Conversion of [15N]ammonia into urea and amino acids in humans and the effect of nutritional status

Hepatic NH3 detoxification by ureagenesis requires an input of aspartate-N, originating either from amino acid-N or NH3-N. The relative importance of these two routes may depend on the nutritional state. To test this, four volunteers were given a liquid diet for 2 d and then on day 3 were either fed every 20 min or fasted. Doses of 15NH4Cl were taken orally every 20 min for 6 h (total 1.5 g) and blood was sampled hourly. Urea-N elimination under fasted conditions was only 0.75 of that for the fed state. Considering the increase in body urea pool during feeding, ureagenesis during fasting was probably closer to 0.6 of that during feeding. Since the [14N15N]urea enrichment was not different between the fed and fasted states, the proportion of the 15NH3 dose converted to urea during fasting was also 0.6 of that during the fed condition. No change in [14N15N]urea and [amide-15N]glutamine enrichment suggested that NH3 enrichment was also not affected by nutritional state. Enrichment of [15N15N]urea was approximately 0.05 that of [14N15N]urea which indicates that 15NH3 can also enter the aspartate route, the importance of which is yet unknown. Both [15N15N]urea and [amino-15N]glutamine enrichment in the fasted state were approximately 1.7 times that in the fed state, indicating increased labelling of precursors and/or increased NH3 flux through the aspartate route. Glutamate, valine, leucine and isoleucine showed comparable increases in enrichment during fasting. Arginine enrichment was unaltered by nutritional state, but was lower than [14N15N]urea, indicating incomplete equilibration with the arginine pool in periportal hepatocytes. The present study indicates that hepatic NH3 detoxification may use the aspartate route, gaining importance in the fasted state. The majority of urea was supplied with only one N atom from NH3, thus provision of the other may have consequences for alternative substrates, in particular amino acids.