Essay title - Investigation of the structure of sterols
The investigation of structure of sterols, which was begun more than 100 years ago, was basically completed only in 1932. Much more interest has been shown in steroids since it was established that this group of compounds includes numerous hormones, substances necessary for the normal life activity if organism which, unlike vitamins, are produced by the organism itself. The investigation of the last few years have shown that steroid hormones, produced by the sex glands and by the adrenal cortex, fulfil various exceptionally important functions in the organism. At the present time it is known that they have an effect on more than sixty different physiological phenomena related to sexual sphere, and on approximately the same number of nonsexual processes. There exist numerous data, indicating the relationship of the steroid hormones to the processes of growth, reproduction, pregnancy and lactation. Steroid hormones are also associated with cancerous psychic disturbances.
The basic chemical characteristics of the female sex hormones of the estrone group and of equilenin is the presence of the aromatic ring. By the reduction of estrone, a mixture of stereosomers is obtained . which possesses the properties of male hormones.
2. Steroids and its different effects
Estrogenic hormones exert an effect on the development of the female sex organs and secondary sex characteristics. In clinical practice estrogenic preparations are used for curing diseases connected with a deficiency in the functions of the ovaries, as well as for stresses resulting from the surgical removal of the latter. They are successfully used in overcoming sterility and climacteric stresses. It is of interest to note that estrogens bring about abundant lactation in sterile cows. In the male hormones the most active is the testosterone.
3. Synthesis of Steroids
Investigations on the synthesis of steroid hormones began around about in year 1940. immediately after their structure was finally clarified. This problem could be solved starting
eiher on the basis of the more complex but accessible cholesterol or other natural substances of this type, or by a complete synthesis of simple compounds. The first method which was found to be the shortest because of the complex structural and stereochemical structure of the steroid molecules.
In spite of the fact that these investigations were partially carried out in industry and that steroid hormones can be obtained at the present time from cholesterol on an industrial scale, the investigations or their complete synthesis have not been curtailed, but are continuing to an even greater extent. One of the causes of the new researches in this direction is the low yields of hormones from cholesterol. In addition, the synthetic method opens up a possibility of obtaining a series of structurally related compounds, the investigation of which may provide an answer to the problem of the relationship between the physiological activity and chemical structure. Finally even though it may not be possible to accomplish a complete synthesis of the natural hormones, still one may axpect to obtain a number of simpler compounds possessing biological activity.
The numerous Investigations undertaken either for the purpose of complete synthesis of steroids, or in order to obtain intermediate products, did not lead to the discovery of new original methods of synthesis of polycyclic compounds. They are all based on the well known methods of Bugo, Darzen, Mannich, Dickman, Diels-Alder and others. Nonetheless, many of these investigations are of general preparative significance, since they have not only improved the above methods, but have considerably broadened the field of their application.
4. Types of Steroids and their functions
The chemical class of steroid is constituted from substances that possess a perhydro-cyclopentan-a-phenanthrene, also called sterane or gonan, structure.
Figure.1 Sterane or Gonan ( Soource : IUPAC , Steroids, 1969)
The history of steroids starts with the compound cholesterol, which is the most common of all steroids.
4.1 Biosynthesis of cholesterol in adrenal cortex.
Cholesterol is mainly synthesised in mammalian organisms in the liver, other places of formation are in the intenstine, adrenal and gonads. Steroids are found in all eucaryontes and are absent in prokaryontes. Of the eucaryonates, only insects and invertebrates are incapable of attaining their own steroid requirement through biosynthesis and have to obtain their steroids from external sources. The biosynthesis of cholesterol is initiated by acetyl-CoA (I), that is subsequently turned over to acetoacetyl-CoA (II).
First the cholesterol esters are hydrolised and transfer of cholesterol by sterol transfer protein in the mitochondria. Steroidogenic acute regulatory protein helps the hydrophobic cholesterol to transfer to the inner mitochondrial memberane, this process is also called the enzymic process.Cholesterol is reserved in lipid droplets as esters and is mainly required for the steroid synthesis. There are some specific biological procedures which are necessary to fulfill it. In the adrenal cortex the steroid synthesis has its own resources and they are known as receptors. The adrenal cells receive two kind of cholesterol which are high density lipoprotein and low density lipoproteins, for lipoprotein complex appoproteins are needed and it helps for the main transfer of cholesterol from liver. Another source and procedure of steroid synthesis is in the adrenal cells itself and it is a biochemical way using acetate.
The figure shows the position of Adrenal glands, its size, cortex, medulla, and pituitary gland in human body. The following is describing the different stages of steroid synthesis in the adrenal cortex.
