Abstract
Traditionally, the enzyme myeloperoxidase (MPO) was almost exclusively associated with the phenomenon of innate immunity. For almost a century, most of the studies that were carried out on this enzyme were directed towards elucidating the intricate biochemical mechanisms involved in bacterial killing [1]. The unique ability of this enzyme to generate the hypochlorite ion by utilizing hydrogen peroxide, which is produced copiously in activated neutrophils, and chloride ions which is also always available in plenty, was elegantly demonstrated in several studies [2]. This was followed by an era in which the focus shifted to the deleterious effects of the same reaction on the host tissues; the acute inflammatory reaction that sets in and the downstream reactions which inevitably follow, causing damage to membrane lipids, proteins and even nucleic acids [3]. In recent times MPO is being implicated in diseases associated with chronic non-microbial pathological processes, which have no direct link with infection, and, in which oxidative stress and inflammation play dominant roles. This article seeks to provide a bird’s eye view of these two aspects of the action of MPO, namely its protective action against micro-organisms and its role in chronic diseases associated with inflammation. MPO is an oxidoreductase (EC No. 1.11.1.7) which is stored in the azurophilic granules of polymorphonuclear neutrophils [4]. It is a strongly cationic hemoprotein with a molecular mass of 114 kDa. It consists of two identical 72 kDa monomers linked by a disulphide bridge. Each monomer is composed of a light chain and a heavy chain which is glycosylated and also contains the heme lodged in a deep cleft [5]. In the native form the heme iron is in the ferric state. The enzymatic reaction catalyzed by MPO has been shown below
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