Arginase deficiency in a 12-year-old boy with mild impairment of intellectual function

Clinical features and neurologic progression of hyperargininemia

Hyperargininemia is an autosomal recessive metabolic disorder caused by a deficiency of enzyme arginase I. It is a rare pan-ethnic disease with a clinical presentation distinct from that of other urea cycle disorders, and hyperammonemic encephalopathy is not usually observed. Hyperargininemia is one of the few treatable causes of pediatric spastic paraparesis, and can be confused with cerebral palsy. We retrospectively evaluated the clinical onset, neurologic manifestations, progression of abnormalities, electroencephalographic abnormalities, and laboratory findings of 16 Brazilian patients with hyperargininemia. Relevant data about the clinical spectrum and natural history of hyperargininemia are detailed. Progressive spastic diplegia constituted the key clinical abnormality in this group, but variability in clinical presentation and progression were evident in our series. Seizures in hyperargininemia may be more common than reported in previous studies. Features distinguishing hyperargininemia from cerebral palsy and hereditary spastic paraplegia are emphasized in this large series of patients.

Identification of two arginases generated by alternative splicing in the silkworm, Bombyx mori

Arginase (EC 3.5.3.1) catalyzes the hydrolysis of arginine to ornithine and urea. Here, we have cloned two arginase cDNAs from the silkworm, Bombyx mori. The analysis of exon/intron structures showed that the two mRNAs named bmarg-r and bmarg-f were generated from a single gene by alternative usage of exons. The bmarg-r and bmarg-f were predicted to encode almost the same amino acid sequences, except that the latter had additional ten N-terminal residues. Recombinant bmARG-r and bmARG-f in Escherichia coli cell lysates were roughly similar to each other in enzymatic characteristics, which did not show large difference from those of arginases assayed by using tissue extracts. Differential RT-PCR experiments and tissue distribution analyses of arginase activity indicated that the bmarg-r gene is expressed in the male reproductive organs, especially in the glandula lacteola and vesicular seminalis, from which it is secreted to the seminal fluid and transferred to the female during copulation, whereas the bmarg-f gene is expressed in the larval and adult nonreproductive organs including the fat body and muscle, where the produced arginase proteins are considered to stay in the cells. Thus, the two silkworm arginase isoforms may have a difference in whether or not the product is excreted out of the cells in which it is synthesized.

Hyperargininemia due to liver arginase deficiency

The urea cycle is a series of six reactions necessary to rid the body of the nitrogen generated by the metabolism, primarily of amino acids, from the diet or released as the result of endogenous protein catabolism. Arginase is the sixth and final enzyme of this cycle. Arginase catalyzes the conversion of arginine to urea and ornithine, the latter recycled to continue the cycle. Hyperargininemia due to arginase deficiency is inherited in an autosomal recessive manner and gene for arginase, designated AI, has been cloned. Unlike the other urea cycle enzymes, a second gene encoding arginase, with similar structural properties and enzyme characteristics, exists and has been named Arginase II (AII). Comprehensive histories and physical examinations confirm a strikingly uniform clinical picture and one notably different from patients with other urea cycle disorders. This condition rarely presents in the neonatal period and first symptoms typically present in children between 2 and 4 years of age. First symptoms are often neurologically based. If untreated, symptoms are progressive with a gradual loss of developmental milestones. With adherence to a dietary and drug regimen, a favorable outcome can be expected, with cessation of further neurological deterioration and in some instances, of improvement. This article summarizes the clinical course of selected patients who represent the full spectrum of presentations of arginase deficiency. In addition to the clinical characterization of this disorder; the biochemical, enzymatic, and molecular evidence of disease is summarized. Treatment and prenatal diagnosis are also discussed.

Functional consequences of the G235R mutation in liver arginase leading to hyperargininemia

Hyperargininemia is a rare autosomal disorder that results from a deficiency in hepatic type I arginase. This deficiency is the consequence of random point mutations that occur throughout the gene. The G235R patient mutation has been proposed to affect the catalytic activity and structural integrity of the protein [D. E. Ash, L. R. Scolnick, Z. F. Kanyo, J. G. Vockley, S. D. Cederbaum, and D. W. Christianson (1998) Mol. Genet. Metab. 64, 243-249]. The G235R (patient) and G235A (control) arginase mutants of rat liver arginase have been generated to probe the effects of these point mutations on the structure and function of hepatic type I arginase. Both mutant arginases were trimeric by gel filtration, but the control G235A mutant had 56% of wild-type activity and the G235R mutant had less than 0.03% activity compared to the wild-type enzyme. The G235R mutant contained undetectable levels of tightly bound manganese as determined by electron paramagnetic resonance, while the G235A mutant had a Mn(II) stoichiometry of 2 Mn/subunit. Molecular modeling indicates that the introduction of an arginine residue at position 235 results in a major rearrangement of the metal ligands that compromise Mn(II) binding.

