Human carbamoyl phosphate synthetase (CPS1), a 1500-residue multidomain enzyme, catalyzes the first step of ammonia detoxification to urea requiring N-acetyl-L-glutamate (NAG) as essential activator to prevent ammonia/amino acids depletion. Here we present the crystal structures of CPS1 in the absence and in the presence of NAG, clarifying the on/off-switching of the urea cycle by NAG. By binding at the C-terminal domain of CPS1, NAG triggers long-range conformational changes affecting the two distant phosphorylation domains. These changes, concerted with the binding of nucleotides, result in a dramatic remodeling that stabilizes the catalytically competent conformation and the building of the ~35 Å-long tunnel that allows migration of the carbamate intermediate from its site of formation to the second phosphorylation site, where carbamoyl phosphate is produced. These structures allow rationalizing the effects of mutations found in patients with CPS1 deficiency (presenting hyperammonemia, mental retardation and even death), as exemplified here for some mutations.

The urea cycle disease carbamoyl-phosphate synthetase deficiency (CPS1D) has been associated with many mutations in the CPS1 gene [Häberle et al., 2011. Hum Mutat 32:579-589]. The disease-causing potential of most of these mutations is unclear. To test the mutations effects, we have developed a system for recombinant expression, mutagenesis, and purification of human carbamoyl-phosphate synthetase 1 (CPS1), a very large, complex, and fastidious enzyme. The kinetic and molecular properties of recombinant CPS1 are essentially the same as for natural human CPS1. Glycerol partially replaces the essential activator N-acetyl-l-glutamate (NAG), opening possibilities for treating CPS1D due to NAG site defects. The value of our expression system for elucidating the effects of mutations is demonstrated with eight clinical CPS1 mutations. Five of these mutations decreased enzyme stability, two mutations drastically hampered catalysis, and one vastly impaired NAG activation. In contrast, the polymorphisms p.Thr344Ala and p.Gly1376Ser had no detectable effects. Site-limited proteolysis proved the correctness of the working model for the human CPS1 domain architecture generally used for rationalizing the mutations effects. NAG and its analogue and orphan drug N-carbamoyl-l-glutamate, protected human CPS1 against proteolytic and thermal inactivation in the presence of MgATP, raising hopes of treating CPS1D by chemical chaperoning with N-carbamoyl-l-glutamate.

The first patients affected by argininosuccinic aciduria (ASA) were reported 60 years ago. The clinical presentation was initially described as similar to other urea cycle defects, but increasing evidence has shown overtime an atypical systemic phenotype with a paradoxical observation, that is, a higher rate of neurological complications contrasting with a lower rate of hyperammonaemic episodes. The disappointing long-term clinical outcomes of many of the patients have challenged the current standard of care and therapeutic strategy, which aims to normalize plasma ammonia and arginine levels. Interrogations have raised about the benefit of newborn screening or liver transplantation on the neurological phenotype. Over the last decade, novel discoveries enabled by the generation of new transgenic argininosuccinate lyase (ASL)-deficient mouse models have been achieved, such as, a better understanding of ASL and its close interaction with nitric oxide metabolism, ASL physiological role outside the liver, and the pathophysiological role of oxidative/nitrosative stress or excessive arginine treatment. Here, we present a collaborative review, which highlights these recent discoveries and novel emerging concepts about ASL role in human physiology, ASA clinical phenotype and geographic prevalence, limits of current standard of care and newborn screening, pathophysiology of the disease, and emerging novel therapies. We propose recommendations for monitoring of ASA patients. Ongoing research aims to better understand the underlying pathogenic mechanisms of the systemic disease to design novel therapies.Carmen Diez-Fernandez and Shaul Lerner contributed equally to this study.

Thioacetamide proved to be a potent necrogenic agent when a single dose of 6.6 mmol/kg was administered intraperitoneally to rats. Its necrogenic ability was assessed on the basis of morphological and biochemical changes. The injury of centrilobular hepatocytes showed a peak of cell death 24 hr after thioacetamide administration; it was followed immediately by the regenerative response. Parallel increases of serum aminotransferases, isocitrate dehydrogenase and gamma-glutamyl transferase activities were observed. Severe liver damage was also evident at 24 hr on the basis of glutathione depletion (29% of control), malondialdehyde production (169%), cytochrome P-450 level decrease (26%) and increased activity of glutathione S-transferase (160%). We checked the regenerative response by determining nuclear DNA content in isolated hepatocytes 0, 6, 12, 18, 24, 36, 48 and 72 hr after thioacetamide administration. Changes in DNA cell distribution between G0-G1, S and G2 + M phases of the cell cycle were observed. The sharp decrease in the percentage of the tetraploid cell population (G2 + M phases) and the abrupt increase of the S-phase cells at 36 and 48 hr suggest transition from adult to fetal in hepatocyte populations obtained 24 and 36 hr after thioacetamide treatment. At 72 hr of treatment, hepatocyte populations showed recovery to adult state. In the shift from the adult to fetal, registered at 24, 36 and 48 hr after thioacetamide administration, mitosis seemed to precede the synthesis of DNA.

