**1. Introduction**

For a long time before the discovery of glutathione S–transferases (GSTs; EC 2.5.1.18), it was a well known fact that some orally administered electrophilic compounds ultimately be‐ come excreted in the urines as a conjugates of N- acetyl cysteine, the so called mercapturic acids. Glutathione was then identified by [1] to be the source of cysteine used for biosynthe‐ sis of the mercapturic acids. As a consequence, the GSTs were discovered as enzymes cata‐ lyzing the first step in the formation of mercapturic acids. The first paper on GSTs was presented by [2], who described the partial purification and some properties of cytosolic rat liver enzymes capable of catalyzing the formation of GSH conjugation with halogenated ar‐ omatic compounds. GSTs form a group of ubiquitous enzymes that catalyze the conjugation between glutathione and several molecules, and play the most important role in the cellular detoxification pathway of endogenous and xenobiotic compounds [3].

GST family classified based on primary structure, substrate specificity and immunological properties. Presently, seven classes of GSTs are recognized in mammals, namely the specific Alpha, Mu, Pi and the common Sigma, Theta, Zeta and Omega. The classification of GSTs into different classes is also reflected in the chromosomal location of the genes. In human, each class is encoded by genes organized into clusters on different chromosomes. For exam‐ ple, the genes of all known class Mu GSTs are clustered on chromosome l, the genes of the class Alpha, Pi and Theta are clustered on chromosomes 6, 11, 22, respectively [4]. Polymor‐ phisms have been identified in the GSTM1, GSTT1 and GSTPl genes coding for enzymes in the μ, θ, and π classes, respectively. The GSTM1 and the GSTT1 genes are polymorphic in humans, and the phenotypic absence of enzyme activity is due to a homozygous inherited deletion of the gene [5-7].

© 2013 Shahein et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Shahein et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Ticks are blood sucking ectoparasites that infest a wide array of species. They are vectors of diseases in humans and other animals. The southern cattle tick, *Rhipicephalus microplus*, transmits the cattle fever pathogen (*Babesia spp*.) and is one of the most important cattle pests. Chemical pesticides continue to be the primary means of control for ectoparasites on livestock. Intensive use of these materials has led to the development of resistance in Rhipi‐ cephalus ticks to all currently used organophosphates [8], synthetic pyrethroids and ami‐ dines [9]. Despite previous studies that suggested increased detoxification [10] and target site insensitivity may contribute to the increased tolerance to acaricides, the mechanisms conferring resistance on ticks are poorly understood.

ing consensus sequences or using the available information from whole genome sequence information. Niranjan Reddy et al. [12] studied the GST superfamily organization in *Ixodes scapularis* using the whole genome sequence information (IscaW1.1, December' 2008) by ap‐ plying different phylogenetic and bioinformatic tools. They identified all the three broad GST classes, the canonical, mitochondrial, and microsomal forms. A total of 35 GST genes belong to five different canonical GST classes, namely Delta (7 genes), Epsilon (5), Mu (14), Omega (3), and Zeta (3 genes) GST classes, and two mitochondrial Kappa class GST genes, and a single microsomal GST gene were found. The analysis of these sequences identified members of the Delta- and Epsilon-classes which are thought to be specific to the Insecta. Surprisingly, Ixodes has lost two of the functionally important gene families, Theta-and Sig‐

Glutathione *S*-Transferase Genes from Ticks http://dx.doi.org/10.5772/52482 269

GSTs had been reported to play a major role in the organophosphate resistance pathway of the *Musca domestica* (Corrnell-HR strain) [13]. On the contrary, Li et al. [14] reported that GSTs play only a minor role against organophosphate toxicity in *R. microplus*. Several GST coding frames had been cloned from *R. microplus* as done by [15] (accession number AAL99403), and [16] described that the activity of this protein is enhanced by organophos‐

Some GST genes were cloned from different tick species and are of the mu class. The conser‐ vation score is represented in figure 1, and three state secondary structure is in figure 2. However, several attempts were carried out to explore the distribution of the different GST classes in ticks. The most widely distributed and economically important; the cattle tick *R. microplus* was used to initiate a study of the genome using an expressed sequence tag (EST) approach [17]. They reported the construction of a gene index named BmiGI from 20417 ESTs derived from a normalized cDNA library. The BmiGI was used to identify genes which

Gurrero et al. [17] reported 15 possible GST coding genes identified from the BmiGI. One of these sequences was reported to be similar to the human GST class Omega 1, and the other clone was similar to mouse GST of Zeta 1 class. The total 15 clones are listed in table 1.

GSTs are dimeric proteins composed of identical or structurally related subunits. Each subu‐ nit has a molecular weight of about 25 kDa and is built of two domains and contains a com‐ plete active site consisting of a highly conserved G-site (GSH binding site) and a divergent H-site (Hydrophobic substrate binding site). The functional soluble enzymatic forms are found in dimers and only subunits within the same class can form heterodimers as found in

The nature of protein folding mechanisms and the manner in which the compact native state is achieved are still not well understood. From a wide range of experiments, it is now evi‐ dent that specific pathways of folding are involved, at least for many proteins. At equilibri‐

**3. Unfolding/refolding of** *Rhipicephalus annulatus* **GST mu class**

alpha subunits, but this would not happen with either pi or mu subunits.

might be involved in the acaricide resistance including GSTs.

ma-GSTs.

phate and coumaphos.

In the past years, significant advancement has been made to determine the potential role of GSTs in toxicology. Besides the well established role of GSTs in detoxification of xenobiotic compounds, it has been observed that GSTs have other intracellular substrates including the metabolites released from cellular molecules. In ticks, GSTs have attracted attention because of their involvement in the defense towards insecticides mainly organophosphates, organo‐ chlorines and cyclodienes. This chapter will give highlight on some of the cloned GST genes in ticks and will discuss and review the folding and unfolding states of a GST mu class from the cattle tick *Rhipicephalus annulatus* distributed in Egypt.
