Heart and Metabolism

Effects of xanthine oxidase inhibition with allopurinol in chronic heart failure

Wolfram Doehner*, Stefan D. Anker*
Department of Clinical Cardiology, National Heart and Lung Institute,
Imperial College School of Medicine, London, UK;
Applied Cachexia Research, Department of Cardiology,
Charité Campus Virchow-Klinikum, Berlin, Germany
Correspondence: Dr Wolfram Doehner, Department of Cardiology, Charité Campus Virchow-Klinikum, Augustenburger Platz 1, 13353 Berlin, Germany.
Tel: +49 30 450553752, fax: +49 30 450553952, e-mail: w.doehner@ic.ac.uk

Abstract

Patients with chronic heart failure (CHF) are characterized by impaired peripheral blood flow and reduced vasodilator capacity. These features are closely related to prominent clinical symptoms such as reduced exercise capacity and early muscle fatigue. Indeed, the symptoms relate more closely to peripheral than to central hemodynamic abnormalities. An important role of the endothelium has been recognized in the regulation of vascular tone and tissue perfusion, and, accordingly, endothelial function and vasodilator reactivity to exercise have been shown to be significantly impaired in patients with CHF. One major factor responsible for the impaired regulation of vascular tone in CHF is the reduced bioavailability of nitric oxide resulting from accelerated degradation of nitric oxide by free oxygen radicals. An important source of increased free oxygen radical load in CHF has been recognized in the enzyme xanthine oxidase. Abnormalities of the xanthine oxidase metabolic pathway may be viewed in the context of derangements within a complex metabolic web. This review will focus on the role of xanthine oxidase as an oxygen radical-generating enzyme and the final step of purine degradation resulting in the formation of uric acid. First results indicating a potential benefit of therapeutically targeting xanthine oxidase in CHF (with allopurinol) will be discussed. n Heart Metab. 2003;19:32–39.

Keywords: Chronic heart failure, xanthine oxidase, free oxygen radicals, vasodilator capacity, allopurinol, insulin resistance, metabolism

Significance of the xanthine oxidase metabolic pathway in chronic heart failure (CHF)
In humans, uric acid forms the metabolic endpoint of purine degradation. The last metabolic steps in this process (from hypoxanthine to xanthine and from xanthine to uric acid) are promoted by the enzyme xanthine oxidoreductase (EC1.1.3.22). This enzyme is a flavoprotein that contains both iron and molybdenum and uses NAD+ as electron acceptor. It exists in two interconvertible forms, xanthine dehydrogenase and xanthine oxidase. In its oxidase form, the enzyme transfers the reducing equivalent generated by oxidation of substrates to molecular oxygen with the resultant production of superoxide anion and hydrogen peroxide (Figure 1). Hydrogen peroxide can be converted to free hydroxyl radicals.

Figure 1. Free oxygen radical production in the xanthine oxidase-mediated reaction from hypoxanthine to xanthine and from xanthine to uric acid.

The generation of free oxygen radicals by xanthine oxidase may have an important pathophysiological role in tissues and in vascular regulation in the setting of chronic heart failure (CHF). It is of interest that in 1968 the cytosolic xanthine oxidase was the first documented putative biological generator of oxygen-derived free radicals [1]. Since then it has been established that xanthine oxidase is a major source of free oxygen radical production in the human body [2–4]. This metabolic pathway is of particular significance in conditions of tissue hypoxia and ischemia-reperfusion [5], as increased degradation of adenosine triphosphate via adenosine leads to an increased substrate load for xanthine oxidase [6]. Accordingly, elevation of serum uric acid has been observed in hypoxic states, such as obstructive pulmonary disease [7], neonatal hypoxia [8, 9], cyanotic heart disease [10, 11], and acute heart failure [12]. Uric acid levels have been shown to increase also in the coronary sinus following consecutive balloon inflations during angioplasty [13, 14] and during coronary bypass operations [15]. Simultaneously, in ischemia-hypoxia, xanthine dehydrogenase is increasingly converted to xanthine oxidase, which further adds to accelerated radical production [2, 16]. In CHF, elevated uric acid levels might therefore be expected, since patients with CHF have impaired uptake of oxygen at rest and during exercise.
In CHF, hyperuricemia is a consistent finding reflecting impaired oxidative metabolism [17, 18]. High serum uric acid levels indicate the degree of xanthine oxidase activation in CHF [19] and occur independently of the effects of diuretics and renal dysfunction [17]. Indirect measurement of endothelium-bound xanthine oxidase has shown increased xanthine oxidase enzyme activity in CHF compared with healthy control subjects [20]. We can therefore conclude that in patients with CHF there is an increased free radical oxygen load [21–23].
In CHF, endothelial dysfunction and reduced vasodilator capacity are consistent findings that relate closely to prominent clinical symptoms such as reduced exercise capacity and early muscle fatigue [24, 25]. Decreased perfusion of skeletal muscle in CHF is neither primarily related to central hemodynamic abnormalities [26, 27] nor to arterial hypotension [28] but, more importantly, to endothelial dysfunction [29] and inflammation [30]. Increased oxidative stress is seen as one major factor responsible for the impaired regulation of vascular tone due to its effect of diminishing vasoactive nitric oxide [31, 32]. Xanthine oxidase-generated free oxygen radicals interact with endothelium-derived nitric oxide to form peroxynitrite (in itself a highly active oxygen radical), starting a cascade of detrimental oxygen radical effects (Figure 2).

