Effect of urea on pH, ammonia, amino acids and lactic acid in the human salivary sediment system incubated with varying levels of glucose.

With concentrations of urea of 0, 0.17, 0.85 or 1.7 per cent (w/v) in salivary sediment (16.7 per cent, v/v), the concentration of glucose varied between 0 and 30 per cent (w/v). The pH of the salivary sediment mixtures remained constant. As glucose was utilized by the salivary sediment, the pH curve of this system was characterized by a rapid fall, followed by a slow rise. In the presence of urea, however, the fall in pH was considerably inhibited and an early pH rise was favoured. Glucose suppressed the formation of NH3 from endogenous sources to an extent almost proportional to its concentration. Glucose also suppressed NH3 formation when urea was present. The effect was optimum near physiologic pH range. Urea favoured the formation of alanine perhaps by transamination or by direct amination of pyruvate involving different pathways. The findings suggest that the inhibition of pH-fall was the result, not only of the interactions between acid and base produced from glucose and urea, respectively, but was largely due to the buffering effect of the products of the metabolism of urea. There appeared to be some metabolic relationship in the formation of alanine and lactate but this did not control pH changes substantially.