Structure and function of xanthine oxidoreductase: where are we now?

Abstract

Xanthine oxidoreductase (XOR) is a complex molybdoflavoenzyme, present in milk and many other tissues, which has been studied for over 100 years. While it is generally recognized as a key enzyme in purine catabolism, its structural complexity and specialized tissue distribution suggest other functions that have never been fully identified. The publication, just over 20 years ago, of a hypothesis implicating XOR in ischemia-reperfusion injury focused research attention on the enzyme and its ability to generate reactive oxygen species (ROS). Since that time a great deal more information has been obtained concerning the tissue distribution, structure, and enzymology of XOR, particularly the human enzyme. XOR is subject to both pre- and post-translational control by a range of mechanisms in response to hormones, cytokines, and oxygen tension. Of special interest has been the finding that XOR can catalyze the reduction of nitrates and nitrites to nitric oxide (NO), acting as a source of both NO and peroxynitrite. The concept of a widely distributed and highly regulated enzyme capable of generating both ROS and NO is intriguing in both physiological and pathological contexts. The details of these recent findings, their pathophysiological implications, and the requirements for future research are addressed in this review.

Introduction

Xanthine oxidoreductase (XOR) is a complex molybdoflavoenzyme that is readily available from cows’ milk, where it forms a major component of the milk fat globule membrane (MFGM) [1], [2]. Largely because of this availability, XOR has been known for 100 years and studied in its essentially pure form for over 60 years [3]. The enzyme has consequently become a model for structural and mechanistic studies of molybdoenzymes in general, involving the application of electron spin resonance, x-ray absorption fine structure, and many other physicochemical techniques [4], [5]. The last few years have seen a dramatic increase in molecular information, with determination of its cDNA and gene sequences and publication of its three-dimensional structure.

XOR is generally recognized as the terminal enzyme of purine catabolism in man, catalyzing the hydroxylation of hypoxanthine to xanthine and of xanthine to urate. Until recently, little was known of the human enzyme, although inherited XOR deficiency, xanthinuria, has long been recognized, and, puzzlingly, known to be asymptomatic [6], [7]. Some 20 years ago, attention was directed toward the physiological and pathological significance of XOR by Granger, McCord, and colleagues, who proposed a role for XOR-derived reactive oxygen species (ROS) in ischemia-reperfusion (IR) injury [8], [9], [10]. Their hypothesis, which generated several hundred publications, undoubtedly helped to stimulate interest in XOR as a source of ROS, not only in many pathological states, but also in signal transduction generally. As recognition of the pathophysiological involvement of XOR increases, so does the need to understand its enzymology, kinetics, and control. As noted above, our structural knowledge of XOR has markedly increased in recent years. Moreover, a great deal of new information has become available concerning species and tissue variations of the enzyme, its substrate specificity, and regulation. For these reasons, it seems to be appropriate to review recent developments and to attempt to inter-relate them.

In general, the present survey focuses on works published in the last 10 years, and this is reflected in the reference list, which, wherever possible, covers earlier work by quoting more recent, especially review, articles. In view of the potential clinical relevance of much of the work described, emphasis is given to studies using mammalian, particularly human, enzyme and tissues.