**1. Introduction**

The paraoxonase (PON) multi-gene family consists of three enzymes: PON1, PON2 and PON3, the genes for which are located adjacent to each other on human chromosome 7q21.22 [1]. The amino acid sequences of the three forms are considerably similar—within the range of 79–95%. All PON genes have nine exons, eight introns and TATA-less promoters. This enzyme came into focus with the notion that PON1 protects low-density lipoproteins (LDL) and high-density lipoproteins (HDL) from lipid peroxidation by the virtue of its arylesterase, lactonase and paraoxonase activities [2]. Paroxonases are HDL-associated, and their antioxidant property plays a vital role in prevention of various microvascular complications due to oxidative stress and also provides protection from various toxic chemicals. All PON family enzymes require calcium to exhibit the action and are able to delay lipid peroxidation in lipoproteins and cell membranes. Thus, polymorphism or any other imbalance in the activity of PON has been suggested to play a role in the pathogenesis of rheumatic diseases, cancers and cardiovascular diseases—i.e. the diseases having a chronic inflammatory component [3, 4]. Although these enzymes have been named paroxonases, the paroxonase activity is significant only for PON1; in others the activity has not been detected. PON1 is a hydrolase and can hydrolyse a wide range of substrates including organophosphorus triester pesticides, lactones, thiolactones, cyclic carbamates, nerve gases like sarin, soman, arylesters, aromatic carboxylic acid and unsaturated aliphatic esters, estrogen-esters, and glucuronide drugs [5].

PON enzymes synthesized in the liver and distributed throughout the human body. They are present in different tissues, and are associated with cell membranes and some lipoproteins, although literature reports free enzyme found in the blood. Paroxonases were named after the ability of PON1 (first to be discovered) to hydrolyze paraoxon, a compound of the organophosphate insecticides class, to the metabolite p-nitrophenol. In vivo, paraoxon, the most toxic form, is an oxidized product of biotransformation of parathion, organophosphate insecticide in the context of which it was found first. As explained above, the name can be said to be a misnomer.

Of the three members of the family, PON1 is the most studied one. It is a circulating Ca2+ dependent enzyme with a molecular mass of 43 kDa and containing 354 amino acids and is classified as an aryldialkyl phosphatase [4]. Synthesis of PON1 is mainly hepatic. From liver, it is secreted into the bloodstream, where it is tightly bound to HDL particles [6]. Structurally, it is a six-bladed beta-propeller with a central tunnel containing two calcium ions—a structural one which is necessary for the conformational stability of the enzyme, and a catalytic one. Addition of EDTA (which removes calcium by complexing with it) resulted in the inactivation of paraoxon and phenyl acetate hydrolysis of PON1, showing that they are Ca2+-dependent activities. However, there was no effect on the ability of PON1 to protect low density lipoprotein (LDL) from oxidation, thus implying that the antioxidant property of PON is independent of Ca2+ [7, 8]. This leads to the possibility of existence of different active sites on PON1 for those dependent on Ca2+-dependent, and for those independent of the same, like protection against oxidation [9]. The amino terminal end of the protein contains hydrophobic amino acid residues that play a role in its binding to HDL and to other proteins such as apoA1 as well as in its self-aggregation. Recently, it has been shown that modulating the hydrophobicity of PON1 can affect organophosphatase activity of the enzyme [10].

A histidine-histidine (His) catalytic dyad is proposed to be involved in the catalytic mechanism of PON1 in which His-115 acts as a general base to deprotonate a single water molecule while His-134 increases His-115 basicity via a proton shuttle mechanism; however, some researchers found that it may participate in the substrate binding and/or orientation [11, 12]. Due to such a wide range of activities as well as being the first to be discovered, PON1 is the most studied one compared to other members. PON1 is thought to play an important role in a variety of disorders including metabolic syndrome, diabetes, atherosclerosis which results in cerebrovascular and cardiovascular events, because it is closely associated with the prevention of oxidative stress and inflammation, and is a determinant of HDL dysfunctionality. There are evidences suggestive of its atheroprotective effects through various mechanisms—maintaining cholesterol homeostasis, regulating cholesterol efflux from macrophages, as an effective xenobiotic metabolizer, and by participating in endothelial homeostasis [5, 13–16].

N-acylhomoserine γ-lactones (AHL) are produced by gram negative bacteria and regulate bacterial virulence and biofilm formation. All three PONs hydrolyse AHL with PON2 having the greatest efficacy, the resulting metabolites are inactive therefore the PON family could be important in preventing bacterial infections [17].