The objective of this work was to characterize the adaptation of cardiac metabolism to a lipid overload in a model of diet-induced obesity (DIO) in mice. After 8 wk dietary treatment, mice receiving a high-fat diet exhibited an increase in the amount of adipose tissue, accompanied by a surge in plasma leptin concentration (from 5.4-16.0 ng/ml). This was associated with: 1) an induction of uncoupling protein-2 (120%), 2) an increase in the phosphorylated form of AMP-activated protein kinase (120%), and 3) a reduction in lactate concentration and lactate dehydrogenase activity in myocardial tissue (40%). Because DIO induces leptin resistance, we analyzed leptin receptor functionality by measuring phospho-signal transducer and activator of transcription 3 in response to acute leptin (1 mg/kg). We observed that leptin receptor signaling remained unaltered within the heart but was fully impaired within the hypothalamus. Taken together, these data show that during DIO development, there is a metabolic shift in the heart aimed at increasing fatty acid oxidation to the detriment of carbohydrates. This effect seems to be leptin-dependent, suggesting that the increased adiposity observed during the onset of obesity might contribute to impairing ectopic lipidic deposition in the heart.

Citrullinemia type 1 is an autosomal recessive urea cycle disorder caused by defects in the argininosuccinate synthetase (ASS) enzyme due to mutations in ASS1 gene. An impairment of ASS function can lead to a wide spectrum of phenotypes, from life-threatening neonatal hyperammonemia to a later onset with mild symptoms, and even some asymptomatic patients exhibiting an only biochemical phenotype. The disease is panethnic. In this update, we report 137 mutations (64 of which are novel), consisting of 89 missense mutations, 19 nonsense mutations, 17 mutations that affect splicing, and 12 deletions. The change p.Gly390Arg is by far the most common mutation and is widely spread throughout the world. Other frequent mutations (p.Arg157His, p.Trp179Arg, p.Val263Met, p.Arg304Trp, p.Gly324Ser, p.Gly362Val, and p.Arg363Trp), each found in at least 12 independent families, are mainly carried by patients from the Indian subcontinent, Turkey, Germany, and Japan. To better understand the disease, we collected clinical data of >360 patients, including all published information available. This information is related to the patients' genetic background, the conservation of the mutated residues and a structural rationalization of the effect of the most frequent mutations. In addition, we review ASS regulation, animal models, diagnostic strategies, newborn screening, and treatment options.

The urea cycle disorder argininemia is caused by a defective arginase 1 (ARG1) enzyme resulting from mutations in the ARG1 gene. Patients generally develop hyperargininemia, spastic paraparesis, progressive neurological and intellectual impairment, and persistent growth retardation. Interestingly, in contrast to other urea cycle disorders, hyperammonemia is rare. We report here 66 mutations (12 of which are novel), including 30 missense mutations, seven nonsense, 10 splicing, 15 deletions, two duplications, one small insertion, and one translation initiation codon mutation. For the most common mutations (p.Thr134Ile, p.Gly235Arg and p.Arg21*), which cluster geographically in Brazil, China, or Turkey, a structural rationalization of their effect has been included. In order to gain more knowledge on the disease, we have collected clinical and biochemical information of 112 patients, including the patients' genetic background and ethnic origin. We have listed as well the missense variants with unknown relevance. For all missense variants (of both known and unknown relevance), the conservation, severity prediction, and ExAc scores have been included. Lastly, we review ARG1 regulation, animal models, diagnostic strategies, newborn screening, prenatal testing, and treatment options.

The influence of aging on the mechanisms of liver injury and regeneration was studied in a model of hepatotoxicity induced in 2-, 6-, 12-, 18- and 30-month-old rats by a sublethal dose of thioacetamide (500 mg/kg body weight), a soft nucleophilic and hepatotoxic compound metabolized by the hepatic microsomal FAD monooxygenase system. Samples-blood and hepatocytes-were obtained at 0, 12, 24, 48, 72 and 96 h following thioacetamide intoxication. Parameters of liver injury in serum (NADPH-isocitrate dehydrogenase (ICDH) activity) indicate that the severity of injury was significantly higher in the adult groups (6 and 12 months old) when compared either with the youngest (2 months old) or oldest (18 and 30 months old) groups. Parameters related to biotransformation, such as microsomal FAD monooxygenase, followed mainly the same pattern of age-dependent changes as those observed for injury. The profile of glutathione-S-transferase activity showed an initial induction parallel to liver injury and opposite to the levels of reduced glutathione and protein -SH groups. Enzyme activities and gene expression of the systems involved in the cell endogenous antioxidant defense, such as Mn- and Cu,Zn-superoxide dismutases (SOD), catalase and glutathione peroxidase (GPX) showed significant age-dependent changes that can be summarized as follows: an increase in all enzyme activities and gene expression and a decreased ability to restore the initial activities following 96 h of thioacetamide. We conclude, first, that the gene expression and activity of the enzymes involved in the intracellular antioxidant defense system increased with aging, which can be considered a consequence of the enhanced oxidative state of the cell (decreased in GSH level); and second, that the lower and delayed response in the aged groups significantly influenced the restoration towards normal of GSH and the antioxidant enzyme activities.