Xanthine oxidoreductase polymorphisms: influence in blood pressure and oxidative stress levels

Inside the genome: understanding genetic influences on oxidative stress

Genetics is a key factor that governs the susceptibility to oxidative stress. In the body, oxidative burden is regulated by the balance between the prooxidant genes that orchestrate processes that produce oxidant species, while the antioxidant genes aid those involved in scavenging these species. Together, the two components aid in maintaining the oxidative balance in the body. Genetic variations can influence the expression and activity of the encoded proteins which can then affect their efficiency in regulating redox processes, thereby increasing the risk of oxidative stress. This review studies single nucleotide polymorphisms (SNPs) that bear relevance to oxidative stress by exploring the variations in the prooxidant genes, such as XDH, CYBA, CYP1A1, PTGS2, NOS, and MAO and antioxidant genes including SOD, CAT, GPX, GSS, GLUL, GSR, GSTM1, GSTM5, GSTP1, TXN and HMOX1. Early identification of individuals at the increased risk of oxidative stress is possible from the assessment of sequence of these genes. Integrating genetic insights into oxidative stress management measures can pave the way for personalized medicine that tailors' healthcare approaches to individual genetic profiles. Effective genetic assessment along with routine quantification of biological markers can improve and monitor treatment strategies, enhancing mitigation approaches that maintain cellular health and promote longevity.

Natural mutations of human XDH promote the nitrite (NO2-)-reductase capacity of xanthine oxidoreductase: A novel mechanism to promote redox health?

Several rare genetic variations of human XDH have been shown to alter xanthine oxidoreductase (XOR) activity leading to impaired purine catabolism. However, XOR is a multi-functional enzyme that depending upon the environmental conditions also expresses oxidase activity leading to both O2·- and H2O2 and nitrite (NO2-) reductase activity leading to nitric oxide (·NO). Since these products express important, and often diametrically opposite, biological activity, consideration of the impact of XOR mutations in the context of each aspect of the biochemical activity of the enzyme is needed to determine the potential full impact of these variants. Herein, we show that known naturally occurring hXDH mutations do not have a uniform impact upon the biochemical activity of the enzyme in terms of uric acid (UA), reactive oxygen species (ROS) and nitric oxide ·NO formation. We show that the His1221Arg mutant, in the presence of xanthine, increases UA, O2·- and NO generation compared to the WT, whilst the Ile703Val increases UA and ·NO formation, but not O2·-. We speculate that this change in the balance of activity of the enzyme is likely to endow those carrying these mutations with a harmful or protective influence over health that may explain the current equipoise underlying the perceived importance of XDH mutations. We also show that, in presence of inorganic NO2-, XOR-driven O2·- production is substantially reduced. We suggest that targeting enzyme activity to enhance the NO2--reductase profile in those carrying such mutations may provide novel therapeutic options, particularly in cardiovascular disease.

Exploring the Multifaceted Nexus of Uric Acid and Health: A Review of Recent Studies on Diverse Diseases

The prevalence of patients with hyperuricemia or gout is increasing worldwide. Hyperuricemia and gout are primarily attributed to genetic factors, along with lifestyle factors like consuming a purine-rich diet, alcohol and/or fructose intake, and physical activity. While numerous studies have reported various comorbidities linked to hyperuricemia or gout, the range of these associations is extensive. This review article focuses on the relationship between uric acid and thirteen specific domains: transporters, genetic factors, diet, lifestyle, gout, diabetes mellitus, metabolic syndrome, atherosclerosis, hypertension, kidney diseases, cardiovascular diseases, neurological diseases, and malignancies. The present article provides a comprehensive review of recent developments in these areas, compiled by experts from the Young Committee of the Japanese Society of Gout and Uric and Nucleic Acids. The consolidated summary serves to enhance the global comprehension of uric acid-related matters.

Xanthine oxidoreductase gene polymorphisms are associated with high risk of sepsis and organ failure

BACKGROUND

Sepsis and associated organ failures confer substantial morbidity and mortality. Xanthine oxidoreductase (XOR) is implicated in the development of tissue oxidative damage in a wide variety of respiratory and cardiovascular disorders including sepsis and sepsis-associated acute respiratory distress syndrome (ARDS). We examined whether single nucleotide polymorphisms (SNPs) in the XDH gene (encoding XOR) might influence susceptibility to and outcome in patients with sepsis.