Expression of arginase isozymes in mouse brain

The two forms of arginase (AI and AII) in man, identical in enzymatic function, are encoded in separate genes and are expressed differentially in various tissues. AI is expressed predominantly in the liver cytosol and is thought to function primarily to detoxify ammonia as part of the urea cycle. AII, in contrast, is predominantly mitochondrial, is more widely expressed, and is thought to function primarily to produce ornithine. Ornithine is a precursor in the synthesis of proline, glutamate, and polyamines. This study was undertaken to explore the cellular and regional distribution of AI and AII expression in brain using in situ hybridization and immunohistochemistry. AI and AII were detected only in neurons and not in glial cells. AI presented stronger expression than AII, but AII was generally coexpressed with AI in most cells studied. Expression was particularly high in the cerebral cortex, cerebellum, pons, medulla, and spinal cord neurons. Glutamic acid decarboxylase 65 and glutamic acid decarboxylase 67, postulated to be related to the risk of glutamate excitotoxic and/or gamma-aminobutyric acid inhibitoxic injury, were similarly ubiquitous in their expression and generally paralleled arginase expression patterns, especially in cerebral cortex, hippocampus, cerebellum, pons, medulla, and spinal cord. This study showed that AI is expressed in the mouse brain, and more strongly than AII, and sheds light on the anatomic basis for the arginine-->ornithine-->glutamate-->GABA pathway.

Subunit-subunit interactions in trimeric arginase. Generation of active monomers by mutation of a single amino acid

The structure of the trimeric, manganese metalloenzyme, rat liver arginase, has been previously determined at 2.1-A resolution (Kanyo, Z. F., Scolnick, L. R., Ash, D. E., and Christianson, D. W., (1996) Nature 383, 554-557). A key feature of this structure is a novel S-shaped oligomerization motif at the carboxyl terminus of the protein that mediates approximately 54% of the intermonomer contacts. Arg-308, located within this oligomerization motif, nucleates a series of intramonomer and intermonomer salt links. In contrast to the trimeric wild-type enzyme, the R308A, R308E, and R308K variants of arginase exist as monomeric species, as determined by gel filtration and analytical ultracentrifugation, indicating that mutation of Arg-308 shifts the equilibrium for trimer dissociation by at least a factor of 10(5). These monomeric arginase variants are catalytically active, with k(cat)/K(m) values that are 13-17% of the value for wild-type enzyme. The arginase variants are characterized by decreased temperature stability relative to the wild-type enzyme. Differential scanning calorimetry shows that the midpoint temperature for unfolding of the Arg-308 variants is in the range of 63.6-65.5 degrees C, while the corresponding value for the wild-type enzyme is 70 degrees C. The three-dimensional structure of the R308K variant has been determined at 3-A resolution. At the high protein concentrations utilized in the crystallizations, this variant exists as a trimer, but weakened salt link interactions are observed for Lys-308.

Laboratory evaluation of urea cycle disorders

The urea cycle disorders (UCDs) represent a group of inherited metabolic diseases with hyperammonemia as the primary laboratory abnormality. Affected individuals may become comatose or die if not treated rapidly. Diagnosis of a UCD requires a high index of suspicion and judicious use of the laboratory. It is important to rule out other conditions causing hyperammonemia that may require different treatment. The astute clinician may suspect a specific UCD in the appropriate clinical setting, but only laboratory results can confirm a specific diagnosis. The importance of the laboratory in helping the clinician to differentiate among various causes of hyperammonemia, in confirming a specific UCD, in carrier testing, and in prenatal diagnostic testing is highlighted in this review.

