N(delta)-Methylated L-arginine derivatives and their effects on the nitric oxide generating system

Arginase Structure and Inhibition: Catalytic Site Plasticity Reveals New Modulation Possibilities

Metalloenzyme arginase is a therapeutically relevant target associated with tumor growth. To fight cancer immunosuppression, arginase activity can be modulated by small chemical inhibitors binding to its catalytic center. To better understand molecular mechanisms of arginase inhibition, a careful computer-aided mechanistic structural investigation of this enzyme was conducted. Using molecular dynamics (MD) simulations in the microsecond range, key regions of the protein active site were identified and their flexibility was evaluated and compared. A cavity opening phenomenon was observed, involving three loops directly interacting with all known ligands, while metal coordinating regions remained motionless. A novel dynamic 3D pharmacophore analysis method termed dynophores has been developed that allows for the construction of a single 3D-model comprising all ligand-enzyme interactions occurring throughout a complete MD trajectory. This new technique for the in silico study of intermolecular interactions allows for loop flexibility analysis coupled with movements and conformational changes of bound ligands. Presented MD studies highlight the plasticity of the size of the arginase active site, leading to the hypothesis that larger ligands can enter the cavity of arginase. Experimental testing of a targeted fragment library substituted by different aliphatic groups validates this hypothesis, paving the way for the design of arginase inhibitors with novel binding patterns.

First detection and quantification of N(δ)-monomethylarginine, a structural isomer of N(G)-monomethylarginine, in humans using MS(3)

The L-arginine metabolites methylated at the guanidino moiety, such as N(G)-monomethyl-L-arginine (LNMMA), asymmetric N(G),N(G)-dimethyl-L-arginine (ADMA), and symmetric N(G),N(G')-dimethyl-L-arginine (SDMA), are long known to be present in human plasma. Far less is known about the structural isomer of LNMMA, N(δ)-monomethyl-L-arginine (δ-MMA). In prior work, it has been detected in yeast proteins, but it has not been investigated in mammalian plasma or cells. In this work, we present a method for the simultaneous and unambiguous quantification of LNMMA and δ-MMA in human plasma that is capable of detecting δ-MMA separately from LNMMA. The method comprises a simple protein precipitation sample preparation, hydrophilic interaction liquid chromatography (HILIC) gradient elution on an unmodified silica column, and triple stage mass spectrometric detection. Stable isotope-labeled D6-SDMA was used as internal standard. The calibration ranges were 25-1000 nmol/L for LNMMA and 5-350 nmol/L for δ-MMA. The intra- and inter-batch precision determinations resulted in relative standard deviations of less than 12% for both compounds with accuracies of less than 6% deviation from the expected values. In a pilot study enrolling 10 healthy volunteers, mean concentrations of 48.0 ± 7.4 nmol/L for LNMMA and 27.4 ± 7.7 nmol/L for δ-MMA were found.

Methylated N(ω)-hydroxy-L-arginine analogues as mechanistic probes for the second step of the nitric oxide synthase-catalyzed reaction

Nitric oxide synthase (NOS) catalyzes the conversion of L-arginine to L-citrulline through the intermediate N(ω)-hydroxy-L-arginine (NHA), producing nitric oxide, an important mammalian signaling molecule. Several disease states are associated with improper regulation of nitric oxide production, making NOS a therapeutic target. The first step of the NOS reaction has been well-characterized and is presumed to proceed through a compound I heme species, analogous to the cytochrome P450 mechanism. The second step, however, is enzymatically unprecedented and is thought to occur via a ferric peroxo heme species. To gain insight into the details of this unique second step, we report here the synthesis of NHA analogues bearing guanidinium methyl or ethyl substitutions and their investigation as either inhibitors of or alternate substrates for NOS. Radiolabeling studies reveal that N(ω)-methoxy-L-arginine, an alternative NOS substrate, produces citrulline, nitric oxide, and methanol. On the basis of these results, we propose a mechanism for the second step of NOS catalysis in which a methylated nitric oxide species is released and is further metabolized by NOS. Crystal structures of our NHA analogues bound to nNOS have been determined, revealing the presence of an active site water molecule only in the presence of singly methylated analogues. Bulkier analogues displace this active site water molecule; a different mechanism is proposed in the absence of the water molecule. Our results provide new insights into the steric and stereochemical tolerance of the NOS active site and substrate capabilities of NOS.

MS³ fragmentation patterns of monomethylarginine species and the quantification of all methylarginine species in yeast using MRM³

Protein arginine methylation is one of the epigenetic modifications to proteins that is studied in yeast and is known to be involved in a number of human diseases. All eukaryotes produce Nη-monomethylarginine (ηMMA), asymmetric Nη1, Nη1-dimethylarginine (aDMA), and most produce symmetric Nη1, Nη2-dimethylarginine (sDMA) on proteins, but only yeast produce Nδ-monomethylarginine (δMMA). It has proven difficult to differentiate among all of these methylarginines using mass spectrometry. Accordingly, we demonstrated that the two forms of MMA have indistinguishable primary product ion spectra. However, the secondary product ion spectra of δMMA and ηMMA exhibited distinct patterns of ions. Using incorporation of deuterated methyl-groups in yeast, we determined which secondary product ions were methylated and their structures. Utilizing distinct secondary product ions, a triple quadrupole multiple reaction monitoring cubed (MRM(3)) assay was developed to measure δMMA, ηMMA, sDMA and aDMA derived from hydrolyzed protein. As a proof-of-concept, δMMA and ηMMA were measured using the MRM(3) method in wild type and mutant strains of Saccharomyces cerevisiae and compared to the total MMA measured using an existing assay. The MRM(3) assay represents the only method to directly quantify δMMA and the only method to simultaneously quantify all yeast methylarginines.