METHODS

We genotyped 28 tag SNPs in XDH gene in the CELEG cohort, including 621 European American (EA) and 353 African American (AA) sepsis patients. Serum XOR activity was measured in a subset of CELEG subjects. Additionally, we assessed the functional effects of XDH variants utilizing empirical data from different integrated software tools and datasets.

RESULTS

Among AA patients, six intronic variants (rs206805, rs513311, rs185925, rs561525, rs2163059, rs13387204), in a region enriched with regulatory elements, were associated with risk of sepsis (P < 0.008-0.049). Two out of six SNPs (rs561525 and rs2163059) were associated with risk of sepsis-associated ARDS in an independent validation cohort (GEN-SEP) of 590 sepsis patients of European descent. Two common SNPs (rs1884725 and rs4952085) in tight linkage disequilibrium (LD) provided strong evidence for association with increased levels of serum creatinine (Padjusted<0.0005 and 0.0006, respectively), suggesting a role in increased risk of renal dysfunction. In contrast, among EA ARDS patients, the missense variant rs17011368 (I703V) was associated with enhanced mortality at 60-days (P < 0.038). We found higher serum XOR activity in 143 sepsis patients (54.5 ± 57.1 mU/mL) compared to 31 controls (20.9 ± 12.4 mU/mL, P = 1.96 × 10- 13). XOR activity was associated with the lead variant rs185925 among AA sepsis patients with ARDS (P < 0.005 and Padjusted<0.01). Multifaceted functions of prioritized XDH variants, as suggested by various functional annotation tools, support their potential causality in sepsis.

CONCLUSIONS

Our findings suggest that XOR is a novel combined genetic and biochemical marker for risk and outcome in patients with sepsis and ARDS.

Update in uric acid, hypertension, and cardiovascular diseases

A direct relationship between serum uric acid levels and hypertension, cardiovascular, renal and metabolic diseases has been reported in many basic and epidemiological studies. Among these, high blood pression is one of the most common features associated with hyperuricemia. In this regard, several small-scale interventional studies have demonstrated a significant reduction in blood pressure in hypertensive or prehypertensive patients on uric acid-lowering drugs. These observation or intervention studies have led to affirm that there is a causal relationship between uric acid and hypertension. While the clinical association between uric acid and high blood pressure is notable, no clear conclusion has yet been reached as to whether lowering uric acid is beneficial to prevent cardiovascular and renal metabolic diseases. Recently, several prospective randomized controlled intervention trials using allopurinol and other uric acid-lowering drugs have been reported, and the results from these trials were almost negative, suggesting that the correlation between hyperuricemia and cardiovascular disease has no causality. However, it is important to note that in some of these recent studies there were high dropout rates and an important fraction of participants were not hyperuricemic. Therefore, we should carry caution in interpreting the results of these studies. This review article presents the results of recent clinical trials using uric acid-lowering drugs, focusing on hypertension and cardiovascular and renal metabolic diseases, and discusses the future of uric acid therapy.

Uric acid in metabolic syndrome: Does uric acid have a definitive role?

Increased serum uric acid (SUA) levels are commonly seen in patients with metabolic syndrome and are widely accepted as risk factors for hypertension, gout, non-alcoholic fatty liver disease, chronic kidney disease (CKD), and cardiovascular diseases. Although some ambiguity for the exact role of uric acid (UA) in these diseases is still present, several pathophysiological mechanisms have been identified such as increased oxidative stress, inflammation, and apoptosis. Accumulating evidence in genomics enlightens genetic variabilities and some epigenetic changes that can contribute to hyperuricemia. Here we discuss the role of UA within metabolism and the consequences of asymptomatic hyperuricemia while providing newfound evidence for the associations between UA and gut microbiota and vitamin D. Increased SUA levels and beneficial effects of lowering SUA levels need to be elucidated more to understand its complicated function within different metabolic pathways and set optimal target levels for SUA for reducing risks for metabolic and cardiovascular diseases.

