Aloof

that conference that included two other proceedings (see Figure I.3), the International Standards for the Prohibited List, for the Laboratories, for Testing and for Therapeutic Use Exemptions (TUE), and models of good practice and guidelines [1-3]. From the three levels of the WADP, only the models of the best practice and guidelines was not mandatory for all Code signatories [3]. The Code and the International standards entered into force on January 2004 in time for the 2004 Athens Olympic Games. At those Games, a significant change within the Prohibited List was the inclusion of gene doping as a prohibited method [1].

Despite the developments in the fight against doping, the beginning of the twenty-first century was also marked by another scandal involving the USA Corporation BALCO (Bay Area Laboratory Co-operative) [9]. BALCO was providing athletes with a new enhancing drug undetectable by the GC/MS screening methods used at that time [9]. That unknown substance was later isolated by the United States Anti-Doping Agency (USADA) from a used syringe and it was found to be a designed anabolic steroid which was called tetrahydrogestrinone (THG) [9]. Alongside with the development of new pharmaceutical enhancing drugs, BALCO also produced combined drugs containing the enhancing substance and its masking agent to prevent an adverse analytical finding [6].

Regarding the WADP, a legal problem still persisted; many governments could not formally take part in legal agreements with nongovernmental entities [1, 16, 26]. This situation was overcome in 2005, with the UNESCO Anti-Doping Convention that led to the first global treaty against doping in sport [16, 26]. The Convention guaranteed the effectiveness of the World Anti-Doping Code and supported the WADA activities, providing the legal framework for governments to act. The Convention came into effect in 2007 [1, 3, 16, 26].

In 2006 WADA started the revision process of the World Anti-Doping Code that was presented and adopted in the third Anti-Doping World Conference in November 2007, held in Madrid [3]. The revised World Anti-Doping Code and its International Standards were implemented in 2009 [2, 3].

As science advances, so the compounds and methods used by athletes that do not respect the spirit of sport advance. Consequently, the list of prohibited substances is forever changing and several compounds and methods are now included. Within these, the use of recombinant peptides, hormone antagonists and modulators, or gene doping seems to be the major future challenge in the fight against doping.

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I.3 THE ROLE OF ENDOGENOUS AND SYNTHETIC AAS IN THE HUMAN BODY

The human body is an extremely complex structure that contains a variety of systems, each one playing a specific role. The perfect functioning of the human body implies that all these systems work together. This means that it has to have the ability to self regulate and that the different organs and tissues that constitute those systems also communicate between themselves. The main control mechanisms, which regulate all aspects of physical life, are the nervous and the endocrine system [27].

With regard to the endocrine system, this consists of glands that release chemical messengers, called hormones, through the blood to act in distant sites [27-29]. Compared with the time of action of the nervous system, the time of action of these compounds in the body is long and, therefore, they are responsible for long-lasting generalized physiological effects, such as growth, development, reproduction and metabolic rate [27, 28].

The hormones will generate different responses depending on the organs in which they operate and although they can be transported through the bloodstream to any cell in the body, only certain cells, called target cells, express highly specific receptors for hormone’s recognition [27, 28]. After the signal recognition, several chemical reactions are triggered inside the target cells, leading to a modification in the output of those cells. A common modification is, for example, the synthesis of specific proteins [27].

Knowing that hormones play a key role in the regulation of the human body, it becomes clear that any alteration of these compounds in the organism will trigger a set of mechanisms that will change the physiological processes of the body.

Hormones can be divided in two classes according to their chemical composition, the steroids and the non steroids hormones that comprises, mainly, proteins and amines [28]. Their effects have long been recognized by athletes that, for several decades, use these compounds to enhance their athletic performance. Within steroids, the male sex hormones, or androgens, and their synthetic derivatives are responsible for the muscular development and strength.

This chapter pretends to illustrate the major aspects concerning the biochemistry of the androgenic anabolic steroids, its mechanism of action and its effects in the human body.

