Gene delivery corrects N-acetylglutamate synthase deficiency and enables insights in the physiological impact of L-arginine activation of N-acetylglutamate synthase.

The urea cycle protects the central nervous system from ammonia toxicity by converting ammonia to urea. N-acetylglutamate synthase (NAGS) catalyzes formation of N-acetylglutamate, an essential allosteric activator of carbamylphosphate synthetase 1. Enzymatic activity of mammalian NAGS doubles in the presence of L-arginine, but the physiological significance of NAGS activation by L-arginine has been unknown. The NAGS knockout (Nags-/-) mouse is an animal model of inducible hyperammonemia, which develops hyperammonemia without N-carbamylglutamate and L-citrulline supplementation (NCG + Cit). We used adeno associated virus (AAV) based gene transfer to correct NAGS deficiency in the Nags-/- mice, established the dose of the vector needed to rescue Nags-/- mice from hyperammonemia and measured expression levels of Nags mRNA and NAGS protein in the livers of rescued animals. This methodology was used to investigate the effect of L-arginine on ureagenesis in vivo by treating Nags-/- mice with AAV vectors encoding either wild-type or E354A mutant mouse NAGS (mNAGS), which is not activated by L-arginine. The Nags-/- mice expressing E354A mNAGS were viable but had elevated plasma ammonia concentration despite similar levels of the E354A and wild-type mNAGS proteins. The corresponding mutation in human NAGS (NP_694551.1:p.E360D) that abolishes binding and activation by L-arginine was identified in a patient with NAGS deficiency. Our results show that NAGS deficiency can be rescued by gene therapy, and suggest that L-arginine binding to the NAGS enzyme is essential for normal ureagenesis.

Effect of arginine on oligomerization and stability of N-acetylglutamate synthase.

N-acetylglutamate synthase (NAGS; E.C.2.3.1.1) catalyzes the formation of N-acetylglutamate (NAG) from acetyl coenzyme A and glutamate. In microorganisms and plants, NAG is the first intermediate of the L-arginine biosynthesis; in animals, NAG is an allosteric activator of carbamylphosphate synthetase I and III. In some bacteria bifunctional N-acetylglutamate synthase-kinase (NAGS-K) catalyzes the first two steps of L-arginine biosynthesis. L-arginine inhibits NAGS in bacteria, fungi, and plants and activates NAGS in mammals. L-arginine increased thermal stability of the NAGS-K from Maricaulis maris (MmNAGS-K) while it destabilized the NAGS-K from Xanthomonas campestris (XcNAGS-K). Analytical gel chromatography and ultracentrifugation indicated tetrameric structure of the MmMNAGS-K in the presence and absence of L-arginine and a tetramer-octamer equilibrium that shifted towards tetramers upon binding of L-arginine for the XcNAGS-K. Analytical gel chromatography of mouse NAGS (mNAGS) indicated either different oligomerization states that are in moderate to slow exchange with each other or deviation from the spherical shape of the mNAGS protein. The partition coefficient of the mNAGS increased in the presence of L-arginine suggesting smaller hydrodynamic radius due to change in either conformation or oligomerization. Different effects of L-arginine on oligomerization of NAGS may have implications for efforts to determine the three-dimensional structure of mammalian NAGS.

A zebrafish model of hyperammonemia.

Hyperammonemia is the principal consequence of urea cycle defects and liver failure, and the exposure of the brain to elevated ammonia concentrations leads to a wide range of neuro-cognitive deficits, intellectual disabilities, coma and death. Current treatments focus almost exclusively on either reducing ammonia levels through the activation of alternative pathways for ammonia disposal or on liver transplantation. Ammonia is toxic to most fish and its pathophysiology appears to be similar to that in mammals. Since hyperammonemia can be induced in fish simply by immersing them in water with elevated concentration of ammonia, we sought to develop a zebrafish (Danio rerio) model of hyperammonemia. When exposed to 3mM ammonium acetate (NH4Ac), 50% of 4-day old (dpf) fish died within 3hours and 4mM NH4Ac was 100% lethal. We used 4dpf zebrafish exposed to 4mM NH4Ac to test whether the glutamine synthetase inhibitor methionine sulfoximine (MSO) and/or NMDA receptor antagonists MK-801, memantine and ketamine, which are known to protect the mammalian brain from hyperammonemia, prolong survival of hyperammonemic fish. MSO, MK-801, memantine and ketamine all prolonged the lives of the ammonia-treated fish. Treatment with the combination of MSO and an NMDA receptor antagonist was more effective than either drug alone. These results suggest that zebrafish can be used to screen for ammonia-neuroprotective agents. If successful, drugs that are discovered in this screen could complement current treatment approaches to improve the outcome of patients with hyperammonemia.

