Differential utilization of systemic and enteral ammonia for urea synthesis in control subjects and ornithine transcarbamylase deficiency carriers

Flux analysis of inborn errors of metabolism

Patients with an inborn error of metabolism (IEM) are deficient of an enzyme involved in metabolism, and as a consequence metabolism reprograms itself to reach a new steady state. This new steady state underlies the clinical phenotype associated with the deficiency. Hence, we need to know the flux of metabolites through the different metabolic pathways in this new steady state of the reprogrammed metabolism. Stable isotope technology is best suited to study this. In this review the progress made in characterizing the altered metabolism will be presented. Studies done in patients to estimate the residual flux through the metabolic pathway affected by enzyme deficiencies will be discussed. After this, studies done in model systems will be reviewed. The focus will be on glycogen storage disease type I, medium-chain acyl-CoA dehydrogenase deficiency, propionic and methylmalonic aciduria, urea cycle defects, phenylketonuria, and combined D,L-2-hydroxyglutaric aciduria. Finally, new developments are discussed, which allow the tracing of metabolic reprogramming in IEM on a genome-wide scale. In conclusion, the outlook for flux analysis of metabolic derangement in IEMs looks promising.

Inborn errors of metabolism: the flux from Mendelian to complex diseases

Inborn errors of metabolism are characterized by dysregulation of the metabolic networks that underlie development and homeostasis, and constitute an important and expanding group of genetic disorders in humans. Diagnostic methods that are based on molecular genetic tools have a limited ability to correlate phenotypes with subtle changes in metabolic fluxes. We argue that the direct and dynamic measurement of metabolite flux will facilitate the integration of environmental, genetic and biochemical factors with phenotypic information. Ultimately, this integration will lead to new diagnostic and therapeutic approaches that are focused on the manipulation of these pathways.

Hyperammonemia increases sensitivity to LPS

Metabolic and cognitive alterations occur during hyperammonemia. Here, we report that chronic hyperammonemia also leads to increased sensitivity to LPS. Sparse-fur mice were challenged i.p. with LPS or saline control and then tested for motivation to investigate a novel juvenile over 24 h. Cytokine, ammonia, and urea concentration were quantified at the peak of sickness (2 h post injection). Chronic hyperammonemic Otc(spf-ash) mice displayed more pronounced and prolonged sickness behavior in response to LPS (P=0.02). LPS significantly (P<0.0001) increased plasma concentrations of TNFalpha, IL-1 beta, IL-6, IL-15, IL-9, IL-2, IL-1 alpha, IL-1 beta, Rantes, MIP1 alpha, MIP1 beta, MCP-1, KC, GM-CSF, G-CSF, Eotaxin, IL-13, and IL-12 in both wild type and Otc(spf-ash) mice. No significant genotype/treatment interactions (P>0.1) were detected for any cytokine. Adult Otc(spf-ash) mice (168+/-41 microM) had four times higher plasma ammonia compared to wild type mice (40 +/- 6 microM) (P=0.002). Two hours after LPS injection, plasma ammonia concentrations tended (P=0.08) to decrease in both wild type and Otc(spf-ash) mice. Learning and memory behaviors were assessed in mice under basal conditions to determine the impact of chronic hyperammonemia on cognition. Otc(spf-ash) mice performed significantly poorer in the two trial Y-maze (P=0.02) and the Morris water maze (P=0.001) than their littermate wild type controls. Taken together, these data indicate that chronic hyperammonemia results in impaired cognition and creates a state of LPS hypersensitivity.

In vivo urea kinetic studies in conscious mice

Stable isotope studies in conscious mice have been limited by the invasive catheterization procedures and relatively large sample size required. We developed minimally invasive catheterization protocols that together with the ability to analyze small samples have allowed for the study of urea kinetics in conscious mice. A single dose of 15N15N-urea followed by multiple sampling in mice (n = 6) showed that a primary pool of urea exchanged rapidly [70.65 +/- 14.96 mmol/(kg x h)] with a secondary pool. The urea entry rate determined with this protocol was 3.36 +/- 0.30 mmol/(kg x h). Continuous infusion of 15N15N-urea (n = 6) achieved plateau enrichment values at 3.3 +/- 0.2.h from which the urea entry rate was determined by isotope dilution [3.24 +/- 0.23 mmol/(kg x h)]. The urea entry rate measured by the single dose or continuous infusion protocol did not differ (P = 0.76). The minimally invasive methods described allow us to study not only ureagenesis and urea cycle disorders in vivo, but also urea transport and transporter function and nitrogen metabolism in general in mouse models. This is especially relevant because mouse targeting technologies will likely facilitate the generation of organ and tissue specific nulls of the various urea cycle enzymes.

Considerations in the Difficult-to-Manage Urea Cycle Disorder Patient

Today, patients with urea cycle disorder (UCD) may survive well beyond infancy. The goal of keeping them in consistent nitrogen balance can be undermined by changing metabolic needs throughout various stages of life, resulting in hyperammonemia in the short term, and poor growth and development in the long term. The specific UCD genotype can affect the risk of metabolic destabilization and management difficulties, as can variable protein tolerance secondary to changing growth demands, biochemical complications, and environmental influences. Preventing catabolic stress is as important as controlling dietary protein intake for avoiding metabolic decompensation. Optimal treatment, specifically pharmacologic therapy, possible branched chain amino acid (BCAA) supplementation, accurate laboratory monitoring, and psychosocial support, requires thorough understanding and careful application of each component.

Clinical consequences of urea cycle enzyme deficiencies and potential links to arginine and nitric oxide metabolism

Urea cycle disorders (UCD) are human conditions caused by the dysregulation of nitrogen transfer from ammonia nitrogen into urea. The biochemistry and the genetics of these disorders were well elucidated. Earlier diagnosis and improved treatments led to an emerging, longer-lived cohort of patients. The natural history of some of these disorders began to point to pathophysiological processes that may be unrelated to the primary cause of acute morbidity and mortality, i.e., hyperammonemia. Carbamyl phosphate synthetase I single nucleotide polymorphisms may be associated with altered vascular resistance that becomes clinically relevant when specific environmental stressors are present. Patients with argininosuccinic aciduria due to a deficiency of argininosuccinic acid lyase are uniquely prone to chronic hepatitis, potentially leading to cirrhosis. Moreover, our recent observations suggest that there may be an increased prevalence of essential hypertension. In contrast, hyperargininemia found in patients with arginase 1 deficiency is associated with pyramidal tract findings and spasticity, without significant hyperammonemia. An intriguing potential pathophysiological link is the dysregulation of intracellular arginine availability and its potential effect on nitric oxide (NO) metabolism. By combining detailed natural history studies with the development of tissue-specific null mouse models for urea cycle enzymes and measurement of nitrogen flux through the cycle to urea and NO in UCD patients, we may begin to dissect the contribution of different sources of arginine to NO production and the consequences on both rare genetic and common multifactorial diseases.