Figure 2. Effects of xanthine oxidase-derived free oxygen radicals on endothelial function. The capillary endothelium is one of the tissues with the highest activity of xanthine oxidase (XO). In CHF, several conditions contribute to increased XO-derived free oxygen radical production. Hypoxia and insulin resistance result in increased substrate supply for the enzyme XO. The balance of the XO/xanthine dehydrogenase (XDH) system is shifted towards increased XO activity. Superoxide anions react with endothelium-derived nitric oxide (NO) to form peroxynitrite (ONOO-) thus initiating a cascade of detrimental oxygen radical effects. Due to NO scavenging, NO-mediated regulation of vascular tone is impaired.

Endothelial dysfunction has been shown to be related to increased scavenging, ie, degradation, of nitric oxide by free oxygen radicals rather than impaired generation of nitric oxide [3]. It should be noted that in humans the tissue with the highest activity of xanthine oxidase (beside the epithelium of the mammary gland) is the capillary endothelium and the endothelium of the small arteries [33, 34].
There is increasing evidence to suggest that the xanthine oxidase metabolic pathway is not merely the final step in purine degradation, with the formation of uric acid as a metabolically inert waste product. In humans the organs with the highest xanthine oxidase activity are the intestine and the liver, with low or undetectable levels in the brain, kidney, lung, and muscle [35]. The localization of xanthine oxidase primarily in the endothelial cells of the capillaries suggests that it is involved in specific functions of the vascular system [33]. Given the capacity to generate free oxygen radicals, this enzyme might have a role in bactericidal defence mechanisms [36, 37], especially at the barrier between intestinal lumen and body tissues. This physiologic mechanism may provide an acute adaptive response to environmental factors. One could hypothesize, however, that long-term stimulation of xanthine oxidase may result in chronic activation of this mechanism leading to maladaptive processes and eventually damaging effects. The latter provides the pathophysiologic link between uric acid generation and a large variety of detrimental processes, including increased cytokine production, cell apoptosis, and endothelial dysfunction, all of which occur in CHF patients [38–40]. Indeed, in a prospective series of studies of patients with CHF, it has been shown that hyperuricemia is a marker of impaired oxidative metabolism and hyperinsulinemia [17], inflammatory cytokine activation [41], and impaired vascular function [42].
The option to therapeutically target raised xanthine oxidase activity in CHF might therefore be an intriguing concept in order to counteract maladaptive chronic upregulation of the xanthine oxidase metabolic pathway. In fact, it has been shown that in CHF patients with hyperuricemia, treatment with the xanthine oxidase inhibitor allopurinol improved endothelial function and peripheral blood flow [43, 44], while markers of free oxygen radical generation were reduced [43]. In a placebo-controlled, randomized, double-blind, crossover study we demonstrated that allopurinol (100 mg once daily), after just 1 week of treatment, reduced uric acid levels by 39% while vasodilator capacity improved in the arm and leg vascular bed by 24% and 23%, respectively [43]. It was found that the treatment-induced reduction of uric acid significantly correlated with the improvement of vasodilator capacity. This might raise the possibility that, independently of the oxygen radicals, uric acid itself may have an adverse effect on the regulation of peripheral vascular tone. This is supported by the finding that high uric acid levels in CHF predict impaired peripheral blood flow and reduced vasodilator capacity [45], independently of renal dysfunction, diuretic dose, and CHF etiology. It has been shown that allopurinol treatment can improve forearm blood flow and endothelial dysfunction also in other conditions such as in type 2 diabetes mellitus and mild hypertension [46]. In the context of reperfusion injury, it is understood that xanthine oxidase-derived free oxygen radicals are a major contributor to impaired blood flow and tissue damage, and that allopurinol may exert protective effects against these reperfusion injuries [47].
Beside its role in affecting peripheral vascular tone, inhibition of xanthine oxidase appears to exert direct myocardial effects in CHF. In animal models, allopurinol reduces myocardial oxygen consumption [48] and improves systolic function [49, 50], resulting in increased myocardial energetic efficiency. Recently, this has been confirmed in human CHF [51]. Although the underlying mechanism is not yet fully uncovered, some authors have suggested a specific effect of allopurinol in sensitizing cardiac myofilaments to Ca2+ [52].

