Polyphenols are plant secondary metabolites which possess many positive effects on human health. Although these beneficial effects could be mediated through an increase in nitric oxide synthase activity, little is known regarding the inhibitory effect of polyphenols on mammal arginase, an enzyme which competes with nitric oxide synthase for their common substrate, L-arginine. The aim of the present study was to determine the potential of a series of polyphenols as mammalian arginase inhibitors and to identify some structure-activity relationships. For this purpose, we first developed a simple and cost-effective colorimetric microplate method using commercially-available mammal bovine liver arginase (b-ARG 1). Among the ten tested polyphenolic compounds [chlorogenic acid, piceatannol, resveratrol, (-)-epicatechin, taxifolin, quercetin, fisetin, caffeic acid, quinic acid, and kaempferol], cholorogenic acid and piceatannol exhibited the highest inhibitory activities (IC = 10.6 and 12.1 µM, respectively) but were however less active as ()-(2-Boronoethyl)-L-cysteine (IC = 3.3 µM), used as reference compound. Enzyme kinetic studies showed that both chlorogenic acid and piceatannol are competitive arginase inhibitors. Structural data identified the importance of the caffeoyl (3,4-dihydroxycinnamoyl)-part and of the catechol function in the inhibitory activity of the tested compounds. These results identified chlorogenic acid and piceatannol as two potential core structures for the design of new arginase inhibitors.
The enzyme arginase catalyses the divalent cation dependent hydrolysis of L-arginine to produce L-ornithine and urea. Two isoforms of arginases have been identified in mammalian (including human) cells. Moreover, some infectious pathogens (e.g. Leishmania) synthesize their own arginase. Work over the last decades has revealed that elevated arginase activity both decreases cellular availability in nitric oxide (NO) by competing with NO synthases (NOS) and increases concentration in L-ornithine, a precursor in the biosynthesis of polyamines which are important for cell differentiation and proliferation. From these data emerged the concept that selective arginase inhibitors might be a valuable strategy for treatment of various diseases associated with decreased NO and/or increased polyamines production. Consistent with this, recent research provides compelling evidence supporting the beneficial effects of arginase inhibitors in cardiovascular diseases (hypertension, ischemia reperfusion injury, atherosclerosis, diabetes mellitus), asthma, cancer, immunologically-mediated diseases or leishmaniasis. Despite active programs to identify potent arginase inhibitors, effective chemical compounds with reliable pharmacokinetics and toxicological properties are rare. The present review summarizes available data on the discovery of new arginase inhibitors from natural origin. Current knowledge on plant-derived compounds or extracts with arginase inhibitory properties as well as available data on structure-activity relationship (SAR) will be presented. Lastly, the present review will open up new prospects in order to improve the discovery of novel arginase inhibitors from natural sources.
Arginases are enzymes that are involved in many human diseases and have been targeted for new treatments. Here a series of cinnamides was designed, synthesized and evaluated in vitro and in silico for their inhibitory activity against mammalian arginase. Using a microassay on purified liver bovine arginase (b-ARG I), (E)-N-(2-phenylethyl)-3,4-dihydroxycinnamide, also named caffeic acid phenylamide (CAPA), was shown to be slightly more active than our natural reference inhibitor, chlorogenic acid (IC50 = 6.9 ± 1.3 and 10.6 ± 1.6 µM, respectively) but it remained less active that the synthetic reference inhibitor Nω-hydroxy-nor-l-arginine nor-NOHA (IC50 = 1.7 ± 0.2 µM). Enzyme kinetic studies showed that CAPA was a competitive inhibitor of arginase with Ki = 5.5 ± 1 µM. Whereas the activity of nor-NOHA was retained (IC50 = 5.7 ± 0.6 µM) using a human recombinant arginase I (h-ARG I), CAPA showed poorer activity (IC50 = 60.3 ± 7.8 µM). However, our study revealed that the cinnamoyl moiety and catechol function were important for inhibitory activity. Docking results on h-ARG I demonstrated that the caffeoyl moiety could penetrate into the active-site pocket of the enzyme, and the catechol function might interact with the cofactor Mn2+ and several crucial amino acid residues involved in the hydrolysis mechanism of arginase. The results of this study suggest that 3,4-dihydroxycinnamides are worth being considered as potential mammalian arginase inhibitors, and could be useful for further research on the development of new arginase inhibitors.
Caffeic acid and related natural compounds were previously described as Leishmania amazonensis arginase (L-ARG) inhibitors, and against the whole parasite in vitro. In this study, we tested cinnamides that were previously synthesized to target human arginase. The compound caffeic acid phenethyl amide (CAPA), a weak inhibitor of human arginase (IC50 = 60.3 ± 7.8 μM) was found to have 9-fold more potency against L-ARG (IC50 = 6.9 ± 0.7 μM). The other compounds that did not inhibit human arginase were characterized as L-ARG, showing an IC50 between 1.3–17.8 μM, and where the most active was compound 15 (IC50 = 1.3 ± 0.1 μM). All compounds were also tested against L. amazonensis promastigotes, and only the compound CAPA showed an inhibitory activity (IC50 = 80 μM). In addition, in an attempt to gain an insight into the mechanism of competitive L-ARG inhibitors, and their selectivity over mammalian enzymes, we performed an extensive computational investigation, to provide the basis for the selective inhibition of L-ARG for this series of compounds. In conclusion, our results indicated that the compounds based on cinnamoyl or 3,4-hydroxy cinnamoyl moiety could be a promising starting point for the design of potential antileishmanial drugs based on selective L-ARG inhibitors.
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