Serum uric acid level is associated with an increase in systolic blood pressure over time in female subjects: Linear mixed-effects model analyses

Whether hyperuricemia is a true risk factor for elevated blood pressure (BP) is controversial, and the sex-specific effects of serum uric acid (SUA) on BP during a follow-up period remain unclear. We investigated whether the association of SUA level with systolic or diastolic BP during a 10-year period differs by sex in a Japanese general population of individuals who received annual health examinations (n = 28,990). After exclusion of subjects who had no BP or SUA data at baseline, a total of 22,994 subjects (male/female: 14,603/8391, age: 47 ± 11 years) were recruited. After adjustment for age; body mass index; BP; SUA level; use of drugs for hyperuricemia and hypertension; diagnosis of diabetes mellitus, dyslipidemia, and chronic kidney disease; family history of hypertension; habits of current smoking and alcohol consumption at baseline; the duration of the observation period; and the interaction between each covariate and the duration of the observation period indicated a significant association of SUA level with change in systolic or diastolic BP over time. There was a significant interaction between sex and SUA level for the change in systolic BP (P = 0.003) but not the change in diastolic BP (P = 0.081). The SUA level at baseline (per 1 mg/dL) was significantly associated with a change in systolic BP over time in females (estimate: 0.073 mmHg/year, P = 0.003) but not in males (estimate: 0.020 mmHg/year, P = 0.160). In conclusion, a high SUA level at baseline is significantly associated with an increase in systolic BP over time in female individuals but not in male individuals.

Uric Acid and Hypertension: Prognostic Role and Guide for Treatment

The relationship between serum uric acid (SUA) and hypertension has been a subject of increasing interest since the 1870 discovery by Frederick Akbar Mahomed. Several epidemiological studies have shown a strong association between high SUA levels and the presence or the development of hypertension. Genetic analyses have found that xanthine oxidoreductase (XOR) genetic polymorphisms are associated with hypertension. However, genetic studies on urate transporters and Mendelian randomization studies failed to demonstrate a causal relationship between SUA and hypertension. Results from clinical trials on the role of urate-lowering therapy in the management of patients with hypertension are not uniform. Our study sought to analyze the prognostic and therapeutic role of SUA in the hypertensive disease, from uric acid (UA) biology to clinical trials on urate-lowering therapies.

Uric Acid and Hypertension: An Update With Recommendations

The association between increased serum urate and hypertension has been a subject of intense controversy. Extracellular uric acid drives uric acid deposition in gout, kidney stones, and possibly vascular calcification. Mendelian randomization studies, however, indicate that serum urate is likely not the causal factor in hypertension although it does increase the risk for sudden cardiac death and diabetic vascular disease. Nevertheless, experimental evidence strongly suggests that an increase in intracellular urate is a key factor in the pathogenesis of primary hypertension. Pilot clinical trials show beneficial effect of lowering serum urate in hyperuricemic individuals who are young, hypertensive, and have preserved kidney function. Some evidence suggest that activation of the renin-angiotensin system (RAS) occurs in hyperuricemia and blocking the RAS may mimic the effects of xanthine oxidase inhibitors. A reduction in intracellular urate may be achieved by lowering serum urate concentration or by suppressing intracellular urate production with dietary measures that include reducing sugar, fructose, and salt intake. We suggest that these elements in the western diet may play a major role in the pathogenesis of primary hypertension. Studies are necessary to better define the interrelation between uric acid concentrations inside and outside the cell. In addition, large-scale clinical trials are needed to determine if extracellular and intracellular urate reduction can provide benefit hypertension and cardiometabolic disease.

Association of plasma xanthine oxidoreductase activity with blood pressure affected by oxidative stress level: MedCity21 health examination registry

Xanthine oxidoreductase (XOR) inhibitor administration reduces uric acid and reactive oxygen species (ROS) production, and also lowers blood pressure (BP). However, the associations of plasma XOR activity, uric acid level, and oxidative stress levels with BP remain unclear. This cross-sectional study included 156 subjects (68 males, 88 females) registered in the MedCity21 health examination registry without anti-hypertensive or anti-hyperuricemic agent administration. Plasma XOR activity was measured using our highly sensitive novel assay, which is unaffected by uric acid in the sample. BP was also determined, and serum uric acid and derivative of reactive oxygen metabolites (d-ROMs) levels were simultaneously measured. Median plasma XOR activity, serum uric acid, d-ROMs, and mean arterial pressure (MAP) values were 25.7 pmol/h/mL, 5.4 mg/dL, 305 Carr U, and 89.0 mmHg, respectively. Multiple regression analysis showed that plasma XOR activity (β = 0.211, p = 0.019), but not serum uric acid (β = 0.072, p = 0.502), was significantly associated with MAP. In subjects with lower but not higher d-ROMs level, an independent association of plasma XOR activity with MAP was observed (β = 0.428, p = 0.001 and β = 0.019, p = 0.891, respectively; p for interaction = 0.046). XOR may contribute to the pathophysiology of higher BP through ROS but not uric acid production, especially in patients with lower oxidative stress.

