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Review Article Argininosuccinate Synthase: at the Center of Arginine Metabolism

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Review Article Argininosuccinate Synthase: at the Center of Arginine Metabolism Int J Biochem Mol Biol 2011;2(1):8-23 www.ijbmb.org /ISSN:2152-4114/IJBMB1090003 Review Article Argininosuccinate synthase: at the center of arginine metabolism Ricci J. Haines, Laura C. Pendleton, Duane C. Eichler Department of Molecular Medicine, University of South Florida, College of Medicine, 12901 Bruce B. Downs Blvd., MDC Box 7, Tampa, Florida 33612, USA. Received September 28, 2010; accepted October 3, 2010; Epub October 8, 2010; Published February 15, 2011 Abstract: The levels of L-arginine, a cationic, semi-essential amino acid, are often controlled within a cell at the level of local availability through biosynthesis. The importance of this temporal and spatial control of cellular L-arginine is highlighted by the tissue specific roles of argininosuccinate synthase (argininosuccinate synthetase) (EC 6.3.4.5), as the rate-limiting step in the conversion of L-citrulline to L-arginine. Since its discovery, the function of argininosucci- nate synthase has been linked almost exclusively to hepatic urea production despite the fact that alternative path- ways involving argininosuccinate synthase were defined, such as its role in providing arginine for creatine and for polyamine biosynthesis. However, it was the discovery of nitric oxide that meaningfully extended our understanding of the metabolic importance of non-hepatic argininosuccinate synthase. Indeed, our knowledge of the number of tissues that manage distinct pools of arginine under the control of argininosuccinate synthase has expanded significantly. Keywords: Argininosuccinate synthase; arginine metabolism; nitric oxide; arginase; urea cycle, arginine recycling Introduction thase participates are linked to the varied uses of the amino acid arginine. There are five major Arginine is required by all tissues in the human pathways in which argininosuccinate synthase body for protein synthesis, and by some tissues plays a key role. These are (a) urea synthesis, for specialized needs. Any de novo biosynthetic (b) nitric oxide synthesis, (c) polyamine synthe- pathway for arginine involves the conversion of sis, (d) creatine synthesis, and (e) the de novo citrulline to arginine catalyzed by argininosucci- synthesis of arginine to maintain serum levels. nate synthase and argininosuccinate lyase. Spe- cifically, argininosuccinate synthase catalyzes In liver, where arginine is hydrolyzed to form the condensation of citrulline and aspartate to urea and ornithine, argininosuccinate synthase form argininosuccinate, the immediate precur- is highly expressed, and hormone and nutrients sor of arginine. Argininosuccinate lyase then constitute the major regulating factors [1]. In splits argininosuccinate to release fumarate contrast, for nitric oxide-producing cells where and arginine (Figure 1). First identified in liver, arginine is the direct substrate for nitric oxide as a rate-limiting enzyme in urea synthesis, ar- synthesis, argininosuccinate synthase is ex- gininosuccinate synthase is now recognized as pressed at relatively low levels, and in most a ubiquitous enzyme in mammalian tissues, and cases, its regulation has been tied to affecting not surprisingly, its expression, localization, and the phenotypic patterns controlled by nitric ox- regulation differs significantly depending on the ide signaling [2]. tissue specific needs for arginine. The subcellular localization of argininosuccinate Argininosuccinate synthase plays an important synthase also varies. For example, in endothe- role as the rate-limiting step in providing argin- lial cells argininosuccinate synthase co-localizes ine for an assortment of metabolic processes, with eNOS at caveolae [3] and outside of the both catabolic and anabolic. Thus, the meta- Golgi [4]. In kidney, subcellular localization is bolic pathways in which argininosuccinate syn- not well-defined, but seems to be cytosolic in Argininosuccinate synthase for arginine metabolism Figure 1. De Novo biosynthetic pathway for arginine. relation to synthesis of arginine for guanidi- remaining three enzymes are located in the cy- noacetate production and for the de novo syn- tosol. Argininosuccinate synthase is located on thesis of arginine for export to maintain serum the outer mitochondrial membrane [5], utilizing arginine levels. In liver, argininosuccinate syn- both citrulline and aspartate to produce argini- thase associates with the outside of the mito- nosuccinate. Argininosuccinate lyase catalyzes chondria to complete the urea cycle, which in- the cleavage of argininosuccinate to produce volves both the mitochondrial and cytosolic arginine and fumarate. Subsequently, arginase compartments [5]. splits the arginine to ornithine