Burden of spinal cord injury-related neuropathic pain in the United States: retrospective chart review and cross-sectional survey.

STUDY DESIGN: Cross-sectional, observational study. OBJECTIVES: Characterize demographic and clinical characteristics, health status, pain, function, productivity and economic burden in spinal cord injury-related neuropathic pain (SCI-NeP) subjects, by pain severity. SETTING: United States. One hundred and three subjects diagnosed with SCI-NeP recruited during routine primary care or specialty physician office visits completed a questionnaire to assess patient-reported outcomes. Physicians completed a case report form on inclusion/exclusion criteria, subject clinical characteristics and health-care resource use (HRU) based on 6-month retrospective chart review. RESULTS: Subjects' mean age was 48.7, 69.9% were male and 48.5% were unable to walk. The most frequently reported comorbidities were sleep disturbance/insomnia (28.2%), depressive symptoms (25.2%) and anxiety (23.3%). Subjects' mean pain severity score was 5.3 (0-10 scale), and 77.7% reported moderate or severe pain. On a 0-10 scale, subjects' reported moderate pain interference with function: mean 5.4. Subjects' health status, as measured by the EuroQol 5-dimensions health-state utility, was 0.49 (-0.11 to 1.00 scale). Pain interference with function and health status were significantly worse among subjects with more severe pain (P<0.0005). Among employed subjects (13.6%), overall work impairment was 38.0%. The proportion of subjects who were prescribed ≥1 medication was 94.2%, and the mean number of physician office visits in past 6 months due to SCI-NeP was 2.2. Total annualized cost per subject was $26 270 (direct: $8636, indirect: $17 634). CONCLUSION: SCI-NeP subjects exhibited high pain levels, despite active management. Pain levels were associated with poor function, low health status and lost productivity. HRU was prevalent, and costs, particularly indirect, were substantial, highlighting unmet need. SPONSORSHIP: This study was supported by Pfizer, Inc.

Recurrent encephalopathy: NAGS (N-acetylglutamate synthase) deficiency in adults.

N-acetyl-glutamate synthase (NAGS) deficiency is a rare autosomal recessive urea cycle disorder (UCD) that uncommonly presents in adulthood. Adult presentations of UCDs include; confusional episodes, neuropsychiatric symptoms and encephalopathy. To date, there have been no detailed neurological descriptions of an adult onset presentation of NAGS deficiency. In this review we examine the clinical presentation and management of UCDs with an emphasis on NAGS deficiency. An illustrative case is provided. Plasma ammonia levels should be measured in all adult patients with unexplained encephalopathy, as treatment can be potentially life-saving. Availability of N-carbamylglutamate (NCG; carglumic acid) has made protein restriction largely unnecessary in treatment regimens currently employed. Genetic counselling remains an essential component of management of NAGS.

1H MRS identifies symptomatic and asymptomatic subjects with partial ornithine transcarbamylase deficiency.

OBJECTIVE: To evaluate brain metabolism in subjects with partial ornithine transcarbamylase deficiency (OTCD) utilizing (1)H MRS. METHODS: Single-voxel (1)H MRS was performed on 25 medically-stable adults with partial OTCD, and 22 similarly aged controls. Metabolite concentrations from frontal and parietal white matter (FWM, PWM), frontal gray matter (FGM), posterior cingulate gray matter (PCGM), and thalamus (tha) were compared with controls and IQ, plasma ammonia, glutamine, and disease severity. RESULTS: Cases ranged from 19 to 59 years; average 34 years; controls ranged from 18 to 59 years; average 33 years. IQ scores were lower in cases (full scale 111 vs. 126; performance IQ 106 vs. 117). Decreased myoinositol (mI) in FWM (p=0.005), PWM (p<0.001), PCGM (p=0.003), and tha (p=0.004), identified subjects with OTCD, including asymptomatic heterozygotes. Glutamine (gln) was increased in FWM (p<0.001), PWM (p<0.001), FGM (p=0.002), and PCGM (p=0.001). Disease severity was inversely correlated with [mI] in PWM (r=-0.403; p=0.046) and directly correlated with [gln] in PCGM (r=0.548; p=0.005). N-Acetylaspartate (NAA) was elevated in PWM (p=0.002); choline was decreased in FWM (p=0.001) and tha (p=0.002). There was an inverse relationship between [mI] and [gln] in cases only. Total buffering capacity (measured by [mI/mI+gln] ratio, a measure of total osmolar capacity) was inversely correlated with disease severity in FWM (r=-0.479; p=0.018), PWM (r=-0.458; p=0.021), PCGM (r=-0.567; p=0.003), and tha (r=-0.345; p=0.037). CONCLUSION: Brain metabolism is impaired in partial OTCD. Depletion of mI and total buffering capacity are inversely correlated with disease severity, and serve as biomarkers.