Genetic polymorphisms associated with reactive oxygen species and blood pressure regulation

Hypertension is the most prevalent cause of cardiovascular disease and kidney failure, but only about 50% of patients achieve adequate blood pressure control, in part, due to inter-individual genetic variations in the response to antihypertensive medication. Significant strides have been made toward the understanding of the role of reactive oxygen species (ROS) in the regulation of the cardiovascular system. However, the role of ROS in human hypertension is still unclear. Polymorphisms of some genes involved in the regulation of ROS production are associated with hypertension, suggesting their potential influence on blood pressure control and response to antihypertensive medication. This review provides an update on the genes associated with the regulation of ROS production in hypertension and discusses the controversies on the use of antioxidants in the treatment of hypertension, including the antioxidant effects of antihypertensive drugs.

Oxidative stress in bisphosphonate-related osteonecrosis of the jaws

OBJECTIVES

To analyze whether oxidative stress (OS) changes are present in patients with bisphosphonate-related osteonecrosis of the jaw (BRONJ) versus controls.

MATERIALS AND METHODS

Oxidative stress was analyzed in serum and unstimulated saliva of three groups: Group 1 consisted of 24 patients who had been treated with intravenous bisphosphonates (ivBPs) and developed BRONJ, group 2 consisted of 20 patients who had received ivBPs and did not develop BRONJ, and group 3 comprised 17 control subjects. Reduced glutathione (GSH), malondialdehyde (MDA), oxidized glutathione (GSSG), and 8-oxo-7,8-dihydro-2-deoxyguanosine (8-oxo-dG) levels, as well as the GSSG/GSH ratio, were measured.

RESULTS

Mean serum and saliva levels of MDA, GSSG, and 8-oxo-dG and the GSSG/GSH ratio were significantly higher in patients with BRONJ than in controls. We found no significant difference in OS according to BRONJ stage, sex, or location in the jaws. Logistic regression analysis revealed that the GSSG/GSH ratio was a significant factor predicting the development of BRONJ (P = 0.01).

CONCLUSIONS

Oxidative stress was detected in patients with BRONJ, and the GSSG/GSH ratio was the most significant OS variable found; it was a significant factor predicting the development of BRONJ.

Vanin-1 T26I polymorphism, hypertension and cardiovascular events in two large urban-based prospective studies in Swedes

BACKGROUND AND AIMS

Vanin-1 (gene name VNN1) is an enzyme with pantetheinase activity generating the amino-thiol cysteamine which is implicated in the regulation of red-ox status through its effect on glutathione. We tested the hypothesis that the rs2294757 VNN1 T26I polymorphism could affect blood pressure (BP) levels, hypertension prevalence, and risk of incident cardiovascular events.

METHODS AND RESULTS

The VNN1 T26I polymorphism was genotyped in 5664 participants of the cardiovascular cohort of the "Malmö Diet and Cancer" (MDC-CVA) study and successively in 17874 participants of the "Malmö Preventive project"(MPP). The incidence of cardiovascular events was monitored for an average of nearly 12 years of follow-up in the MDC-CVA and for 25 years in the MPP. Both before and after adjustment for sex, age and BMI in the MDC-CVA the polymorphism had a mild lowering effect on diastolic BP and hypertension, especially in females. However in MPP no effect on BP phenotypes was detectable. Before and after adjustment for major cardiovascular risk factors, the hazard ratio for incident ischemic stroke and coronary events in the MDC-CVA was not significantly different in carriers of different genotypes.

CONCLUSIONS

Our data do not support a major role for the VNN1 T26I variant in determining BP level and incident ischemic events.

