Oxidation of urate in human skeletal muscle during exercise

Abstract

The purpose of the present study was to investigate whether high metabolic stress to skeletal muscle, induced by intensive exercise, would lead to an oxidation of urate to allantoin in the exercised muscle. Seven healthy male subjects performed short term (4.39 ± 0.04 [±SE] min) exhaustive cycling exercise. Muscle samples were obtained from m. v. lateralis before and during the first few minutes after the exercise. Venous blood samples were obtained before and up to 45 min after the exercise. The concentration of urate in muscle decreased from a resting level of 0.26 ± 0.023 to 0.084 ± 0.016 μmol·g⁻¹ w.w. (p < .05) during the exercise and then rapidly increased during recovery to reach the resting level within 3 min after exercise. The concentration of allantoin in the muscle increased from a resting value of 0.03 ± 0.007 to 0.10 ± 0.014 μmol·g⁻¹ w.w. immediately after exercise (p < .05) and then decreased to 0.079 ± 0.002 μmol·g⁻¹ w.w. during the first 3 min after exercise (p < .05). Plasma urate levels increased slowly from 305 ± 16 to 426 ± 20 μmol·liter⁻¹ at 45 min in recovery (p < .05). Plasma allantoin was 11.9 ± 2.6 μmol·liter⁻¹ at rest and by 5 min the level was more than twofold higher and remained elevated throughout recovery (p < .05). The present results indicate that urate is oxidized to allantoin in the muscle during exercise, probably due to generation of free radicals. Furthermore, the findings support the suggested importance of urate as a free radical scavenger in vivo. Copyright © 1996 Elsevier Science Inc.

Introduction

Urate has been proposed to be a potent scavenger of free radicals in human and animal tissues. In the presence of ascorbate, urate may scavenge hydroxyl radicals, and thereby this purine provides one of the few biological defenses against this highly reactive oxygen species.1, 2 In addition, urate has been shown in vitro to scavenge peroxides and hypochlorous acid as well as other radicals.2, 3 Several oxidation products of urate have been identified; these include allantoin, parabanic acid, and oxonic/oxaluric acid. Of these, allantoin has been reported to be the predominant oxidation product from the reaction of urate with different radical species.[2]

Studies of urate turnover in humans have revealed that a significant quantity of urate disappears from the body by pathways other than renal excretion. Because humans lack uricase, the enzyme responsible for the oxidation of urate to allantoin, it is believed that the only enzymatic pathway for uric acid oxidation is via uricase present in enteric bacteria.[4] Alternatively, it has been proposed that allantoin present in humans may originate from the free radical induced oxidation of urate.5, 6 Although several in vitro studies have shown that urate may act as an effective antioxidant,5, 7 there are to date few indications of such a role for urate in humans in vivo. Some of the only evidence has been provided by Becker and coworkers,[6] who have shown a release of small amounts of allantoin in the venous effluent from the lungs. In addition, elevations in synovial fluid allantoin in patients with rheumatoid arthritis were interpreted as evidence for the free radical induced oxidation of urate.[8] It is not known, however, how readily allantoin crosses cell membranes. It is quite conceivable that allantoin's highly polar character may retard its release into the plasma, in which case plasma levels may not be a simple reflection of the extent of its cellular formation. Thus, direct measurements of tissue allantoin may be the most sensitive way to establish if urate functions as a radical scavenger in vivo.

It has been hypothesized that short term, high intensity exercise may lead to generation of free radicals in muscle due to metabolic stress caused by the very high rate of energy turnover.[9] This hypothesis has remained untested thus far because studies on free radical formation in exercising muscle to date have focused on moderate exercise of longer duration.[10] In addition, the investigations that have been conducted in the area of free radical generation during exercise have led to inconclusive results that can be attributed to the use of insensitive and/or nonspecific markers for radical formation in vivo, such as plasma and muscle thiobarbituric reactive substances (TBARS). Thus, there are many uncertainties in the area of free radical generation in muscle during exercise, and there is an evident need for markers of radical reactions.

In the present paper, the hypothesis that exercise-induced metabolic stress could lead to the nonenzymatic oxidation of urate to allantoin in the muscle was tested by measuring urate and allantoin in muscle and plasma before and after intense exercise.