Genotypic heterogeneity and clinical phenotype in triple A syndrome: a review of the NIH experience 2000-2005.

Triple A syndrome (AAAS, OMIM#231550) is an autosomal recessive condition characterized by adrenal insufficiency, achalasia, alacrima, neurodegeneration and autonomic dysfunction. Mutations in the AAAS gene on chromosome 12q13 have been reported in several subjects with AAAS. Over the last 5 years, we have evaluated six subjects with the clinical diagnosis of AAAS. Three subjects had mutations in the AAAS gene-- including one novel mutation (IVS8+1 G>A)-- and a broad spectrum of clinical presentations. However, three subjects with classic AAAS did not have mutations in the AAAS gene on both alleles. This finding supports the notion of genetic heterogeneity for this disorder, although other genetic mechanisms cannot be excluded.

Fatal presentation of ornithine transcarbamylase deficiency in a 62-year-old man and family studies.

Ornithine transcarbamylase deficiency (OTCD) resulting from deficiency of the mitochondrial enzyme OTC shows extensive phenotypic heterogeneity influenced by allelic heterogeneity and modifying environmental influences such as protein intake. We report the fatal late-onset presentation of OTCD in a 62-year-old man with the V337L mutation, a previous presentation in his grandson and negative clinical and biochemical screening of the proband's three daughters.

Clinical spectrum of succinic semialdehyde dehydrogenase deficiency.

Succinic semialdehyde dehydrogenase (SSADH) deficiency is a rare autosomal recessive disorder affecting CNS gamma-aminobutyric acid (GABA) degradation. SSADH, in conjunction with GABA transaminase, converts GABA to succinate. In the absence of SSADH, GABA is converted to 4-OH-butyrate. The presence of 4-OH-butyrate, a highly volatile compound, may be undetected on routine organic acid analysis. Urine organic acid testing was modified at the authors' institution in 1999 to screen for the excretion of 4-OH-butyrate by selective ion monitoring gas chromatography-mass spectrometry in addition to total ion chromatography. Since then, five patients with 4-hydroxybutyric aciduria have been identified. The authors add the clinical, neuroimaging, and EEG findings from a new cohort of patients to 51 patients reported in the literature with clinical details. Ages ranged from 1 to 21 years at diagnosis. Clinical findings include mild-moderate mental retardation, disproportionate language dysfunction, hypotonia, hyporeflexia, autistic behaviors, seizures, and hallucinations. Brain MRI performed in five patients at the authors' institution revealed symmetric increased T2 signal in the globus pallidi. SSADH deficiency is an under-recognized, potentially manageable neurometabolic disorder. Urine organic acid analysis should include a sensitive method for the detection of 4-hydroxybutyrate and should be obtained from patients with mental retardation or neuropsychiatric disturbance of unknown etiology.

Biological aspects of neuroblastomas identified by mass screening in Quebec.