Different impacts of cardiovascular risk factors on oxidative stress

The objective of the study was to evaluate oxidative stress (OS) status in subjects with different cardiovascular risk factors. With this in mind, we have studied three models of high cardiovascular risk: hypertension (HT) with and without metabolic syndrome, familial hypercholesterolemia (FH) and familial combined hyperlipidemia (FCH) with and without insulin resistance. Oxidative stress markers (oxidized/reduced glutathione ratio, 8-oxo-deoxyguanosine and malondialdehide) together with the activity of antioxidant enzyme triad (superoxide dismutase, catalase, glutathione peroxidase) and activation of both pro-oxidant enzyme (NAPDH oxidase components) and AGTR1 genes, as well as antioxidant enzyme genes (CuZn-SOD, CAT, GPX1, GSR, GSS and TXN) were measured in mononuclear cells of controls (n = 20) and patients (n = 90) by assessing mRNA levels. Activity of some of these antioxidant enzymes was also tested. An increase in OS and pro-oxidant gene mRNA values was observed in patients compared to controls. The hypertensive group showed not only the highest OS values, but also the highest pro-oxidant activation compared to those observed in the other groups. In addition, in HT a significantly reduced antioxidant activity and mRNA induction of antioxidant genes were found when compared to controls and the other groups. In FH and FCH, the activation of pro-oxidant enzymes was also higher and antioxidant ones lower than in the control group, although it did not reach the values obtained in hypertensives. The thioredoxin system was more activated in patients as compared to controls, and the highest levels were in hypertensives. The increased oxidative status in the presence of cardiovascular risk factors is a consequence of both the activation of pro-oxidant mechanisms and the reduction of the antioxidant ones. The altered response of the main cytoplasmic antioxidant systems largely contributes to OS despite the apparent attempt of the thioredoxin system to control it.

Polymorphisms of antioxidant enzymes, blood pressure and risk of hypertension

OBJECTIVE

To assess the association of single-nucleotide polymorphisms (SNPs) in genes codifying for antioxidant enzymes to blood pressure (BP) values and risk of hypertension.

METHODS

Population-based study including 1388 participants (704 women) older than 18 years in which 300 were untreated hypertensive patients. In 335 untreated hypertensive patients referred to one hypertension clinic, the study was replicated. Thirty-five SNP throughout 13 genes were analyzed using SNPlex. In a subgroup of hypertensive patients, the amount of 8-oxo-deoxyguanosine and GPX activity levels was measured in mononuclear cells.

RESULTS

In the general population, genotypes with the G allele of the c.172G>A polymorphism in the SOD3 gene and those with the T allele of the c.-20C>T polymorphism in the CAT gene were associated with significant lower values of BP. Likewise, these genotypes were associated with less risk for hypertension after adjusting for confounder variables. Haplotypes in both genes increased the strength of associations. In the hypertensive patients, the same alleles of the two polymorphisms were associated with lower BP values too. In addition, two others, the CT-TT genotypes of the c.891C>T polymorphism in the GPX1 gene and the CT-CC genotypes of the c.-793T>C polymorphism of the TXN gene were also significantly associated to lower BP values. Furthermore, the CC genotype of the c.891C>T polymorphism in the GPX1 gene was associated with higher values of 8-oxo-dG and GPX activity levels as compared to those for the CT-TT genotype.

CONCLUSIONS

The results of the present study support the influence of antioxidant enzyme genes in BP values and hypertension risk.

Functional characterization of genetic polymorphisms identified in the promoter region of the xanthine oxidase gene

Xanthine oxidase (XO) catalyzes the oxidation of endogenous and exogenous purines and pyrimidines. In the present study, we investigated polymorphisms in the promoter region of the XO gene. Sequence variations in the 5'-flanking region were screened using denaturing high-performance liquid chromatography (DHPLC) on DNA samples from 196 unrelated Japanese individuals. Thirteen polymorphisms were identified and 13 haplotypes were classified by haplotype analysis. The promoter activities of these polymorphisms were measured by luciferase assay in the human hepatoma cell lines HepG2 and Huh-7. Transcriptional activity was significantly lower in cell lines transfected with the reporter construct containing 5-kb upstream fragments with -1756T than in those with wild-type -1756C. Our results indicate that genetic variation in the promoter region of XO may determine interindividual differences in XO gene expression.

Uric acid and hypertension: cause or effect?

Uric acid was first associated with primary hypertension in 1874, yet its role in this condition remains unclear. Historically, uric acid was thought to be a secondary response to hypertension or its associated conditions. However, more recent experimental and clinical studies suggest that uric acid could have a contributory role in the pathogenesis of elevated blood pressure. More studies are needed to help dissect the potential mechanisms by which uric acid could initiate this response. It remains possible that uric acid is a marker for xanthine oxidase-associated oxidants and that the latter could be driving the hypertensive response. However, the weight of the evidence suggests that uric acid is a true modifying and possibly causal factor for human primary hypertension. Hence, early management of hyperuricemia might delay the development of essential hypertension.

