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Protein Dissimilation by Human Salivary-sediment Bacteria

Department of Preventive and Community Dentistry, The University of Melbourne, 711 Elizabeth Street, Melbourne, 3000, Australia

P.F. Riley

Department of Preventive and Community Dentistry, The University of Melbourne, 711 Elizabeth Street, Melbourne, 3000, Australia

Proteins of known composition and structural characteristics were incubated (1.0 mglmL) with re-suspended salivary sediment (2.5% vl v) in a lactate-salt medium with an initial pH of 5.2 for two hr at 37°C. Hydrolysis of the proteins was monitored by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Hydrogen ion, amines, and ammonia were measured by use of a combined pH electrode, high performance liquid chromatography, and glutamate dehydrogenase, respectively. Of the proteins studied, the caseins _s1, β, and K and the histones H1 and H3 were extensively hydrolyzed by the salivary-sediment bacteria. The hydrolysis of these proteins was attributed to their relative lack of tertiary (folded) structure. The only amine detected was the polyamine putrescine arising from the catabolism of arginine following the hydrolysis of the arginine-rich histone H3. None of the other proteins extensively hydrolyzed by salivary sediment, although containing arginyl and lysyl residues, served as substrates for putrescine or cadaverine production. Pre-hydrolysis of the arginine-rich histone H3 and poly-L-arginine with trypsin resulted in a marked increase in putrescine produced, suggesting that the salivary-sediment proteolytic activity was not "trypsin-like". Incubation of salivary-sediment bacteria with the caseins and the histone H3 resulted in an increase in ammonium ion concentration and an associated decrease in hydrogen ion concentration. The increase in ammonium ion concentration not attributed to arginine hydrolysis was correlated with the content of glutaminyl plus asparaginyl residues of the proteins. The results suggest that amido nitrogen, in the form of glutaminyl and asparaginyl residues of salivary and dietary proteins, is a potential source of nitrogen for oral bacteria and may also play a role in plaque pH homeostasis.

Journal of Dental Research, Vol. 68, No. 2, 124-129 (1989)
DOI: 10.1177/00220345890680020501


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