Hyperuricemia as a novel prognostic marker
In addition to its involvement in the regulation of vascular tone, there is increasing evidence to suggest that the xanthine oxidase metabolic pathway has prognostic significance. Recently, it has been shown that in CHF patients high uric acid levels are a predictor of impaired survival, independently of and better than other well-established parameters such as clinical status, exercise capacity, parameters of kidney function, and diuretic therapy [53]. Data have indicated a stepwise increase of mortality risk in parallel with rising uric acid levels (Figure 3).

Figure 3. Risk ratios in CHF subgroups with elevated uric acid levels. A graded relationship is shown between serum uric acid levels and survival in 294 patients with CHF; risk ratios are compared with uric acid £400 mmol/L (unpublished data, proceedings of the Working Group on Myocardial Function of the ESC, Isola, 2003).

This is in line with the findings of a recent retrospective study that examined the effect of allopurinol in CHF on mortality and hospitalization [54]. It was observed that in these patients long-term high-dose allopurinol (³300 mg/day) may be associated with better all-cause mortality (adjusted relative risk 0.59, 95% CI 0.37–0.95, P < 0.05) than low-dose allopurinol (<300 mg/day), assuming a dose-related effect of allopurinol. Treatment with allopurinol is, however, not free of problems. It can induce gout attacks, kidney dysfunction, or skin reactions, and further work is required to confirm the potential benefit of this treatment.

The xanthine oxidase metabolic pathway as part of a complex metabolic web
Abnormal regulation of the xanthine oxidase pathway may be seen in the context of more complex metabolic derangements. Epidemiologic studies have documented an increase in serum uric acid as a characteristic finding in patients with obesity and hypertension [55–57], hypertriglyceridemia [58], and coronary artery disease [59, 60]. The metabolic syndrome is a constant etiologic factor in these diseases and, indeed, recent studies have identified a significant relation between hyperuricemia and insulin resistance — the widely accepted causative factor of the metabolic syndrome [56, 61, 62]. The relationship between hyperuricemia and insulin resistance persisted when differences in age, sex, body mass index, and waist-to-hip ratio were taken into account, and were also seen in normal healthy individuals [63, 64]. Based on these data, an expanded definition of the insulin resistance syndrome was suggested that added hyperuricemia to the cluster of metabolic abnormalities comprising the syndrome [65].
The pathophysiologic connection of hyperuricemia and the insulin resistance syndrome has been hypothesized to result from the accumulation and diversion of glycolytic intermediates towards the pentose phosphate pathway [66].
Its increased metabolic turnover results in accumulation of phosphoribosylpyrophosphate [67], in itself a key precursor of the purine de novo synthesis. Increased substrate supply is followed by upregulated purine metabolism and hence purine production. One branch point for diversion of glycolytic intermediates towards the pentose phosphate pathway is controlled by the enzyme glyceraldehyde 3-phosphate dehydrogenase (GA3PDH). This enzyme catalyzes the only oxidative step in glycolysis and its activity is regulated by insulin [68, 69]. Thus the link between insulin resistance and uric acid production could be mediated by impairment of GA3PDH. Although further work is needed to fully uncover the etiological relationship between hyperuricemia and insulin resistance, the significance of high uric acid levels as an additional risk factor within the metabolic
syndrome has repeatedly been confirmed [70–73].

Summary
The traditional view of CHF as a mere pumping disorder has shifted to a more complex approach including hormonal, immune, and metabolic aspects, and secondary changes. Increasing data suggest that the xanthine oxidase metabolic pathway is a significant contributor to the pathophysiology of CHF, with both symptomatic and prognostic implications.
Preliminary results suggest a beneficial effect of lowering uric acid by xanthine oxidase
inhibition. Whether one could also use uricosuric treatments (to increase excretion of uric acid) or newer, more selective xanthine oxidase inhibitors potentially with fewer side effects in chronic or acute heart failure needs to be established in further studies.

REFERENCES

1: J Biol Chem. 1968 Nov 10;243(21):5753-60.