As mentioned before above about enzymes, because of enzymatic process the chain cleavage enzyme also cytochrome enzymes are transferred then to the pregnenolone. This cytochrome enzymes are used for hydroxilations of the steroids.
Pregnenolone then have to reach to endoplasmic reticulum membranes and there by the conversion process of hydroxysteroid and enzyme , progesterone is formed.
Aldosterone is thought to be biosynthesized from corticosteron. There is a use of 18-hydroxycorticosterone as a help and the process is to be named as dehydrogenation reaction.
An androgen, male sex hormone synthesized in the testes, responsible for secondary male sex characteristics, produced from progesterone
An estrogen, principal female sex hormone, produced in the ovary, responsible for secondary female sex characteristics
Progesteron is hydroxylated at 17α position. Some of the 17α hydroprogesterone is converted to androstenedione by 17αhydroxylase, thereafter, 11 deoxycortisol or 11 deoxycorticosterone moves back to mitochondria which is necessary for 11ß hydroxylation catalysed by 11ß hydroxylating cytochrome p-450 to form cortisol.
whilst 17α-hydroxypregnenolone can be converted to androgens and cortisol. There is, however, considerable interconversion between these two pathways and it should be noted that the final stage in cortisol synthesis takes place back in the mitochondria.
In functional terms, the adrenal cortex is, therefore, not a single endocrine gland since it secretes different steroids with widely different activities and functions. This is achieved by differential expression of enzymes resulting in functional zonation that has anatomical correlates.
All the steroid hormones exert their action by passing through the plasma membrane and binding to intracellular receptors. The mechanism of action of the thyroid hormones is similar; they interact with intracellular receptors. Both the steroid and thyroid hormone-receptor complexes exert their action by binding to specific nucleotide squences in the DNA of responsive genes. These DNA sequences are identified as hormone response elements, HREs. The interaction of steroid-receptor complexes with DNA leads to altered rates of transcription of the associated genes.
5. The mechanism of initiating biological responses
Steroid hormones penetrate the memberane of their target cells due to their lipophility and are then bound by specific steroid receptors. These hormone receptors are present in small
numbers in the endocytic plasma and show high affinity and specify to their ligands. It is assumed that the binding of the hormones cuases several actions in the receptors, such as an allosteric change(allosteric effects are characterized by the change of properties of an anzyme or receptor such as its affinity to its subtrate, that can lead to different effects: a protein from the heat schock protein family dissociates from it, the receptor can now be phosporylized its affinity for specific nucleotide sequences of the DNA (desoxyribonucleic acid) increases and it dimerizes. The conjugation of the steroid to its receptor is a prerequisite for the cell response, these components are ineffective by themselves. The hormone-receptor complex diffuses in to the nucleas and bonds to short nucleotide sequences on the DNA felix that are called hormone response elements (HRE). The HRE control the transcription of the DNA to RNA (ribonucleic acid) by repressing dormant DNA portions. These hormone response
elements are different for each sort of receptor-hoemone complex. i.e each complex can bond to only one specific sequence of the DNA helix. The activation of the HRE through the bonded hormone receptor complex starts the transcription process in which adjacent DNA sections are unfurled and the information needed for proteinbiosynthesis copied to mRNA (messenger ribonucleic acid). The m RNA then diffuse out of the nucleolus to the ribosomes, where they are translated into the corresponding amino acids that finally make up the response protein that is the cell´s answer to the chemical message induced by the arrival of the steroid hormone.
6. Human Sex hormones
Steriod hormones are a class of lipophilic messenger substances that control metabolism, growth and reproduction in all mammals, Significant steroid hormones are testosterone 2, progesterone 7, estrone 13, estradiol 14, aldosterone 28, and cortisole 19. The first five of them are sex hormones, the latter one is an important adrenal hormone.They are secreted from endocrinal glands into the plasma, which transports them to their destinations. When hormones reach their targets, they initiate certain physiological and biochemical responses such as the biosynthesis of proteins,
Physiological effects of the gestagenes :
Contrary to other sex hormones, gestagenes evolve biological functions only in the female organisms. Gestagenes are produced in the corpus luteum before conception, then in the placenta during pregnancy. A continual high level of progesterone during pregnancy
inhibits further ovulation by a negative feedback effect on the secretion of gonatotropines. All gestagenes can initiate in cooperatin with estrigens the secretion of luteinzing (LH) and follicile simulating hormones (FSH) that ultimately initiate ovulation. Gestagenes alone are incapable of starting these process, they are effective in the combined synergetic presence of estrogens.