Molecular basis of hyperargininemia: structure-function consequences of mutations in human liver arginase

Hyperargininemia is a rare autosomal recessive disorder that results from a deficiency of hepatic type I arginase. At the genetic level, this deficiency in arginase activity is a consequence of random point mutations throughout the gene that lead to premature termination of the protein or to substitution mutations. Given the high degree of sequence homology between human liver and rat liver enzymes, we have mapped both patient and nonpatient mutations of the human enzyme onto the structure of the rat liver enzyme to rationalize the molecular basis for the low activities of these mutant arginases. Mutations identified in hyperargininemia patients affect the structure and function of the enzyme by compromising active-site residues, packing interactions in the protein scaffolding, and/or quaternary structure by destabilizing the assembly of the arginase trimer.

Cloning and characterization of the mouse and rat type II arginase genes

Two forms of arginase, both catalyzing the hydrolysis of arginine to ornithine and urea, are found in animals ranging from amphibians to mammals. In humans, inherited deficiency of hepatic or type I arginase results in hyperargininemia, a syndrome characterized by periodic episodes of hyperammonemia, spasticity, and neurological deterioration. In these patients, a second extrahepatic or type II arginase activity is significantly increased, an induction that may partially compensate for the lack of AI activity and apparently mitigates some of the clinical effects of the condition. Cloning and characterization of the human AII cDNA was recently accomplished. The cloning, sequencing, and partial characterization of the mouse and rat AII cDNAs are reported herein. The DNA sequences predicted polypeptides of 354 amino acids, including a N-terminal mitochondrial import signal. Sequence homology to the human type II arginase, arginase activity data, and immunoprecipitation with an anti-AII antibody confirm the identity of these cloned genes as rodent extrahepatic type II arginases.

Argininemia: a treatable genetic cause of progressive spastic diplegia simulating cerebral palsy: case reports and literature review

Argininemia, a rare autosomal recessive urea cycle disorder, is caused by a deficiency of arginase, with resulting elevated plasma arginine and ammonia levels. Reports to date have focused little on the neurology of this disorder or the efficacy of treatments. A MEDLINE search revealed 25 previously reported cases, to which we have added two brothers who presented with late onset progressive spastic diplegia. Though their degree of enzyme deficiency was comparable, the severity of their phenotypic abnormalities differed substantially. With dietary therapy, both showed improved cognitive and motor function. Late metabolic crises occurred in both, resulting in death of the less severely affected brother. Based on analysis of our clinical database, we report on the full spectrum of neurologic abnormalities seen in argininemia with particular focus on the accompanying progressive spastic diplegia and its response to treatment; progressive decline in head growth; distinctive neuroradiologic findings; and life-threatening later complications. Current and potential future therapies and long-term outcome are summarized.

Comparative properties of arginases

Arginase is a primordial enzyme, widely distributed in the biosphere and represented in all primary kingdoms. It plays a critical role in the hepatic metabolism of most higher organisms as a cardinal component of the urea cycle. Additionally, it occurs in numerous organisms and tissues where there is no functioning urea cycle. Many extrahepatic tissues have been shown to contain a second form of arginase, closely related to the hepatic enzyme but encoded by a distinct gene or genes and involved in a host of physiological roles. A variety of functions has been proposed for the "extrahepatic" arginases over the last three decades. In recent years, interest in arginase has been stimulated by a demonstrated involvement in the metabolism of the ubiquitous and multifaceted molecule nitric oxide. Molecular biology has begun to furnish new clues to the disparate functions of arginases in different environments and organisms. Comparative studies of arginase sequences are also beginning to elucidate the comparative evolution of arginases, their molecular structures and the nature of their catalytic mechanism. Further studies have sought to clarify the involvement of arginase in human disease. This review presents an outline of the current state of arginase research by giving a comparative overview of arginases and their associated properties.

Plasma arginase concentration measured by an enzyme-linked immunosorbent assay (ELISA) in normal adult population

Human liver arginase has many biological effects on lymphocytes, macrophages, liver cells, and tumor cells, in addition to its major role in the liver urea cycle. We have developed a sandwich enzyme-linked immunosorbent assay (ELISA) method to quantitate arginase concentrations in plasma that can be applied to various body fluids. The sensitivity was 2.5 ng/mL. The coefficients of variation were good both in intra- and inter-assay. Using this method to study the stability of an arginase preparation, we found that plasma arginase was stable for only 1 or 2 days even at temperatures as low as 4 degrees C. The mean plasma level was 41.0 +/- 3.3 ng/mL (mean +/- SE) in 143 normal subjects. There was no age difference in the general population and in the male group. However, in the female group, the plasma arginase level increased with age (p = 0.05). Its biological significance was unclear. As a whole, the ELISA method for the measurement of arginase concentration in the body fluid is convenient and reliable.