BACKGROUND: Neuroblastoma has several characteristics that suggest that preclinical diagnosis might improve outcome. Therefore, the Quebec Neuroblastoma Screening Project was undertaken from 1989 to 1994 to examine infants at 3 weeks and 6 months by measuring urinary catecholamine metabolites. PROCEDURE: Over the 5-yr period, 45 tumors were detected by screening, 20 were identified clinically prior to the third week, and 64 were identified clinically at a later time. We analyzed available tumors for Shimada histopathology, tumor ploidy, MYCN copy number and serum ferritin. RESULTS: Of the tumors detected by screening, only 2 of 45 tested had unfavorable histology, 2 of 45 had diploid or tetraploid DNA content, 0 of 43 had MYCN amplification, and 4 of 44 had elevated serum ferritin. All of these patients are alive and well. The 20 patients detected prior to the 3-week screen had similar biological characteristics. In contrast, of the patients detected clinically after 3 weeks of age, 19 of 51 testedhad unfavorable histology, 25 of 66 had diploid or tetraploid tumors, 12 of 56 had MYCN amplification, and 14 of 54 had elevated ferritin. CONCLUSIONS: The difference between the screened and clinically detected cases was highly significant for each biological variable. Preliminary data on other biological variables, such as neurotrophin expression and allelic loss on 1 p in these patients are consistent with the above findings. These data suggest that mass screening for neuroblastoma at or before 6 months of age detects almost exclusively tumors that have favorable biological characteristics, many of which might have regressed spontaneously. Thus, continued mass screening for neuroblastoma at 6 months is unlikely to accomplish its intended goal, and should probably be discontinued.

Crystallization and preliminary X-ray crystallographic studies of wild-type human ornithine transcarbamylase and two naturally occurring mutants at position 277.

Wild-type human ornithine transcarbamylase (OTCase) and two mutants (R277Q and R277W) that cause 'late-onset' hyperammonemia were crystallized and a preliminary structure determination was carried out. The unliganded wild-type enzyme crystallizes in the cubic space group I23, with unit-cell parameters a = b = c = 203.4 A. R277Q crystallizes in two crystal forms under the same crystallization conditions. One crystal form is isomorphous to that of unliganded wild-type crystals, with unit-cell parameters a = b = c = 202.2 A. The second form also belongs to a cubic space group, P4(3)32, but has unit-cell parameters a = b = c = 139.8 A. R277W crystals are isomorphous to the second crystal form of R277Q, with unit-cell parameters a = b = c = 138.7 A. None of these crystal forms is isomorphous to other crystal forms of OTCase that have been studied. The structures in both crystal forms have been solved using molecular replacement. In the first crystal form there are two monomers in the asymmetric unit, corresponding to a solvent content of 75%. Because of its high molecular and crystal symmetry and the presence of non-crystallographic symmetry, this structure could not be solved with AMoRe or X-PLOR, but was solved successfully with COMO. There is only one monomer in the asymmetric unit in the second crystal form, corresponding to a solvent content of 62%. This structure was successfully solved with AMoRe.

Human ornithine transcarbamylase: crystallographic insights into substrate recognition and conformational changes.

Two crystal structures of human ornithine transcarbamylase (OTCase) complexed with the substrate carbamoyl phosphate (CP) have been solved. One structure, whose crystals were prepared by substituting N-phosphonacetyl-L-ornithine (PALO) liganded crystals with CP, has been refined at 2.4 A (1 A=0.1 nm) resolution to a crystallographic R factor of 18.4%. The second structure, whose crystals were prepared by co-crystallization with CP, has been refined at 2.6 A resolution to a crystallographic R factor of 20.2%. These structures provide important new insights into substrate recognition and ligand-induced conformational changes. Comparison of these structures with the structures of OTCase complexed with the bisubstrate analogue PALO or CP and L-norvaline reveals that binding of the first substrate, CP, induces a global conformational change involving relative domain movement, whereas the binding of the second substrate brings the flexible SMG loop, which is equivalent to the 240s loop in aspartate transcarbamylase, into the active site. The model reveals structural features that define the substrate specificity of the enzyme and that regulate the order of binding and release of products.

Proceedings of a consensus conference for the management of patients with urea cycle disorders.

In an effort to develop standards for the treatment of patients with urea cycle disorders, a consensus conference was held in Washington, DC, from April 27-29, 2000. Conference participants included physicians, scientists, nurses, dieticians, and a genetic counselor, all experts in their various medical fields in these diseases. Representatives from the Food and Drug Administration and the National Urea Cycle Disorders Foundation, a parents support group, also participated in the conference. The goals set forth for the conference were to (1) reach a consensus on diagnostic and therapeutic guidelines for urea cycle disorders with the most up-to-date information and the experience of experts in the field, (2) establish a collaborative network of health care professionals to advance the cause of patients with urea cycle disorders in the areas of clinical management and research, and (3) provide help to health care providers in the recognition and management of these complex disorders by publishing the proceedings of the conference in a widely read journal. The articles that follow this introduction represent the current state of knowledge on the topics addressed in the conference and a summary of the discussions that followed each of the presentations. With input from all the participants, we tried to cover those topics that were believed to be the most relevant both to the experts and to patients. As the reader will appreciate, many unresolved and controversial issues pertaining to treatment have yet to be studied by rigorous scientific methods. On the other hand, there are many issues on which the panel agreed. In many instances the availability of reliable information on the respective topics determined whether consensus could be reached.

