Plasma xanthine oxidoreductase activity in Japanese patients with type 2 diabetes across hospitalized treatment

Aims/Introduction

Xanthine oxidoreductase (XOR) is an enzyme that catalyzes hypoxanthine and xanthine to xanthine and uric acid, respectively. Plasma XOR activity has recently been measured in humans. However, limited information is known about plasma XOR activity in patients with type 2 diabetes mellitus, and its changes after short‐term glycemic control treatment.

Materials and Methods

We enrolled 28 Japanese patients (10 men/18 women) with type 2 diabetes mellitus who were hospitalized to undergo medical treatment for diabetes. Plasma XOR activity, quantified using triple quadrupole mass spectrometry and liquid chromatography, and other clinical parameters were examined at admission and 2 weeks after treatment during hospitalization. Changes in plasma XOR activity after treatment during hospitalization and associated clinical parameters were assessed.

Results

At the time of admission, the median plasma XOR activity was 83.1 pmol/h/mL, with a wide range of 14.4–1150 pmol/h/mL. Multiple regression analysis identified serum aspartate transaminase and alanine transaminase levels as significant and independent factors correlating with the baseline plasma XOR. Two weeks of treatment during hospitalization was associated with a significant decrease in plasma XOR activity. Changes in serum aspartate transaminase were also the only significant and independent factor correlating with changes in plasma XOR activity.

Conclusions

A close relationship was observed between plasma XOR activity and liver transaminases in patients with type 2 diabetes mellitus, cross‐sectionally, and also across treatment during hospitalization.

New insights into purine metabolism in metabolic diseases: role of xanthine oxidoreductase activity

Xanthine oxidoreductase (XOR) consists of two different forms, xanthine dehydrogenase and xanthine oxidase (XO), and is a rate-limiting enzyme of uric acid production from hypoxanthine and xanthine. Uric acid is the end product of purine metabolism in humans and has a powerful antioxidant effect. The lack of ascorbic acid, known as vitamin C, in hominoids has been thought to cause a compensatory increase in uric acid as an antioxidant by unfunctional gene mutation of uricase to a pseudogene. Because XO is involved in an increase in reactive oxygen species (ROS) by generating superoxide and hydrogen peroxide, inadequate activation of XOR promotes oxidative stress-related tissue injury. Plasma XOR activity is associated with obesity, smoking, liver dysfunction, hyperuricemia, dyslipidemia, insulin resistance, and adipokines, indicating a novel biomarker of metabolic disorders. However, XOR activity in adipose tissue is low in humans unlike in rodents, and hypoxanthine is secreted from human adipose tissue. The concentration of hypoxanthine, but not xanthine, is independently associated with obesity in a general population, indicating differential regulation of hypoxanthine and xanthine. Treatment with an XOR inhibitor can decrease uric acid for preventing gout, reduce production of XO-related ROS, and promote reutilization of hypoxanthine and ATP production through the salvage pathway. It has recently been suggested that discontinuation of an XOR inhibitor causes adverse cardiovascular outcomes as XOR inhibitor withdrawal syndrome, possibly due to cardiac disturbance of conduction and contraction by reduced ATP production. New insights into purine metabolism, including the role of XOR activity in the past 5 yr, are mainly discussed in this review.

U-shaped relationship between serum uric acid level and decline in renal function during a 10-year period in female subjects: BOREAS-CKD2

