Effect of urea concentration on its utilization, on the pH and the formation of ammonia and carbon dioxide in a human salivary sediment system

Abstract

Experiments were initiated to study the metabolism of urea in a salivary sediment system in which the sediment concentration was 16.7 per cent (v/v). The pH, utilization of urea and formation of NH 3 and CO 2 at each of several urea concentrations, between 0 and 10 per cent (w/v), were measured before and during incubation of the system for 4 hr at 37 °C. At low levels of urea, the urea was completely utilized before the end of the 4-hr experimental period, and the pH rose and subsequently fell. At higher levels, the urea was not used up and the pH rapidly rose to an asymptote. Ammonia and CO 2 formation both increased with increase in the initial urea concentration. Both at low and high substrate levels, NH 3 and CO 2 progressively accumulated, the latter much more than the former since most of the urea-N was “stored” whereas most of the urea-C was not. At the low urea levels, nitrogen storage occurred as the urea was utilized but N was released as NH 3 after the urea had disappeared from the medium. No urease activity was observed in the system and it was postulated that urea breakdown occurred by a reverse ornithine cycle. Radioautographs of two-dimensional paper chromatograms of alcohol extracts of sediment showed the formation and depletion of an unidentified 14C-labelled compound during the early part of the 4-hr incubation and the formation of some lactic and succinic acid during the later part. The formation of these two acids indicated that pathways of urea catabolism are linked to pathways involved in the catabolism of carbohydrate.