Absence of erythrocyte arginase protein in Japanese patients with hyperargininemia

In Japan, hyperargininemia has been reported in only 5 unrelated families and four patients are alive at present. In this study we examined arginase protein in erythrocytes of these Japanese patients using two analytical methods of immunoblotting and two-dimensional gel electrophoresis. Immunoblotting study with anti-E. coli-expressed human liver arginase rabbit IgG revealed lack of cross-reacting materials in the erythrocyte lysates from these patients. On two-dimensional gels, arginase protein was detected in any control subject, but it was completely absent in all the patients studied. These results suggest that either arginase protein in erythrocytes is not produced or it is structurally labile in these patients.

Effect of an adjacent base on detection of a point mutation by restriction enzyme digestion

While routinely mapping point mutations within the arginase locus of a collection of hyperargininemic patients, we discovered that a base immediately outside a restriction endonuclease recognition site (TaqI) can eliminate cleavage of this site by this enzyme. The genetic lesion lay in a base immediately flanking a TaqI recognition site within exon 8 of the arginase locus and abolished cutting by approximately 80%. We wish to emphasize the necessity of heeding subtle cues frequently encountered while generating restriction enzyme data, because neither Southern blot maps nor endonuclease digestion of polymerase chain reaction amplified products of exon 8 accurately predicted where the point mutation lay. To our knowledge, this is the first instance of inhibition of cleavage by flanking bases occurring on natural (nonsynthetic) DNA substrates, i.e., within the clinical setting of characterization of a human genetic disorder.

Guanidino compounds that are increased in hyperargininemia inhibit GABA and glycine responses on mouse neurons in cell culture

The effects of arginine, homoarginine, alpha-keto-delta-guanidinovaleric acid and argininic acid (guanidino compounds that were found to be increased in hyperargininemia) were evaluated on responses to gamma-aminoburtyric acid (GABA) and glycine (Gly) on mouse neurons in primary dissociated cell culture. GABA and Gly were applied iontophoretically and intracellular microelectrode recording techniques were used. The guanidino compounds rapidly and reversibly inhibited both GABA and Gly responses. The guanidino compounds inhibited GABA responses in a concentration-dependent manner and inhibited Gly responses at a concentration of 10 mM. Argininic acid was the most potent in reducing inhibitory amino acid responses, followed in decreasing potency by alpha-keto-delta-guanidinovaleric acid, homoarginine and arginine. The guanidino compounds were equally potent in decreasing Gly and GABA responses. Co-application of CGS 9896, a benzodiazepine receptor antagonist, did not antagonize the guanidino compound-induced inhibition of GABA responses. These findings suggest that the guanidino compounds inhibited responses to the inhibitory neurotransmitters GABA and Gly by blocking the chloride channel. This effect might underlie the in vivo epileptogenicity of some of the guanidino compounds and might contribute to the pathogenesis of seizures in hyperargininemia.

Expression of human liver arginase in Escherichia coli. Purification and properties of the product

Arginase is an enzyme that catalyses the hydrolysis of arginine to urea and ornithine. It is abundantly present in the liver of ureotelic animals (i.e. those whose excretion is characterized by the excretion of uric acid as the chief end-product of nitrogen metabolism), but its purification has hitherto not been simple, and the yield not high. Starting with a partially truncated cDNA for human liver arginase recently made available, we constructed an expression plasmid that had tandemly linked tac promotors placed upstream of a full-length cDNA. By selecting Escherichia coli strain KY1436 as the host micro-organism, we established an efficient system for the production of human liver arginase protein. Chromatographies on CM-Sephadex G-150, DEAE-cellulose and Sephadex G-150, followed by preparative agar-gel electrophoresis, yielded 10 mg of apparently homogeneous enzyme protein from 1 g (wet wt.) of E. coli cells. E. coli-expressed human liver arginase had chemical, immunological and most catalytic properties indistinguishable from those of purified human erythrocyte arginase. However, E. coli-expressed arginase was a monomer of Mr 35,000, whereas the purified erythrocyte arginase was trimer of Mr 105,000. They differed also in pH- and temperature-stabilities. Gel-filtration experiments with these two purified arginases under various conditions, as well as with unfractionated human liver and erythrocyte cytosol preparations, indicated that the native form of human arginase should be of Mr 35,000, and that the trimeric appearance of human erythrocyte arginase after purification was an artifact of the purification procedures. It was thus concluded that, in Nature, the liver and erythrocyte arginases are identical proteins.