While hyperuricemia is recognized as a risk factor for chronic kidney disease (CKD), the risk of CKD in subjects with a low level of serum uric acid (UA) remains controversial. Here, we examined whether the association of CKD risk with serum UA level differs depending on the sex and age of subjects in a general population. Of subjects who received annual health checkups, we enrolled 6,779 subjects (male/female: 4,454/2,325; age: 45 ± 9 years) with data from a 10-year follow-up after excluding subjects taking anti-hyperuricemic drugs and those with CKD at baseline. During the follow-up period, 11.4% of the males and 11.7% of the females developed CKD. A significant interaction of sex, but not age, with the effect of baseline UA level on CKD risk was found. A restricted cubic spline analysis showed a U-shaped association of the baseline UA level with the risk of CKD in females. Multivariable Cox proportional hazard analyses for females showed that baseline UA levels in the 5th quintile (Q5, ≥5 mg/dL; HR: 1.68) and the 1st quintile (Q1, ≤3.5 mg/dL; HR: 1.73) were independent risk factors for CKD when compared with UA levels in the 4th quintile (Q4, 4.5-4.9 mg/dL). In males, restricted cubic spline analysis indicated increased CKD risk in subjects with a higher baseline UA level but not in those with a low UA level. In conclusion, a low UA level is a significant risk factor for CKD in females, while an elevated UA level increases the risk of CKD in both sexes.

Differential regulation of hypoxanthine and xanthine by obesity in a general population

AIMS/INTRODUCTION

Uric acid is synthesized by oxidation of hypoxanthine and xanthine using a catalyzing enzyme, xanthine oxidoreductase (XOR), which can be a source of reactive oxygen species. Plasma XOR activity is a metabolic biomarker associated with obesity, hyperuricemia, liver dysfunction and insulin resistance. However, it has recently been reported that XOR activity in fat tissue is low in humans, unlike in rodents, and that hypoxanthine is secreted from human fat tissue.

MATERIALS AND METHODS

The associations of obesity with hypoxanthine, xanthine and plasma XOR activity were investigated in 484 participants (men/women: 224/260) of the Tanno-Sobetsu Study.

RESULTS

Levels of hypoxanthine, xanthine and plasma XOR activity were significantly higher in men than in women. In 59 participants with hyperuricemia, 11 (men/women: 11/0) participants were being treated with an XOR inhibitor and had a significantly higher level of xanthine, but not hypoxanthine, than that in participants without treatment. In all of the participants, hypoxanthine concentration in smokers was significantly higher than that in non-smokers. Stepwise and multivariate regression analyses showed that body mass index, smoking habit and xanthine were independent predictors of hypoxanthine after adjustment of age, sex and use of antihyperuricemic drugs. Whereas, alanine transaminase, hypoxanthine and plasma XOR activity were independent predictors for xanthine, and alanine transaminase, triglycerides and xanthine were independent predictors for plasma XOR activity.

CONCLUSIONS

The concentration of hypoxanthine, but not that of xanthine, is independently associated with obesity and smoking habit, indicating differential regulation of hypoxanthine and xanthine in a general population.

Plasma xanthine oxidoreductase (XOR) activity in patients who require cardiovascular intensive care

Hyperuricemia is known to be associated with adverse outcomes in cardiovascular intensive care patients, but its mechanisms are unknown. A total of 569 emergency department patients were prospectively analyzed and assigned to intensive care (ICU group, n = 431) or other departments (n = 138). Uric acid (UA) levels were significantly higher in the intensive care patients (6.3 [5.1-7.6] mg/dl vs. 5.8 [4.6-6.8] mg/dL). The plasma xanthine oxidoreductase (XOR) activity in the ICU group (68.3 [21.2-359.5] pmol/h/mL) was also significantly higher than that in other departments (37.2 [15.1-93.6] pmol/h/mL). Intensive care patients were divided into three groups according to plasma XOR quartiles (Q1, low-XOR, Q2/Q3, normal-XOR, and Q4, high-XOR group). A multivariate logistic regression model showed that lactate (per 1.0 mmol/L increase, OR 1.326; 95%, CI 1.166-1.508, p < 0.001) and the Acute Physiology and Chronic Health Evaluation II score (per 1.0 point increase, OR 1.095, 95% CI 1.034-1.160, p = 0.002) were independently associated with the high-XOR group. In-hospital mortality was significantly higher in the high-XOR group (n = 28, 26.2%) than in the normal- (n = 11, 5.1%) and low- (n = 9, 8.3%) XOR groups. The high-XOR group (vs. normal-XOR group) was independently associated with the in-hospital mortality (OR 2.934; 95% CI 1.170-7.358; p = 0.022). Serum UA levels and plasma XOR activity were high in patients admitted to intensive care. The enhanced XOR activity may be one of the mechanisms under which hyperuricemia was associated with adverse outcomes in patients requiring cardiovascular intensive care.

