In the current study, we demonstrated associations among the plasma XOR activity, xanthine, and hypoxanthine levels in patients with type 2 diabetes. Previous studies have shown an association between the plasma XOR activity and the severity of cardiovascular disease [8, 14, 17].
In this study, a significant correlation was observed between the plasma XOR activity and plasma xanthine level, but not the plasma hypoxanthine level. In contrast, the plasma xanthine levels were correlated with the plasma hypoxanthine levels. Since hypoxanthine is a precursor of xanthine, this association between hypoxanthine and xanthine might be easy to conceive. Significant associations between the plasma levels of hypoxanthine and xanthine, and between the plasma xanthine and XOR activity were also reported by another study conducted in the general population [29]. The plasma XOR activity and xanthine levels, but not the plasma hypoxanthine level, were correlated with the BMI, unlike the results of a previous study [29]. Hypoxanthine is a known marker of hypoxia [32, 33] and a recent study revealed that the secretion of hypoxanthine from the adipose tissue increased in response to local hypoxia [34]. It is possible that the production of hypoxanthine in the adipose tissue and other tissues differs from that under chronic hyperglycemia conditions in type 2 diabetes.
It is well known that the liver is the main source of XOR, and that hepatic damage caused by an infection and a variety of toxic agents is associated with elevation of the serum XOR activity [35]. Furthermore, many clinical trials have shown correlation between the degree of liver dysfunction and the plasma XOR activity [13, 36]. In fact, the plasma XOR activity level was strongly correlated with the serum transaminase and γ-GTP levels in the present study. The plasma xanthine, but not plasma hypoxanthine, level was also correlated with the serum transaminase and γ-GTP levels. The plasma XOR activity also tended to show a weak correlation with the Fib4-index, a marker of hepatic fibrosis [37], although it did not reach statistical significance. However, interestingly, on the other hand, the plasma xanthine concentration showed a significant positive correlation with the Fib4-index. It is unknown whether plasma XOR activity is associated with liver fibrosis. Recently, the contribution of MiR-218-XOR-ROS pathway in the development of non-alcoholic steatohepatitis (NASH) was reported in vitro and in animal model [38]. These results indicate that both the plasma xanthine and plasma XOR activity are possibly associated with severity of liver fibrosis, as well as the degree of liver dysfunction.
A previous study showed the existence of a relation between the plasma XOR activity and the severity of dyslipidemia in the general population [36]. In this study also, we found a significant positive correlation between the serum triglyceride levels and plasma XOR activity, and a negative correlation between the serum HDL-C levels and plasma XOR activity. Moreover, there were also significant correlations between the plasma xanthine and hypoxanthine and serum HDL-C levels. The plasma XOR activity was correlated with multiple parameters indicative of insulin resistance, such as the fasting IRI, fasting C-peptide, HOMA-IR, and urinary C-peptide levels during the day in the current study. Thus, the plasma XOR activity may be a marker of the severity of metabolic syndrome.
The plasma XOR activity showed no correlation with the fasting glucose level or percent HbA1c in the patients with type 2 diabetes. Previous studies in which patients with type 1 diabetes or subjects from the general population were enrolled, a positive correlation was observed between the plasma XOR activity and the percent HbA1c [15, 39]. The association between glycemic control and plasma XOR activity remains controversial. In this study, the percent glycated albumin showed a weak, but significant negative correlation with the plasma XOR activity. In a previous study of hemodialysis patients with type 2 diabetes, the percent glycated albumin showed a positive and independent association with the plasma XOR activity [16]. Our discrepant results could be explained by the fact that the patients in the aforementioned study were on maintenance hemodialysis and the backgrounds of the patients were different from those in our study: 1) the enrolled individuals in our study were hospitalized because of poor glycemic control, and 2) we excluded patients with severe renal dysfunction (eGFR < 30 mL/min/1.73 m2) 3) hypoalbuminemia in patients with hemodialysis. Since the percent glycated albumin is a precursor of advanced glycation end products and enhances oxidative stress, it is possible that this result reflected oxidative stress, at least in part.
The results of our multiple regression analysis identified the serum levels of ALT, AST, and triglyceride as independent indicators of the plasma XOR activity in patients with type 2 diabetes. A previous study also showed associations between the serum levels of liver enzymes and the plasma XO activity level in patients with type 2 diabetes or metabolic syndrome [13]. When the serum levels of transaminases were excluded from the possible determinants, fasting IRI and HOMA-IR were found to be independent predictors of the plasma XOR activity. These findings suggest that both liver dysfunction and insulin resistance are associated with the elevation of the plasma XOR activity in patients with type 2 diabetes.
We also found a weak, but significant negative correlation between the plasma XOR activity and the duration of diabetes, indicating that the plasma XOR activity could decrease with the development of diabetes. This might be consistent with the finding that the CVR-R and sensory nerve conduction velocity, a parameter of diabetic neuropathy, showed a positive correlation with the plasma XOR activity. However, since there was no significant correlation between the plasma XOR activity and the motor nerve conduction velocity or CVR-R, and patients with impaired vibration perception threshold or blunted Achilles tendon reflex did not show decreased plasma XOR activity, further investigation is needed to elucidate the association between plasma XOR activity and diabetic neuropathy.
In the present study, no association was observed between the plasma XOR activity and the presence of diabetic nephropathy or diabetic retinopathy. Since we excluded patients with progressive nephropathy from the study and the mean duration of diabetes was 10 years in the patients enrolled in this study, further studies would be needed to clarify these relationships.
However, notably, patients with CVD showed significantly reduced plasma XOR activity as compared to patients without CVD. Our results seem to be inconsistent with those of previous studies which showed elevated plasma XO activity in patients with coronary artery disease [8] or coronary artery spasm [40]. Multiple factors, such as chronic hyperglycemia, increased oxidative stress, and other pathogenetic factors in patients with type 2 diabetes could explain this discrepant result. No significant correlation was observed between the plasma XOR activity and the brachial-ankle pulse wave velocity (baPWV) or ankle-brachial index (ABI) (data not shown). To the best of our knowledge, there have been no previous reports about the relationship between the plasma XOR activity and diabetic vascular complications, these results give a new insight into the pathogenesis of diabetes and associated diseases. Furthermore, our findings might be of practical value if/when XOR inhibitors are used in patients with the expectation of protective effects against oxidative stress; a renoprotective effect of XOR inhibitors has already been reported in patients with CKD.