Alternative pathway therapy for urea cycle disorders: twenty years later.

Alternative pathway therapy is currently an accepted treatment approach for inborn errors of the urea cycle. This involves the long-term use of oral sodium phenylbutyrate, arginine supplements, or both, depending on the specific enzyme deficiency, and treatment of acute hyperammonemic crises with intravenous sodium benzoate/sodium phenylacetate plus arginine. A review of 20 years of experience with this approach illustrates the strengths and limitations of this treatment. It has clearly decreased the mortality and morbidity from these disorders, but they remain unacceptably high. The medications are generally well tolerated, but severe accidental overdosage has been reported because of the infrequent use of the medication. There is also a difference in their metabolism between newborns and older children that must be addressed in determining dosage. To avoid these complications it is recommended that drug levels in blood be monitored routinely and that very specific treatment protocols and oversight be followed to avoid overdoses. Finally, it must be acknowledged that alternative pathway therapy has limited effectiveness in preventing hyperammonemia and must be combined with effective dietary management. Therefore in children with neonatal-onset disease or in those with very poor metabolic control, liver transplantation should be considered. There should also be the continued search for innovative therapies that may offer a more permanent and complete correction, such as gene therapy.

The clinically variable R40H mutant ornithine carbamoyltransferase shows cytosolic degradation of the precursor protein in CHO cells.

Ornithine carbamoyltransferase (OCT) deficiency is now frequently found in adults with hyperammonaemia affected by mutations that cause partial deficiency of this urea cycle enzyme. One of these mutations (R40H) has occurred in several families and has been found also in asymptomatic relatives. To better understand the phenotypic heterogeneity of this recurrent mutation, we investigated the biological properties of the mutant protein. Using 35S labelling, the import and processing of the R40H mutant OCT protein was investigated in intact CHO cells and in isolated rat liver mitochondria and compared to the wild type and R141Q mutant that causes complete enzyme deficiency. The R40H OCT protein seems to be imported and processed by the mitochondria in a manner similar to that of wild type. However, it is consistently degraded to a smaller fragment in the intact cells, unlike the wild type and R141Q mutant. The mature form of the enzyme is not susceptible to degradation. These data, obtained in CHO cells, suggest that deficiency in OCT enzymatic function conferred by the R40H mutation is likely caused by enhanced degradation of the preprotein in the cytosol. We propose therefore that variation in the rate of OCT turnover is responsible for the heterogeneity of the clinical phenotype in these patients.

Expression of wild-type and mutant human ornithine transcarbamylase genes in Chinese hamster ovary cells and lack of dominant negative effect of R141Q and R40H mutants.

Chinese hamster ovary cultured cells were transformed to continuously express wild-type and two mutant ornithine transcarbamylase genes, R141Q and R40H. In addition, these cells were transfected to transiently express the same genes. The R141Q mutation abolishes the enzymatic activity, and the amount of "mature" protein present in transfected cells is equivalent to the wild type. The R40H mutation causes a reduction of enzymatic activity to approximately 26 to 35% of wild type concomitant with a significant reduction in the amount of protein present. Transfection with wild-type and mutant genes together in various proportions did not reveal dominant negative effects of the two mutations studied. This expression system can be used to examine the deleterious effect of private mutations or lack thereof in families with ornithine transcarbamylase deficiency as well as evaluate the potential dominant negative effects of gene delivery for treatment of ornithine transcarbamylase deficiency.

Genotype spectrum of ornithine transcarbamylase deficiency: correlation with the clinical and biochemical phenotype.