Independent association of plasma xanthine oxidoreductase activity with serum uric acid level based on stable isotope-labeled xanthine and liquid chromatography/triple quadrupole mass spectrometry: MedCity21 health examination registry

Background

We developed a novel high-sensitive assay for plasma xanthine oxidoreductase (XOR) activity that is not affected by the original serum uric acid level. However, the association of plasma XOR activity with that level has not been fully examined.

Methods

This cross-sectional study included 191 subjects (91 males, 100 females) registered in the MedCity21 health examination registry. Plasma XOR activity was determined using our assay for plasma XOR activity with [13C2,15N2] xanthine and liquid chromatography/triple quadrupole mass spectrometry. Serum levels of uric acid and adiponectin, and visceral fat area (VFA) obtained by computed tomography were measured, and insulin resistance was determined based on the homeostasis model assessment (HOMA-IR) index.

Results

The median values for uric acid and plasma XOR activity were 333 μmol/L and 26.1 pmol/h/mL, respectively. Multivariable linear regression analysis showed a significant and positive association of serum uric acid level (coefficient: 26.503; 95% confidence interval: 2.06, 50.945; p = 0.035) with plasma XOR activity independent of VFA and HOMA-IR, and also age, gender, alcohol drinking habit, systolic blood pressure, estimated glomerular filtration rate (eGFR), glycated hemoglobin A1c, triglyceride, and adiponectin levels. The “gender*XOR activity” interaction was not significant (p = 0.91), providing no evidence that gender modifies the relationship between plasma XOR activity and serum uric acid level.

Conclusions

Plasma XOR activity was found to be positively associated with serum uric acid level independent of other known confounding factors affecting that level, including gender difference, eGFR, adiponectin level, VFA, and HOMA-IR.

Relation of Plasma Xanthine Oxidoreductase Activity to Coronary Lipid Core Plaques Assessed by Near-Infrared Spectroscopy Intravascular Ultrasound in Patients With Stable Coronary Artery Disease

Previous studies reported that elevated serum uric acid level was associated with greater coronary lipid plaque. Xanthine oxidoreductase (XOR) is a rate-limiting enzyme in purine metabolism and is believed to play important roles in coronary atherosclerosis. However, the relation between XOR and coronary lipid plaque is unclear. Patients with stable coronary artery disease who underwent elective percutaneous coronary intervention under near-infrared spectroscopy intravascular ultrasound (NIRS-IVUS) guidance were prospectively included. They were divided into 3 groups according to plasma XOR activities based on a previous report: low, normal, and high. Quantitative coronary angiography and gray-scale IVUS were analyzed. The primary end point was coronary lipid plaques in a nontarget vessel assessed by NIRS-IVUS with lipid core burden index (LCBI) and maximum LCBI in 4 mm (maxLCBI_4mm). Out of 68 patients, 26, 31, and 11 patients were classified as low, normal, and high XOR activity groups. Quantitative coronary angiography demonstrated that the high XOR activity group had longer lesion length, smaller minimum lumen diameter, and higher percentage of diameter stenosis in a nontarget vessel among the 3 groups. Gray-scale IVUS analysis also showed smaller lumen area in the high XOR activity group than the others. LCBI (102.1 ± 56.5 vs 65.6 ± 48.5 vs 55.6 ± 37.8, p = 0.04) and maxLCBI_4mm (474.4 ± 171.6 vs 347.4 ± 181.6, 294.0 ± 155.9, p = 0.04) in a nontarget vessel were significantly higher in the high XOR group than in the normal and low groups. In conclusion, elevated XOR activity was associated with coronary lipid-rich plaque in a nontarget vessel in patients with stable coronary artery disease.