Dimethylarginine-dimethylaminohydrolase-2 (DDAH-2) does not metabolize methylarginines

Free endogenous methylarginines, N(ω)-monomethyl-L-arginine (L-NMMA) and N(ω),N(ω')-dimethyl-L-arginine (ADMA), inhibit NO synthases (NOSs) and are metabolized by dimethylargininedimethylaminohydrolase (DDAH). A postulated metabolism has been shown several times for DDAH-1, but the involvement of DDAH-2 in the degradation of ADMA and L-NMMA is still a matter of debate. Determination of the isoform-specific DDAH protein expression profiles in various porcine tissue types shows a correlation of DDAH activity only with DDAH-1 levels. DDAH activity (measured as L-citrulline formation from the conversion of methylarginines and alternative DDAH substrates) was detected in DDAH-1-rich porcine tissue types, that is, kidney, liver, and brain, but not in DDAH-2-rich porcine fractions, that is, spleen and thyroid. Furthermore, several ex vivo studies showed DDAH activity to be important for L-citrulline formation in porcine tissue and indicated the absence of an endogenous DDAH inhibitor in porcine tissue. This study provides new insights into tissue distributions as well as substrate selectivity for both DDAH isoforms. Although DDAH-1 is known to metabolize the endogenous NOS inhibitors L-NMMA and ADMA, a physiological function for DDAH-2 has yet to be determined. Hence, determining DDAH activity by methylarginine conversion is not suitable for analyzing isoform selectivity of DDAH-1 inhibitors as postulated.

Prodrug design for the potent cardiovascular agent Nω-hydroxy-L-arginine (NOHA): synthetic approaches and physicochemical characterization

N(ω)-Hydroxy-L-arginine (NOHA)--the physiological nitric oxide precursor--is the intermediate of NO synthase (NOS) catalysis. Besides the important fact of releasing NO mainly at the NOS-side of action, NOHA also represents a potent inhibitor of arginases, making it an ideal therapeutic tool to treat cardiovascular diseases that are associated with endothelial dysfunction. Here, we describe an approach to impart NOHA drug-like properties, particularly by wrapping up the chemically and metabolically instable N-hydroxyguanidine moiety with different prodrug groups. We present synthetic routes that deliver several more or less highly substituted NOHA derivatives in excellent yields. Versatile prodrug strategies were realized, including novel concepts of bioactivation. Prodrug candidates were primarily investigated regarding their hydrolytic and oxidative stabilities. Within the scope of this work, we essentially present the first prodrug approaches for an interesting pharmacophoric moiety, i.e., N-hydroxyguanidine.

Reduction of Nω-hydroxy-L-arginine by the mitochondrial amidoxime reducing component (mARC)

NOSs (nitric oxide synthases) catalyse the oxidation of L-arginine to L-citrulline and nitric oxide via the intermediate NOHA (Nω-hydroxy-L-arginine). This intermediate is rapidly converted further, but to a small extent can also be liberated from the active site of NOSs and act as a transportable precursor of nitric oxide or potent physiological inhibitor of arginases. Thus its formation is of enormous importance for the nitric-oxide-generating system. It has also been shown that NOHA is reduced by microsomes and mitochondria to L-arginine. In the present study, we show for the first time that both human isoforms of the newly identified mARC (mitochondrial amidoxime reducing component) enhance the rate of reduction of NOHA, in the presence of NADH cytochrome b5 reductase and cytochrome b5, by more than 500-fold. Consequently, these results provide the first hints that mARC might be involved in mitochondrial NOHA reduction and could be of physiological significance in affecting endogenous nitric oxide levels. Possibly, this reduction represents another regulative mechanism in the complex regulation of nitric oxide biosynthesis, considering a mitochondrial NOS has been identified. Moreover, this reduction is not restricted to NOHA since the analogous arginase inhibitor NHAM (Nω-hydroxy-Nδ-methyl-L-arginine) is also reduced by this system.

Arylazoamidoximes and related compounds as NO-modulators

Three amidinoarylhydrazines 1, three arylazoamidines 2, and nine arylazoamidoximes 3 have been synthesized and investigated for their potential to function as nitric oxide (NO) modulators. In-vitro studies demonstrated that 2 and 3 inhibited platelet aggregation (2c, IC(50 )= 3 microM) which could also be shown in vivo by inhibition of thrombus formation in arterioles (3a, 22%). Moreover, for all compounds antihypertensive effects were examined in vivo with SHR rats, with 2a being the most potent candidate by lowering blood pressure by 19%. However, no common underlying mechanism of action could be shown. Some of these compounds released HNO non-enzymatically. Incubations with NO synthase isoforms (NOSs) revealed, that compounds 1 to 3 were weak substrates for NOSs but arylazoamidoximes 3 remarkably elevated the NOSs activity in the presence of L-arginine (3h, up to fivefold). In addition, we examined effects on arginase and dimethylarginine dimethylaminohydrolase (DDAH), two further enzymes involved in the complex regulation of NO biosynthesis, to elucidate whether the observed in-vivo effects can be traced back to their modulation. Furthermore, the metabolic fate of arylazoamidoximes 3 was addressed by investigation of a possible N-reductive biotransformation. In summary, novel NO-modulating compound classes are presented, among which arylazoamidoximes 3 are potent activators of NOS isoforms, and arylazoamidines 2 exert in-vivo effects by unknown mechanisms.