Natural antioxidants and hypertension: promise and challenges

Hypertension reigns as a leading cause of cardiovascular morbidity and mortality worldwide. Excessive reactive oxygen species (ROS) have emerged as a central common pathway by which disparate influences may induce and exacerbate hypertension. Potential sources of excessive ROS in hypertension include nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, mitochondria, xanthine oxidase, endothelium-derived NO synthase, cyclooxygenase 1 and 2, cytochrome P450 epoxygenase, and transition metals. While a significant body of epidemiological and clinical data suggests that antioxidant-rich diets reduce blood pressure and cardiovascular risk, randomized trials and population studies using natural antioxidants have yielded disappointing results. The reasons behind this lack of efficacy are not completely clear, but likely include a combination of (1) ineffective dosing regimens, (2) the potential pro-oxidant capacity of some of these agents, (3) selection of subjects less likely to benefit from antioxidant therapy (too healthy or too sick), and (4) inefficiency of nonspecific quenching of prevalent ROS versus prevention of excessive ROS production. Commonly used antioxidants include Vitamins A, C and E, L-arginine, flavanoids, and mitochondria-targeted agents (Coenzyme Q10, acetyl-L-carnitine, and alpha-lipoic acid). Various reasons, including incomplete knowledge of the mechanisms of action of these agents, lack of target specificity, and potential interindividual differences in therapeutic efficacy preclude us from recommending any specific natural antioxidant for antihypertensive therapy at this time. This review focuses on recent literature evaluating naturally occurring antioxidants with respect to their impact on hypertension.

Increased oxidative stress levels and normal antioxidant enzyme activity in circulating mononuclear cells from patients of familial hypercholesterolemia

Familial hypercholesterolemia (FH) is a clinical condition with high risk for developing atherosclerosis. Increased oxidative stress (OS) and FH have been related to atherosclerosis, but no data are available on levels of OS and antioxidant enzyme activity in circulating mononuclear cells (CMCs) from FH patients. Circulating mononuclear cells are important mediators in atherosclerosis development, and chronically increased blood OS present in FH can induce modification in CMC activity. The objective of the study was to analyze the OS levels in CMCs from FH patients and controls. We have selected 30 nonrelated FH index patients and 30 normoglycemic and normocholesterolemic controls matched by age, sex, body mass index, abdominal circumference, and homeostasis model assessment index. Production of free radicals was analyzed by measurement of xanthine oxidase activity in plasma, reduced and oxidized glutathione (GSH and GSSG, respectively), and malonyldialdehyde in levels CMCs. Antioxidant status was analyzed by measuring antioxidant enzyme activity as superoxide dismutase, catalase, and glutathione peroxidase. We have found that FH patients showed significantly higher xanthine oxidase and malonyldialdehyde enzyme activities, as well as increased GSSG and lower GSH values resulting in a higher GSSG/GSH ratio. These data indicate a higher free radical production in plasma and increased OS levels in CMCs from patients than from controls. No significant differences were found in superoxide dismutase, catalase, and glutathione peroxidase activities between both groups. These data show an important alteration of OS regulation in FH and the absence of antioxidant response in CMCs mediated by some of the major antioxidant enzymes.

The molecular sources of reactive oxygen species in hypertension

In both animal models and humans, increased blood pressure has been associated with oxidative stress in the vasculature, i.e. an excessive endothelial production of reactive oxygen species (ROS), which may be both a cause and an effect of hypertension. In addition to NADPH oxidase, the best characterized source of ROS, several other enzymes may contribute to ROS generation, including nitric oxide synthase, lipoxygenases, cyclo-oxygenases, xanthine oxidase and cytochrome P450 enzymes. It has been suggested that also mitochondria could be considered a major source of ROS: in situations of metabolic perturbation, increased mitochondrial ROS generation might trigger endothelial dysfunction, possibly contributing to the development of hypertension. However, the use of antioxidants in the clinical setting induced only limited effects on human hypertension or cardiovascular endpoints. More clinical studies are needed to fully elucidate this so called "oxidative paradox" of hypertension.