Role of Glutamine in Urea Synthesis

Abstract

IN 1939, it was suggested by one of us¹ that the amide-nitrogen of glutamine was a source of urea-nitrogen and that urea formation was paralleled by a continuous re-synthesis of glutamine. Evidence has now been obtained for a reaction between bicarbonate (as the source of urea-carbon) and the two nitrogenous groups of glutamine. This appears to represent the first of the sequence of reactions leading to urea synthesis. The evidence for this step is based on the observation that when liver slices are incubated with glutamine in absence of added bicarbonate, not only was urea synthesis practically suppressed but also neither the glutamine-amide-nitrogen nor the amino-nitrogen of glutamine was utilized to any extent. On the other hand, in the presence of bicarbonate a considerable synthesis of urea occurred with the simultaneous utilization of amide-nitrogen and of amino-nitrogen (Table 1). The small quantities of ammonia derived from glutamine and formed during the incubation were unaffected by bicarbonate and therefore appear to be unrelated to urea synthesis. That glutamine rather than ammonia or ammonium glutamate acted as a direct source of urea-nitrogen was demonstrated in several ways: first, by the fact that the rate of urea synthesis from glutamine was directly proportional to the substrate concentration, whereas this was not so when ammonia served as a substrate (Fig. 1). A slight inhibitory effect of ammonia was observed at the highest concentrations tested. Secondly, inhibition of glutam-inase by sulphophthaleins had no effect on the synthesis, whereas activation of glutaminase with pyruvate almost completely inhibited urea formation. Finally, evidence was obtained by paper chromatography for a considerable formation of glutamine from ammonium salts in liver slices. These observations provide evidence for the role of glutamine, as distinct from ammonium glutamate, in urea synthesis and suggest that urea formation from ammonia may depend upon a transitory formation of glutamine.