Molecular basis of argininemia. Identification of two discrete frame-shift deletions in the liver-type arginase gene

Argininemia results from a deficiency of arginase (EC 3.5.3.1), the last enzyme of the urea cycle in the liver. We examined the molecular basis for argininemia by constructing a genomic library followed by cloning and DNA sequencing. Discrete mutations were found on two alleles from the patient, a product of a nonconsanguineous marriage. There was a four-base deletion at protein-coding region 262-265 or 263-266 in exon 3 that would lead to a reading-frame shift after amino acid residue 87 and make a new stop codon at residue 132. The other was a one-base deletion at 77 or 78 in exon 2 that would lead to a reading-frame shift after residue 26 and make a stop codon at residue 31. For confirmation, genomic DNAs from the patient and from her parents were amplified by the polymerase chain reaction method. The patient was shown to be a compound heterozygote, inheriting an allele with the four-base deletion from the father and the other allele with the one-base deletion from the mother. These data seem to be the first evidence of a case of argininemia caused by two different deletion mutations.

Differential expression of the two human arginase genes in hyperargininemia. Enzymatic, pathologic, and molecular analysis

Previous studies in our laboratory and others have demonstrated in humans and other mammals two isozymes of arginase (AI and AII) that differ both electrophoretically and antigenically. AI, a cytosolic protein found predominantly in liver and red blood cells, is believed to be chiefly responsible for ureagenesis and is the one missing in hyperargininemic patients. Much less is known about AII because it is present in far smaller amounts and localized in less accessible deep tissues, primarily kidney. We now report the application of enzymatic and immunologic methods to assess the independent expression and regulation of these two gene products in normal tissue extracts, two cultured cell lines, and multiple organ samples from a hyperargininemic patient who came to autopsy after an unusually severe clinical course characterized by rapidly progressive hepatic cirrhosis. AI was totally absent (less than 0.1%) in the patient's tissues, whereas marked enhancement of AII activity (four times normal) was seen in the kidney by immunoprecipitation and biochemical inhibition studies. Immunoprecipitation-competition and Western blot analysis failed to reveal presence of even an enzymatically inactive cross-reacting AI protein, whereas Southern blot analysis showed no evidence of a substantial deletion in the AI gene. Induction studies in cell lines that similarly express only the AII isozyme indicated that its activity could be enhanced severalfold by exposure to elevated arginine levels. Our findings suggest that the same induction mechanism may well be operative in hyperargininemic patients, and that the heightened AII activity may be responsible for the persistent ureagenesis seen in this disorder. These data lend further support to the existence of two separate arginase gene loci in humans, and raise possibilities for novel therapeutic approaches based on their independent manipulation.

Effects of alpha-keto-delta-guanidinovaleric acid on inhibitory amino acid responses on mouse neurons in cell culture

The experimentally proven convulsant alpha-keto-delta-guanidinovaleric acid (alpha-K-delta-GVA) was applied to mouse spinal cord neurons in primary dissociated cell culture to assess its effects on postsynaptic gamma-aminobutyric acid (GABA)- and glycine (GLY)-responses. Intracellular microelectrode recording techniques were used. alpha-K-delta-GVA reversibly inhibited both GABA- and GLY-responses in a concentration-dependent manner. The effect of alpha-K-delta-GVA on GABA-responses was not antagonized by co-application of the benzodiazepine receptor antagonist CGS 9896. The results suggest that alpha-K-delta-GVA inhibited responses to the inhibitory neurotransmitters GABA and GLY by blocking the chloride channel. This action might underlie the convulsant effect of this compound in rabbit. The possible pathophysiological importance of alpha-K-delta-GVA in hyperargininemic patients is discussed.

Isolation of human liver arginase cDNA and demonstration of nonhomology between the two human arginase genes

A human liver cDNA library was screened by colony hybridization with a rat liver arginase cDNA. The number of positive clones detected was in agreement with the estimated abundance of arginase message in liver, and the identities of several of these clones were verified by hybrid-select translation, immunoprecipitation, and competition by purified arginase. The largest of these human liver arginase cDNAs was then used to detect arginase message on northern blots at levels consistent with the activities of liver arginase in the tissues and cells studied. The absence of a hybridization signal with mRNA from a cell line expressing only human kidney arginase demonstrated the lack of homology between the two human arginase genes and indicated considerable evolutionary divergence between these two loci.