and urea, which ensures that the arginine generated by the ac- The objective of this discussion is to highlight tions of argininosuccinate synthase and argini- the varied metabolic processes that involve ar- nosuccinate lyase is, for the most part, directed gininosuccinate synthase, its essential role in to urea production [8] (Figure 2). Urea is ex- regulating unique cellular pools of available creted and the ornithine is transported back arginine, and the levels of control that regulate into the mitochondria to complete the cycle. its expression. Immunocytochemical studies by Cohen & Kudal Argininosuccinate synthase and the urea cycle [5] showed that argininosuccinate synthase and argininosuccinate lyase are located in the im- The urea cycle, also known as the Krebs urea mediate vicinity of the mitochondria, predomi- cycle or ornithine cycle, was first discovered in nantly next to the cytoplasmic surface of the 1932 by Hans Krebs [6]. The cycle is comprised outer membrane. This finding confirmed previ- of five enzymes and is essentially localized to ous biochemical studies [9-11] suggesting that the liver in mammals, although the kidney does these enzymes are organized in situ next to the share some capacity to produce urea. The first outer membrane of mitochondria. Perhaps our two enzymes of the urea cycle, carbamoyl phos- best understanding as to how argininosuccinate phate synthetase I and ornithine transcar- synthase associates with hepatic mitochondria bamoylase are located within the mitochondrial comes from the findings related to adult-onset matrix of periportal hepatocytes [7], while the type II citrullinemia (CTLN2). Unlike classical 9 Int J Biochem Mol Biol 2010:2(1):8-23 Argininosuccinate synthase for arginine metabolism Figure 2. Urea cycle type I citrullinemia (CTLN1) in children where ated with one another such that substrates and the deficiency of argininosuccinate synthase products can be shuttled from one enzyme to results from a mutation in the gene [12], in another by channeling [11]. Support for chan- CTLN2 there is no mutation in the argininosucci- neling comes from studies demonstrating the nate synthase gene, yet there is still a marked channeling of the extramitochondrial ornithine deficiency in liver argininosuccinate synthase to the mitochondrial ornithine transcarbamoy- protein [13]. To account for the liver specific lase via its transporter [15], from studies show- deficiency of argininosuccinate synthase in ing the co-localization of carbamoyl phosphate CTLN2, Saheki et al. [13] showed association of synthetase I and ornithine transcarbamoylase CTLN2 with a defect in SLC25A13 gene that with the inner mitochondrial membrane [16], encodes the mitochondrial Ca2+-dependent as- from labeling studies demonstrating channeling partate/glutamate transporter known as citrin of citrulline from ornithine transcarbamoylase in [14]. Saheki’s findings suggested that citrin the mitochondria to argininosuccinate synthase forms a complex with the three “soluble” cyto- in the cytoplasm, and from studies demonstrat- plasmic enzymes of the urea cycle, including ing channeling of arginine between argininosuc- argininosuccinate synthase. This interaction cinate lyase and arginase [10]. with citrin may be direct or mediated through some other protein or proteins. Nevertheless, Thus the spatial relationship of argininosucci- the loss of spatial control attributable to the nate synthase relative to ornithine transcar- mutated citrin strongly correlated with reduction bamoylase in the mitochondria, and arginino- of liver argininosuccinate synthase protein in succinate lyase in the cytosol, together with the CTLN2, possibly through destabilization and/or relatively high expression levels for each of degradation [13]. these enzymes, ensures that both substrate and product, and in particular arginine, do not Urea cycle enzymes appear to be closely associ- readily exchange with other cellular pools, or 10 Int J Biochem Mol Biol 2010:2(1):8-23 Argininosuccinate synthase for arginine metabolism Figure 3. Citrulline-nitric oxide cycle even with extracellular sources. Moreover, it is not surprising that expression of arginino- hepatocytes do not easily exchange arginine succinate synthase, relative to urea cycle func- with serum, evidenced by their limited capacity tion, is highly regulated and governed by to take up arginine [17]. As a result, any other changes in dietary protein intake, and by hor- intracellular pool of arginine, as well as serum mones such as glucagon, insulin, and glucocor- arginine, is protected [18, 19]. ticoids [1]. There are also studies in cultured hepatocytes demonstrating nutrient control over The significance of this arrangement can be argininosuccinate synthase expression where illustrated by the grave outcomes observed dur- the addition of arginine was suppressive and ing liver failure,
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