L-γ-glutamyl-L-cysteinylglycine, is in high concentration in most tissue cells, and this tripeptide may be more readily available as a source of hydrogen sulfide formation in the pockets. The ability of 37 different species of oral bacteria to utilize glutathione in hydrogen sulfide formation was studied. Of these species, only 2 species of Peptostreptococcus and 5 species of Fusobacterium formed high amounts of hydrogen sulfide from glutathione within 24 h. Since the initial rate of hydrogen sulfide formation was more than 5 times higher in Peptostreptococcus micros than in any of the other bacterial species, the kinetics of sulfide formation from glulathione by P. micros was further elucidated. The formation of sulfide followed quite closely hyperbolic Michaelis-Menten kinetics. The maximal initial rate of sulfide formation (Vmax) was 163 ±2 nmol sulfide per minute per milligram of cellular protein. Half maximal initial rate (Km) was obtained at 7.4±0.8 μM glutathione. The initial rate of sulfide formation from L-cysteine was much slower and was almost proportional to L-cysteine concentration. This difference in kinetics of sulfide formation between glutathione and L-cysteine strongly suggested that glutathione was actively transported into the cell, whereas the transport of L-cysteine was more or less controlled by diffusion. The sulfide formation from the dipeptide L-cysteinylglycine also followed quite closely hyperbolic Michaelis-Menten kinetics. The maximal initial rate of sulfide formation (Kmax) from L-cysteinylglycine was 75 ± 2 nmol sulfide per minute per milligram of cellular protein. Half maximal initial rate (Km) was obtained at 26 ± 4 μM L-cysteinylglycine. Hydrogen sulfide may readily be formed from glutathione by P. micros in periodontal pockets. This is the first time that experimental data indicates active transport of di-and tripeptides in anaerobic bacteria.