I.3.1 STRUCTURE

As derivatives of cholesterol, the androgens, in which is included the natural male hormone testosterone, possess the backbone of perhydrocyclopentanophenanthrene ring system [30]. The carbon atoms of the anabolic steroids are numbered and the tetracyclic rings are labelled as shown in Figure I.4 for testosterone. Naturally occurring androgens contain in their structure angular methyl groups, which are the methyl groups attached to the C-10 and C-13 carbon atoms, corresponding to the

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ring junction [30]. Other common substitutions are the hydroxyl or carbonyl group at C-3 and C-17 carbon atoms. In addition, an alkyl side chain may be found at the C-17 carbon atom [30].

Figure I.4 – Steroid`s carbon atoms numbering and ring labelling system.

I.3.2 BIOSYNTHESIS AND SECRETION OF ANDROGENS

In the human body, the biosynthesis of most steroid hormones occurs mainly in a specific set of tissues, which are the adrenal cortex gland and the gonads that comprises the testes and ovaries.

Within these specific tissues the biological pathways to produce the steroid hormones are common.

Nevertheless, the secretion of steroid hormones is different in the different tissues, mainly due to the different distribution, level of expression and activities of the involved enzymes [28].

In normal men, the production rate of testosterone is approximately 3-10 mg/ day of which more than 95% is produced in the testes, whereas in women this value is about 10 times lower and it’s mainly secreted in the adrenal cortex [1, 28].

Regarding other androgens, the steroid 5α-dihydrotestosterone (DHT), a more potent androgen than testosterone, is secreted in the testes but most of the circulating DHT is formed in the peripheral tissues by conversion of testosterone, catalysed by the enzyme 5α-reductase [1, 31]. The aromatisation of the A ring of testosterone, catalysed by the enzyme aromatase, to produce 17β-estradiol in the brain and in the male reproductive organs has important consequences in the sexual dimorphism [32, 33].

Epitestosterone, which is the epimer of testosterone, is also thought to be primarily formed in the testes [1].

Dehydroepiandrosterone (DHEA) and androstenedione that can be precursors of other androgens are mostly synthesised in the adrenal cortex as well as the 11β-hydroxyandrostenedione and its metabolites [1, 34, 35].

It is important to stress that steroidogenesis of some of these androgens has also been reported to take place in peripheral tissues, either by interconversion or by complete biosynthesis [34, 35].

CH₃

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I.3.2.1 BIOSYNTHESIS OF CHOLESTEROL

Cholesterol is the biochemical precursor of all steroid hormones [36]. In this chapter it is presented the de novo synthesis of cholesterol, although cholesterol may be obtained from other sources, such as low-density lipoprotein (LDL) and high-density lipoprotein (HDL) as well as from the hydrolysis of esterified cholesterol stored as lipid droplets [1, 37].

The elucidation of the biosynthetic pathway to cholesterol was achieved in the 1960s and it starts with the biosynthesis of mevalonic acid [30, 36]. The first step in the mevalonate pathway involves a Claisen-type condensation between two molecules of acetyl-CoA, catalysed by the enzyme acetoacetyl-CoA thiolase, to form acetoacetyl-CoA [36, 38]. The second step is catalysed by the enzyme hydroxymethylglutaryl-CoA synthase, and it involves an aldol-type reaction between acetoacetyl-CoA and another molecule of acetyl-CoA to produce the hydroxymethylglutaryl-CoA [36, 38]. Mevalonic acid is formed after hydroxymethylglutaryl-CoA reduction with two molecules of NADPH. This last step is catalised by the enzyme hydroxymethylglutaryl-CoA reductase and is the rate-limiting step in cholesterol biosynthesis [30, 36].

The next step in the Mevalonate pathway is the conversion of mevalonic acid, a six carbon compound, into the five carbon isopentenyl pyrophosphate (IPP), after successive phosphorylation and decarboxylation [30, 36, 38]. Following its formation, isopentenyl pyrophosphate suffers an enzymatic isomerisation that gives rise to dimethylallyl pyrophosphate (DMAPP), establishing an equilibrium that makes both compounds available to the cell (see Figure I.5).

Figure I.5 – Biosynthetic pathway to form IPP and DMAPP.

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In document UNIVERSIDAD COMPLUTENSE DE MADRID (página 137-140)