Plasma Xanthine Oxidoreductase (XOR) Activity in Cardiovascular Disease Outpatients

Background: The mechanisms of the increased plasma xanthine oxidoreductase (XOR) activity in outpatients with cardiovascular disease were unclear.

Methods and Results: A total of 372 outpatients were screened, and 301 outpatients with cardiovascular disease were prospectively analyzed. Blood samples were collected from patients who visited a daily cardiovascular outpatient clinic. Patients with diabetes mellitus (DM) were significantly more likely to be classified into the high-XOR group (≥100 pg/h/mL; 50%) than the low-XOR group (<100 pmol/h/mL; 28.7%). On multivariate logistic regression analysis, DM (OR, 2.683; 95% CI: 1.441–4.996) was independently associated with high plasma XOR activity in all cohorts. In the diabetic cardiovascular disease patients (n=100), median body mass index (BMI) in the high-XOR group (28.0 kg/m²; IQR, 25.2–29.4 kg/m², n=32) was significantly higher than in the low-XOR group (23.6 kg/m²; IQR, 21.2–25.7 kg/m², n=68), and BMI was independently associated with high plasma XOR activity (OR, 1.340; 95% CI: 1.149–1.540). Plasma hydrogen peroxide was significantly higher in DM patients with high plasma XOR activity and obesity (>22 kg/m²) than in other patients.

Conclusions: DM with obesity is one of the mechanisms of XOR enhancement in cardiovascular disease. The increase of XOR is a possible pathway for the production of reactive oxygen species in obese cardiovascular disease patients with DM.

Annual change in plasma xanthine oxidoreductase activity is associated with changes in liver enzymes and body weight

Xanthine oxidoreductase (XOR), an enzyme of uric acid formation from hypoxanthine and xanthine, is recognized as a source of oxidative stress. Plasma activity of XOR has been reported to be a biomarker of metabolic disorders associated with obesity, liver dysfunction, insulin resistance, hyperuricemia and adipokines. We investigated longitudinal change in plasma XOR activity, which was determined by using mass spectrometry and liquid chromatography to detect [¹³C₂, ¹⁵N₂]-uric acid using [¹³C₂, ¹⁵N₂]-xanthine as a substrate, in 511 subjects (male/female: 244/267) of the Tanno-Sobetsu Study in the years 2016 and 2017. Plasma XOR activity in a basal state was significantly higher in men than in women, but no significant sex difference was observed in annual change in plasma XOR activity. Annual change in plasma activity of XOR was positively correlated with changes in each parameter, including body weight (r = 0.203, p < 0.001), body mass index, diastolic blood pressure, aspartate transaminase (AST) (r = 0.772, p < 0.001), alanine transaminase (r = 0.647, p < 0.001), γ-glutamyl transpeptidase, total cholesterol, triglycerides, uric acid, fasting glucose and HbA1c. Multivariate regression analysis demonstrated that change in AST and that in body weight were independent predictors of change in plasma XOR activity after adjustment of age, sex and changes in each variable with a significant correlation without multicollinearity. In conclusion, annual change in plasma XOR activity is independently associated with changes in liver enzymes and body weight in a general population. Improvement of liver function and reduction of body weight would decrease plasma XOR activity and its related oxidative stress as a therapeutic strategy.

Independent links between plasma xanthine oxidoreductase activity and levels of adipokines

AIMS/INTRODUCTION

Xanthine oxidoreductase (XOR) is a rate-limiting enzyme that catalyzes uric acid formation in the purine metabolism, is involved in an increase in reactive oxygen species. Plasma XOR activity has been shown to be associated with obesity, smoking, liver dysfunction, hyperuricemia, dyslipidemia and insulin resistance.

