Isolation and characterization of a novel monomeric zinc- and heme-containing protein from bovine brain

Pharmacological activation of dimethylarginine dimethylaminohydrolase (DDAH) activity by inorganic nitrate and DDAH inhibition by NG-hydroxy-L-arginine, Nω,Nω-dimethyl-L-citrulline and Nω,Nω-dimethyl-Nδ-hydroxy-L-citrulline: results and overview

Asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA) are endogenous inhibitors of nitric oxide (NO) synthase. SDMA is excreted in the urine without major metabolization. About 10% of daily produced ADMA are excreted unchanged in the urine. The major elimination route of ADMA (about 90%) involves its hydrolysis to dimethylamine (DMA) and L-citrulline by dimethylarginine dimethylaminohydrolase (DDAH) and excretion of DMA in the urine. High circulating and low excretory concentrations of ADMA are considered risk factors. Experimentally, DDAH activity can be inhibited by SH-specific agents such as inorganic and organic mercury compounds, and by S-nitrosothiols which block the SH group of a particular cysteine moiety of DDAH that is essential for its hydrolytic activity. Alternatively, DDAH activity can be inhibited by organic compounds that compete with the substrate ADMA for DDAH. Arginine analogs that contain substituents on guanidine nitrogen atom(s) (N^G) represent a class of DDAH inhibitors. In the present study, we investigated the effects of physiological and natural amino acid derivatives of L-arginine and L-citrulline as well as of nitrate and nitrite, the major circulating and excretory metabolites of NO and NO donating drugs. Here, we report for the first time that the physiological N^G-hydroxy-L-arginine, an isolable intermediate in NO synthesis, inhibits recombinant DDAH-1 activity (IC₅₀ ≈ 100 µM). Two plant L-citrulline derivatives, i.e., N^ω,N^ω-dimethyl-L-citrulline and N^ω,N^ω-dimethyl-N^δ-hydroxy-L-citrulline (connatin), were found to inhibit almost completely hepatic DDAH activity in vitro in rat homogenate at a concentration of 100 µM each. At pharmacological concentrations (i.e., 1 mM), inorganic nitrate, but not inorganic nitrite, was found to increase rat liver DDAH activity. In urine of 18 patients with Becker's muscular dystrophy, nitrate was found to correlate closely with DMA (Spearman, r = 0.73, p = 0.002). In summary, N^G-hydroxy-L-arginine, N^ω,N^ω-dimethyl-L-citrulline and N^ω,N^ω-dimethyl-N^δ-hydroxy-L-citrulline are novel inhibitors of DDAH activity. This article provides an overview of amino acid-based DDAH inhibitors and discusses potential underlying inhibition mechanisms.

Zn(II)-free dimethylargininase-1 (DDAH-1) is inhibited upon specific Cys-S-nitrosylation

The endogenous nitric oxide synthase inhibitors L-N(omega)-methylarginine and L-N(omega),N(omega)-dimethylarginine are catabolized by the enzyme dimethylargininase. Dimethylargininase-1 from bovine brain contains one tightly bound Zn(II) coordinated by two cysteine sulfur and two lighter ligands. Activity measurements showed that only the apo-enzyme is active and that the holo-enzyme is activated by zinc removal. In this work, the effect of NO on dimethylargininase-1 structure and its activity was investigated using 2-(N,N-dimethylamino)-diazenolate-2-oxide as an NO source. The results showed that whereas the holo-form was resistant to S-nitrosylation, the apo-form could be modified. The results of absorption spectroscopy, mass spectrometry, and fluorometric S-NO quantification revealed that two of five cysteine residues reacted with NO yielding cysteine-S-NO. The modification reaction is specific, because by liquid chromatography/mass spectrometry experiments of digested S-NO-dimethylargininase-1, cysteines 221 and 273 could be identified as cysteine-NO. Because Zn(II) protects the enzyme against nitrosation, it is suggested that both cysteines are involved in metal binding. However, specific cysteine-S-NO formation occurred in the absence of a characteristic sequence motif. Based on a structural model of dimethylargininase-1, the activation of both cysteines may be accomplished by the close proximity of charged residues in the tertiary structure of the enzyme.