Ornithine transcarbamylase (OTC) deficiency, a partially dominant X-linked disorder, is the most common inherited defect of the urea cycle. Previous reports suggested a variable phenotypic spectrum, and several studies documented different "private" mutations in the OTC genes of patients. Our laboratory identified disease-causing mutations in 157 families with OTC deficiency, 100 of which came to medical attention through a hemizygous propositus and in 57 the index case was a heterozygous female. We correlated the genotype with age of onset, liver OTC activity, incorporation of nitrogen into urea, and peak plasma ammonia levels. The "neonatal onset" group has a homogeneous clinical and biochemical phenotype, whereas the "late onset" group shows an extremely wide phenotype; 60% of the mutations are associated exclusively with acute neonatal hyperammonemic coma. The remaining mutations caused a nonuniform phenotype ranging from severe disease to no symptoms; 31% of the mutations in the OTC gene occur in CpG dinucleotides (methylation-mediated deamination), and none of them accounted for more than 4% of the total. Eighty-six percent of the mutations represented single-base substitutions and 68% of the substitutions were transitions. G-to-A and C-to-T transitions were the most frequent substitutions (34 and 21%, respectively) whereas C-to-A, A-to-C, C-to-G, and T-to-A transversions were the least common (1.5-3%). Twenty percent of propositi and 77% of propositae carried new mutations. Forty percent of female germinal mutations were in CpG dinucleotides whereas this number appears much smaller in male germinal mutations. These data allow classification of patients with OTC deficiency into at least two groups who have discordant disease course and prognoses. In addition, they improve our understanding on the origin of mutations in the OTC gene and allow better counseling of affected families.

Crystal structure of human ornithine transcarbamylase complexed with carbamoyl phosphate and L-norvaline at 1.9 A resolution.

The crystal structure of human ornithine transcarbamylase (OTCase) complexed with carbamoyl phosphate (CP) and L-norvaline (NOR) has been determined to 1.9-A resolution. There are significant differences in the interactions of CP with the protein, compared with the interactions of the CP moiety of the bisubstrate analogue N-(phosphonoacetyl)-L-ornithine (PALO). The carbonyl plane of CP rotates about 60 degrees compared with the equivalent plane in PALO complexed with OTCase. This positions the side chain of NOR optimally to interact with the carbonyl carbon of CP. The mixed-anhydride oxygen of CP, which is analogous to the methylene group in PALO, interacts with the guanidinium group of Arg-92; the primary carbamoyl nitrogen interacts with the main-chain carbonyl oxygens of Cys-303 and Leu-304, the side chain carbonyl oxygen of Gln-171, and the side chain of Arg-330. The residues that interact with NOR are similar to the residues that interact with the ornithine (ORN) moiety of PALO. The side chain of NOR is well defined and close to the side chain of Cys-303 with the side chains of Leu-163, Leu-200, Met-268, and Pro-305 forming a hydrophobic wall. C-delta of NOR is close to the carbonyl oxygen of Leu-304 (3.56 A), S-gamma atom of Cys-303 (4.19 A), and carbonyl carbon of CP (3.28 A). Even though the N-epsilon atom of ornithine is absent in this structure, the side chain of NOR is positioned to enable the N-epsilon of ornithine to donate a hydrogen to the S-gamma atom of Cys-303 along the reaction pathway. Binding of CP and NOR promotes domain closure to the same degree as PALO, and the active site structure of CP-NOR-enzyme complex is similar to that of the PALO-enzyme complex. The structures of the active sites in the complexes of aspartate transcarbamylase (ATCase) with various substrates or inhibitors are similar to this OTCase structure, consistent with their common evolutionary origin.

Fatal hyperammonemia after orthotopic lung transplantation.

BACKGROUND: A case of fatal hyperammonemia complicating orthotopic lung transplantation was previously reported. OBJECTIVE: To describe the incidence, clinical features, and treatment of hyperammonemia associated with orthotopic lung transplantation. DESIGN: Retrospective cohort analysis. SETTING: Academic medical center and lung transplantation center in Philadelphia, Pennsylvania. PATIENTS: 145 sequential adult patients who underwent orthotopic lung transplantation. MEASUREMENTS: Plasma ammonium levels. RESULTS: Six of the 145 patients who had had orthotopic lung transplantation developed hyperammonemia, all within the first 26 days after transplantation. The 30-day post-transplantation mortality rate was 67% for patients with hyperammonemia compared with 17% for those without hyperammonemia (P = 0.01). Development of major gastrointestinal complications (P = 0.03), use of total parenteral nutrition (P < 0.001), and lung transplantation for primary pulmonary hypertension (P = 0.045) were associated with hyperammonemia. CONCLUSIONS: Hyperammonemia is a potentially fatal event occurring after orthotopic lung transplantation. It is associated with high nitrogen load, concurrent medical stressors, primary pulmonary hypertension, and hepatic glutamine synthetase deficiency.