MATERIALS AND METHODS

The association between plasma XOR activity, measured by using liquid chromatography and mass spectrometry, and levels of adipokines, including adiponectin, fatty acid-binding protein 4 (FABP4) and fibroblast growth factor 21 (FGF21), was investigated in 282 participants (male/female: 126/156) of the Tanno-Sobetsu Study who were not taking medication.

RESULTS

Women had lower plasma XOR activity than did men. Smoking habit was associated with increased activity. Plasma XOR activity was positively correlated with concentrations of FABP4 (r = 0.192, P < 0.001) and FGF21 (r = 0.208, P < 0.001), homeostasis model assessment of insulin resistance as an index of insulin resistance and uric acid, and was negatively correlated with adiponectin level (r = -0.243, P = 0.001). Multivariate regression analyses showed that levels of adiponectin, FABP4 and FGF21 were independent determinants of plasma XOR activity after adjusting age, sex, uric acid and homeostasis model assessment of insulin resistance. With additional adjustment of smoking habit, the level of FABP4, but not that of adiponectin or FGF21, remained as an independent predictor of plasma XOR activity.

CONCLUSIONS

Plasma XOR activity was independently associated with levels of adipokines in a general population of individuals not taking medication.

Fatty Acid-Binding Protein 4 in Cardiovascular and Metabolic Diseases

Fatty acid-binding proteins (FABPs), a family of lipid chaperones, contribute to systemic metabolic regulation via several lipid signaling pathways. Fatty acid-binding protein 4 (FABP4), known as adipocyte FABP (A-FABP) or aP2, is mainly expressed in adipocytes and macrophages and plays important roles in the development of insulin resistance and atherosclerosis in relation to metabolically driven low-grade and chronic inflammation, referred to as ‘metaflammation’. FABP4 is secreted from adipocytes in a non-classical pathway associated with lipolysis and acts as an adipokine for the development of insulin resistance and atherosclerosis. Circulating FABP4 levels are associated with several aspects of metabolic syndrome and cardiovascular disease. Ectopic expression and function of FABP4 in cells and tissues are also related to the pathogenesis of several diseases. Pharmacological modification of FABP4 function by specific inhibitors, neutralizing antibodies or antagonists of unidentified receptors would be novel therapeutic strategies for several diseases, including obesity, diabetes mellitus, atherosclerosis and cardiovascular disease. Significant roles of FABP4 as a lipid chaperone in physiological and pathophysiological conditions and the possibility of FABP4 being a therapeutic target for metabolic and cardiovascular diseases are discussed in this review.

Plasma xanthine oxidoreductase activity in patients with decompensated acute heart failure requiring intensive care

Aims

Plasma xanthine oxidoreductase (XOR) activity during the acute phase of acute heart failure (AHF) requires further elucidation.

Methods and results

One hundred eighteen AHF patients and 231 control patients who attended a cardiovascular outpatient clinic were prospectively analysed. Blood samples were collected within 15 min of admission from AHF patients (AHF group) and control patients who visited a daily cardiovascular outpatient clinic (control group). Plasma XOR activity was compared between the two groups, and factors independently associated with extremely elevated XOR activity were identified using a multivariate logistic regression model. Plasma XOR activity in the AHF group (median, 104.0 pmol/h/mL; range, 25.9–423.5 pmol/h/mL) was significantly higher than that in the control group (median, 45.2 pmol/h/mL; range, 19.3–98.8 pmol/h/mL). The multivariate logistic regression model showed that serum uric acid (per 1.0 mg/dL increase, odds ratio: 1.280; 95% confidence interval: 1.066–1.536; P = 0.008) and lactate levels (per 1.0 mmol/L increase, odds ratio: 1.239; 95% confidence interval: 1.040–1.475; P = 0.016) were independently associated with high plasma XOR activity (>300 pg/h/mL) during the acute phase of AHF.