Structural and functional characterization of the Zn(II) site in dimethylargininase-1 (DDAH-1) from bovine brain. Zn(II) release activates DDAH-1

L-N(omega),N(omega)-dimethylarginine dimethylaminohydrolase-1 (DDAH-1) is a Zn(II)-containing enzyme that, through hydrolysis of side-chain methylated l-arginines, regulates the activity of nitric-oxide synthase. Herein we report the structural and functional properties of the Zn(II)-binding site in DDAH-1 from bovine brain. Activity measurements of the native and metal-free enzyme have revealed that the endogenously bound Zn(II) inhibits the enzyme. Native DDAH-1 could be fully or partially activated using various concentrations of phosphate, imidazole, histidine, and histamine, a process that is paralleled by the release of Zn(II). The slow activation of the enzyme by the bulky complexing agents EDTA and 1,10-phenantroline suggests that the Zn(II)-binding site is partially buried in the protein structure. The apparent Zn(II)-dissociation constant of 4.2 nm, determined by 19F NMR using the chelator 5F-BAPTA (1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid), lies in the range of intracellular free Zn(II) concentrations. These results suggest a regulatory role for the Zn(II)-binding site. The coordination environment of the Zn(II) in DDAH-1 has been examined by Zn K-edge x-ray absorption spectroscopy. The extended x-ray absorption fine structure observed is consistent with Zn(II) being coordinated by 2 S and 2 N (or O) atoms. The biological implications of these findings are discussed.

Dimethylargininase, a nitric oxide regulatory protein, in Alzheimer disease

In this study, we show that dimethylargininase, a zinc protein involved in the regulation of nitric oxide synthase, is specifically elevated in neurons displaying cytoskeletal abnormalities and oxidative stress in Alzheimer disease (AD) while none of this enzyme was found in neurons in age-matched control cases. Seen in the context of earlier studies showing widespread nitric oxide related damage in AD and the role of dimethylargininase to activate nitric oxide synthetase, through catalytic removal of its endogenous inhibitors, these findings indicate major alterations in nitric oxide regulation in AD. Further, that low levels of zinc specifically inhibit dimethylargininase may provide a link between the numerous studies showing specific abnormalities in zinc and oxidative stress. Finally, our results provide additional evidence that oxidative stress- and nitric oxide-mediated events play important roles in the pathogenesis of AD.

Characterization of dimethylargininase from bovine brain: evidence for a zinc binding site

Dimethylargininase (EC 3.5.3.18) is involved in the regulation of the levels of the natural occurring free arginine derivatives L-Nomega,Nomega-dimethylarginine and L-Nomega-methylarginine, which are reversible inhibitors of nitric oxide synthase. A dimethylargininase has been isolated from bovine brain tissue and was characterized by using immunological, kinetic, and spectroscopic techniques. Western blot analysis using polyclonal antibodies revealed that the enzyme is widely distributed in bovine with the highest relative concentrations found in brain and kidney tissue. A similar tissue distribution has also been reported for the other so far isolated dimethylargininase from rat kidney [Ogawa, T., Kimoto, M., and Sasaoka, K. (1989) J. Biol. Chem. 264, 10205-10209]. The bovine enzyme is a monomeric, globular protein (molecular mass approximately 31.2 kDa) containing one tightly bound Zn2+ ion, which can be removed by dialysis against 1,10-phenanthroline. The determination of kinetic constants for both the native (holo-protein) and the zinc-depleted (apo-protein) enzyme at 37 degrees ¿C established that the dimethylargininase is not a zinc hydrolase. The specific activity was 0.66 unit/mg for the holo-protein and 0.19 unit/mg for the apo-protein. The secondary structure determination of the native enzyme by circular dichroism revealed 41% alpha-helix and 32% beta-sheet and beta-turn structure. In the apo-enzyme, a small, but significant decrease in the alpha-helical content (5%) was observed, consistent with a marked decrease in enzymatic activity to 30%. Upon preincubation of both enzyme forms at 50 degrees C, only the holo-enzyme showed a residual enzymatic activity. In thermostability studies, a 7 degrees C lower apparent Tm value was observed for the apo-enzyme compared to the 66 degrees C for the holo-enzyme, suggesting that the zinc ion has a structure-stabilizing role. Besides the tightly bound zinc, additional Zn2+ ions inhibit the enzyme competitively with a Ki value of 2.0 microM. A possible interrelationship between dimethylargininase and nitric oxide synthase is discussed.

Studying noncovalent protein complexes by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry has been used to study protein interactions driven by noncovalent forces. The gentleness of the electrospray ionization process allows intact protein complexes to be directly detected by mass spectrometry. Evidence from the growing body of literature suggests that the ESI-MS observations for these weakly bound systems reflect, to some extent, the nature of the interaction found in the condensed phase. Stoichiometry of the complex can be easily obtained from the resulting mass spectrum because the molecular weight of the complex is directly measured. For the study of protein interactions, ESI-MS is complementary to other biophysical methods, such as NMR and analytical ultracentrifugation. However, mass spectrometry offers advantages in speed and sensitivity. The experimental variables that play a role in the outcome of ESI-MS studies of noncovalently bound complexes are reviewed. Several applications of ESI-MS are discussed, including protein interactions with metal ions and nucleic acids and subunit protein structures (quaternary structure).