The Conversion of Citrulline to Arginine

Enzyme inhibition can be reversed by supplementation of arginine or citrulline.34 106 Pathovars of P. syringae were shown to inhibit Escherichia coli growth, an effect reversed by L-arginine, but not by L-citrulline or L-glutamine.121 This suggested that the site of action of the toxin produced is involved in the conversion of citrulline to arginine in the urea cycle. 

Adenylosuccinate formed by adenylosuccinate synthetase is cleaved by adenylosuccinate lyase to form AMP. The reaction steps are illustrated in Fig. 1. Included in the sequence is the additional reaction catalyzed by AMP deaminase. These three enzymes have been suggested to function in a cyclic process termed the purine nucleotide cycle 7,8). The two-step conversion of IMP to AMP is very similar to both the conversion of citrulline to arginine, which involves formation of argininosuccinate as an intermediate, and formation of 5-amino-imidazole 4-carboxamide ribonucleotide from 5-aminoimidazole 4-carboxylate ribonucleotide as part of IMP biosynthesis. Adenylosuccinate lyase is a dual function enzyme catalyzing the cleavage of both adenylosuccinate and 5-aminoimidazole 4-N-succinocarboxamide ribonucleotide. 

The substrate structures and chemical reactions participating in the conversion of citrulline to arginine are given in Fig. 3. The intermediate formed in the conversion of ornithine to citrulline was found by Grisolia and Cohen in 1952. The structure of this compound was shown by chemical synthesis to be carbamylphosphate by Jones, Spector and Lipmann in 1955. Subsequent advances on carbamyl phosphate synthetase and ornithine transcarbamylase in the laboratories of P.P. Cohen, S. Grisolia, P. Reichard, M. E. Jones and M. Marshall have contributed to the enzymology and elucidation of the reaction mechanisms (cf. Fig. 3). The two enzymes are localized within the liver mitochondria, the last three enzymes of the cycle are in the cytosol. 

Cohen, P. P. and Hayano, M. (1946) The conversion of citrulline to arginine (transimination) by tissue slices and homogenates. Urea synthesis by liver homogenates. J. Biol. Chem. 166, 239-250 251-259. 

The conversion of citrulline to argininosuccinate and the subsequent breakdown to fumarate and arginine take place in the cytosol (Fig. 24-11). The ureido... 

The obscure stages in the cycle are (i.) the conversion of ornithine into citrulline, and (ii.) the conversion of citrulline into arginine. According to Krebs (1936), the first of these depends on the combination between COj and the 8-amino group of citrulline to form a oarbamino compound,... 

The conversion of ornithine to arginine via citrulline was first proposed for mammalian liver by Krebs and Henseleit (1932). The mechanistic and enzymatic details have since been thoroughly established (Ratner, 1954). The first real evidence for this pathway in plants was the isolation of Neurospora mutants... 

The reaction a source of arginine required for the formation of guanido-acetic acid by transamidination with glycine in the kidney. The source of citruUine in the kidney is unknown. It is remarkable that the interaction of citrulline with AS or GL to form arginine apparently does not occur in liver, although conversion of citrulline to arpnine is a highly probable intermediary step in the ornithine cycle of urea formation (c/. Gomall and Hunter, 80). 

The enzymes involved in the conversion of citrulline (R0NHCONH2) to arginine (RoNHCNHNH2) appear not to be associated with mitochondria. This conversion has been studied extensively by Sarah Ratner" ... 

PAD4 is a Ca -dependent enzyme that catalyzes the conversion of protein arginine residues to citrulline. (a) Ribbon representation of the PAD4/benzoyl-L-arginine amide complex (PDB code Iwda) in the presence of the ions (green spheres). 

Schematic illustration of the conversion of L-arginine to NO plus L-citrulline by the enzyme NO synthase. Conversion requires the presence of NADPH, calcium (Ca), calmodulin (CM), and O2. Calcium complexes with CM and the Ca-CM complex binds to the enzyme. The asterisk signifies the basic amino nitrogen atom that undergoes oxidation and cleavage to form NO. Both of the basic amino nittogens are equivalent and either nitrogen can be incorporated into NO.

Arginine deiminases catalyze the conversion of arginine residues to citrulline residues in proteins. Deregulation of protein arginine deiminase 4 (PAD4) is... 

In some instances the type of inhibition has been found to be isozyme specific. For example, inducibly expressed isozymes (iNOS) and constitutively expressed isozymes (cNOS) of nitric oxide synthase (NOS) all catalyze the conversion of L-arginine to L-citrulline and nitric oxide (Equation 17.34). 

NO has a short half-life (2-30 s) and therefore the direct measurement of NO production is impractical. In aqueous solutions, however, it rapidly reacts with oxygen to form the stable water-soluble metabolites nitrite and nitrate (Figure 1). The concentration of these ions, which can be measured via a variety of methods, is used as a measure of the tissue content of NO and/or the synthesis of NO by cultured cells (Hevel and Marietta, 1994 Archer, 1993). An alternative method used to quantify NOS activity is to directly measure the catalytic activity of a cell or tissue extract (i.e., the conversion of radioactive arginine to citrulline Hevel and Marietta, 1994 Archer, 1993). While both methods are sensitive measurements of the overall capacity of a cell or tissue extract to synthesize NO, neither one provides a true kinetic... 

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