Measuring in vivo ureagenesis with stable isotopes

Abstract

Stable isotopes have been an invaluable adjunct to biomedical research for more than 70years. Indeed, the isotopic approach has revolutionized our understanding of metabolism, revealing it to be an intensely dynamic process characterized by an unending cycle of synthesis and degradation. Isotopic studies have taught us that the urea cycle is intrinsic to such dynamism, since it affords a capacious mechanism by which to eliminate waste nitrogen when rates of protein degradation (or dietary protein intake) are especially high. Isotopes have enabled an appreciation of the degree to which ureagenesis is compromised in patients with urea cycle defects. Indeed, isotopic studies of urea cycle flux correlate well with the severity of cognitive impairment in these patients. Finally, the use of isotopes affords an ideal tool with which to gauge the efficacy of therapeutic interventions to augment residual flux through the cycle.

Conflict of interest statement

Conflict of Interest statement: The authors declare that there are no conflicts of interest.

Copyright 2010 Elsevier Inc. All rights reserved.

Figures

Figure 1

Left: Appearance of [15N]urea in blood after oral administration of 15NH4Cl (~ 0.37 mmol/kg; 98 atom % excess) to health controls (closed circles); asymptomatic heterozygotes with ornithine transcarbamylase deficiency (open circles); symptomatic heterozygotes (open boxes); or a severely affected hemizygote with neonatal onset disease (open triangles). Data are mean ± SEM at each time point. Right: Appearance of label in blood [5-15N]glutamine after administration of the 15NH4Cl. Symbols same as those for the left panel. The “absolute” values refer to the product of the blood concentration of either urea or glutamine and the isotopic enrichment (atom % excess) at each time point. Adapted from Yudkoff et al [12].

Figure 2

A patient with N-acetylglutamate synthetase deficiency was given an oral dose of [1-13C]acetate and samples subsequently were taken at the indicated times for analysis of 13C enrichment (atom % excess) in breath 13CO2 (Panel A), plasma [13C]urea (Panel B) and the absolute concentration of [13C]urea in blood (Panel C). The study was performed both before (open circles) and after a 3 day course of treatment with N-carbamylglutamate. From Caldovic et al [16].

Figure 3

Isotopic abundance (atom % excess) in 13CO2 in expired air in healthy adults who received an infusion of NaH13CO3 (0.089 mmol/kg/hr; 98 atom % excess) for a period of 300 minutes. The solid horizontal line denotes the mean isotopic abundance from 10 to 300 minutes. During the initial 10 minutes of the experiment each subject also received a priming dose (0.083 mmol/kg) of the labeled bicarbonate. At 90 minutes each individual received an oral dose of NCG (100 mg/kg).

Fig. 4

The concentration of [13C]urea in plasma during the constant infusion of NaH13CO3. The line represents linear regression analysis from 10 to 150 minutes. At 90 minutes each subject received an oral dose (50 mg/kg) of N-carbamylglutamate. The closed circles correspond to the period during which the linear regression was performed. The open circles are points from 150 until 300 minutes and correspond to the post-treatment values. The time from 10 until 150 minutes is included in the regression because this is a period of drug absorption. The number at the top of each graph (μmol/min/liter) correspond to the rate of [13C]urea appearance in blood.