Homoarginine and inhibition of human arginase activity: kinetic characterization and biological relevance

Abstract

The inhibition of arginase, resulting in higher arginine (ARG) availability for nitric oxide synthesis, may account for the putative protective effect of homoarginine (HOMOARG) against atherosclerosis and cardiovascular disease. However, uncertainty exists regarding the significance of HOMOARG-induced arginase inhibition in vivo. A novel UPLC-MS method, measuring the conversion of ARG to ornithine (ORN), was developed to determine arginase 1 and arginase 2 inhibition by HOMOARG, lysine (LYS), proline (PRO), agmatine (AG), asymmetric dimethylarginine (ADMA), symmetric dimethylarginine (SDMA), and NG-Monomethyl-L-arginine (L-NMMA). Plasma HOMOARG, ARG and ORN concentrations were further measured in 50 healthy older adults >65 years (27 males and 23 females). HOMOARG inhibited arginase 1 with IC50 and Ki values of 8.14 ± 0.52 mM and 6.1 ± 0.50 mM, and arginase 2 with IC50 and Ki values of 2.52 ± 0.01 mM and 1.73 ± 0.10 mM, respectively. Both arginase isoforms retained 90% activity vs. control when physiological HOMOARG concentrations (1–10 µM) were used. In partial correlation analysis, plasma HOMOARG was not associated with ARG (P = 0.38) or ARG/ORN ratio (P = 0.73) in older adults. Our results suggest that arginase inhibition is unlikely to play a significant role in the reported cardio-protective effects of HOMOARG.

Homoarginine and all‐cause mortality: A systematic review and meta‐analysis

“…41,42 Arginase inhibition would increase the availability of the substrate L-arginine for NO synthase, resulting in a further increase in NO synthesis and, hence, anti-atherosclerotic effects. Tommasi et al 13 showed that both human arginase isoforms, 1 and 2, retained 90% activity in the presence of physiological homoarginine concentrations (1-10 lmol/L). However, the concentrations of homoarginine shown to be a substrate for NO synthase, or to inhibit NO synthase activity, were significantly higher than the observed physiological concentrations (1-10 lmol/L).…”

“…12,41,42 Furthermore, the biological and clinical significance of arginase inhibition by homoarginine has been recently challenged. Tommasi et al 13 showed that both human arginase isoforms, 1 and 2, retained 90% activity in the presence of physiological homoarginine concentrations (1-10 lmol/L).…”

“…3 Therefore, the identification of endogenous factors and drugs that exert salutary effects on endothelial function remains a priority in cardiovascular research. 12,13 Circulating homoarginine concentrations have shown inverse associations with surrogate markers of vascular and cardiac dysfunction. [6][7][8][9][10] Although it has been speculated that the protective effects of homoarginine are at least partly mediated by its interactions with NO synthase and arginase, 2 key enzymes in the L-arginine and NO metabolic pathways, resulting in increased NO synthesis, 6,11 significant uncertainty still exists regarding its exact biological effects.…”

“…[6][7][8][9][10] Although it has been speculated that the protective effects of homoarginine are at least partly mediated by its interactions with NO synthase and arginase, 2 key enzymes in the L-arginine and NO metabolic pathways, resulting in increased NO synthesis, 6,11 significant uncertainty still exists regarding its exact biological effects. 12,13 Circulating homoarginine concentrations have shown inverse associations with surrogate markers of vascular and cardiac dysfunction. [14][15][16][17][18] Therefore, one can hypothesize that higher homoarginine concentrations might translate into better clinical outcomes in subjects with various degrees of cardiovascular risk.…”

Arginine Derivatives in Cerebrovascular Diseases: Mechanisms and Clinical Implications

“…Homoarginine has been implicated to play a role in vascular function [90,[99][100][101]. Correspondingly, epidemiological studies revealed an inverse association of hArg with aortic wall thickness and aortic plaque burden [93], an inverse correlation of hArg/ADMA ratio with aortic intima-media thickness [58] and a link between hArg/SDMA ratio with internal carotid artery stenosis and unfavorable outcome after stroke [71,98].…”