Conclusions

Plasma XOR activity was extremely high in patients with severely decompensated AHF. This would be associated with a high lactate value and would eventually lead to hyperuricaemia in patients with AHF.

Insulin Resistance Associated with Plasma Xanthine Oxidoreductase Activity Independent of Visceral Adiposity and Adiponectin Level: MedCity21 Health Examination Registry

BACKGROUND

Higher levels of uric acid production have been reported in individuals with visceral fat obesity, and obesity is known to enhance xanthine oxidoreductase (XOR) activity, although the precise mechanism remains unclear. We investigated the associations of visceral fat area (VFA), serum adiponectin level, and insulin resistance with plasma XOR activity using our novel highly sensitive assay based on [¹³C₂,¹⁵N₂] xanthine and liquid chromatography/triple quadrupole mass spectrometry.

METHODS

This cross-sectional study included 193 subjects (92 males and 101 females) registered in the MedCity21 health examination registry. Plasma XOR activity, serum adiponectin level, and VFA obtained by computed tomography were measured, and insulin resistance was determined based on the homeostasis model assessment (HOMA-IR) index.

RESULTS

The mean values for VFA, log HOMA-IR, and log plasma XOR activity were 76.8 ± 45.8 cm², 0.14 ± 0.30, and 1.50 ± 0.44 pmol/h/mL, respectively. Multiple regression analysis showed that HOMA-IR was significantly (p=0.020) associated with plasma XOR activity independent of other factors, including VFA and adiponectin level, as well as age, sex, alcohol drinking habit, smoking habit, alanine transaminase, HbA1c, and eGFR. The "sex∗HOMA - IR" interaction was not significant (p=0.020) associated with plasma XOR activity independent of other factors, including VFA and adiponectin level, as well as age, sex, alcohol drinking habit, smoking habit, alanine transaminase, HbA1c, and eGFR. The ".

CONCLUSIONS

Our results indicate that insulin resistance is associated with plasma XOR activity and that relationship is independent of visceral adiposity and adiponectin level, suggesting that the development of insulin resistance resulting from increased visceral adiposity and/or reduced serum adiponectin contributes to increased uric acid production by stimulating XOR activity.

Unexpected high plasma xanthine oxidoreductase activity in female subjects with low levels of uric acid

Hypouricemia is a high-risk factor of exercise-induced acute kidney injury (EIAKI) probably through a lack of an antioxidant effect of uric acid. Xanthine oxidoreductase (XOR) is an enzyme that catalyzes the formation of uric acid from hypoxanthine and xanthine, leading to an increase in superoxide and reactive oxygen species. Activation of XOR has been proposed to promote oxidative stress-related tissue injury. We measured plasma XOR activity by a sensitive and accurate assay using a combination of liquid chromatography and triple quadrupole mass spectrometry in subjects with relatively low levels of uric acid (≤4.0 mg/dL) who were recruited from 627 subjects (male/female: 292/335) in the Tanno-Sobetsu Study, a population-based cohort. The numbers of subjects with uric acid ≤4.0 mg/dL, ≤3.0 mg/dL and ≤2.0 mg/dL were 72 (11.5%, male/female: 5/67), 13 (2.1%, all females) and 2 (0.3%, both females), respectively. Plasma XOR activities in 5 male subjects were below the median value of the 292 male subjects. In 12 (17.9%) of the 67 female subjects with uric acid ≤4.0 mg/dL, plasma XOR activities were above the upper quartile value of the 335 female subjects. Eleven of the 12 female subjects with high plasma XOR activity and a low uric acid level had liver dysfunction and/or insulin resistance. In conclusion, unexpected high plasma XOR activities were found in some female subjects with relatively low levels of uric acid. Measurement of plasma XOR activity may help to identify hypouricemic patients with a high risk for EIAKI.