The concentration of testosterone in plasma has been determined as approximately 7 μg/l. About 80% of this amount is produced in the testes, 10% in the adrenals and the remaining 10% stems from peripherally circulating androsteindione. The concentration of testosterone is much lower in females ca.0.3 μg/l. Testosterone is able to diffuse readily into cells but there is much controversy over which portion of the circulating testosterone is available to tissues. It circulates in blood bound to three proteins. In both sexes the greater proportion is bound to sex hormone binding globlin . Only about 1% of the total circulating testosterone in women and about 2% in men remains unbound.
Androgens establish and maintain the secondary male sex functions. Some specific functions are:
- Function of the reproductive organs (prostate, vesicular gland), Spermatogenesis.
- Secondary sex properties such as hair growth, deep voice, sex specific enzyme patterns,
- Sexual desire
- Formation of the primary sexual organs
They also possess several sexually unspecific properties:
- Protein metabolism, (anabolic effect)
- Effect on bone maturing and growth
- Male behaviour pattern
- Constitution of muscles skin, function of sebaceous glands
Estrogens act on the same organs as the gestagens. The target organs is for both hormones classes the uterus, cervix, vagina and the mamma. Estrogens induct in all mammals estrus by controlling the secreting of the luteinizing hormone (LH) and follicile simulating Hormone (FSH), the hormones that directly initiate ovulatin. Furthermore estrogens maintain growth of the endometrium in the uterus and control the composition of the cervical mucus, during ovulation it becomes more liquified and enables sperm cells to penetrate into the cervix.
7. Steroid functionality in neurosystem
Steroids are produced by the gonads and adrenal glands (fig.2) are producing steroids. Brain, spinal cord and in nervous system receives steroid with the help of blood. There are some main neurosteroids developed in the brain which helps for some main human behavior like sexual behavior and memory, this neurosteroids are called allopregnanolone and pregnenolone sulfate. Pregnenolone sulphate is affecting the deficits related to age in satial memory performance and in different models of amnesia as protective effect. Anticonsultant, anxiolytic, antidepressive and sedative effects in the brain and in the memory are affected by psychopharmacological effects of steroids. The psychopharmacological is modelled by the neurosteransmitter receptors.There are many sites where active novo steroid synthesis is taking place because of the Steroidogenic acute regulatory protein (StAR). The experiments on human and mouse are also therefore evidence for the presence of mRNA and protein in glia and nerous and also neurosteroidgenesis and StAR are results for cholesterol cleavage enzyme. Thus StAr is playing a main role in producing neurosteroids and the change in StAR mainly effects steroidogenesis in gonadal and adrenal.
As we discussed earlier neurosteroids are responsible for physiological process in the central nervous system and its amount in the brain varies according to the situation and behaviours of human ex. Anger,stress, sex etc.which are effected by the hippocampal pregnenolonee. There are main sources of neurosteroid which includes pregnenolone and dehydroepiandrosterone, their sulfates and tetrahydroderivative of progesterone. The synthesis of steroid from above sources takes place in central and peripheral nervous system, partly in myelinating glial cells and cholesterol(steroidat) precursors. Pregnenolone dehydroepiandrosterone their sulfates and tetrahydroderivative of progesterone are playing a role as allosteric modulators of neurotransmitter receptors where progesterone receptors are placed in surface central glial cells. Progesterone is a neurosteroid and its receptors are also found at the border of glial cells. Progesterone helps myelination threw activating the appearance of genes coding for myelin proteins.
It is very much of big interest to know more about neurosteroids concentration and its changes in the nervous system which helps to cure some major treatments regarding functional disturbances in the nervous system. In the early development the steroid helps the plasticity of neurous and glial cells and also important later in the adult nervous system.
8. Biosynthesis and metabolism of steroids hormones
In humans and mammals steroid hormones are produced in particular glands under the stimulation of protein hormones (gonadotrophins) secreted in the anterior pituitary (figure 2). Cholesterol is the precursor of all steroid hormones. Androgens and estrogens can be synthesised from pregnolone. Corticoid hormones can also be formed from pregnenolone and progesterone.Sex hormones are predominantly synthesised in the ovaries and in the tests, and during pregnancy also in the placenta. The Corticoids are synthesised in the adrenal cortex (major hormone : cortisol), where the C21 precursors pregnenolone, progesterone and 17alpha-hydroxyprogesterone as well as some adrenal androgens such as dehydroepiandrosterone and androstenedione, androsterone and the estrogen estrone are also detectable. Estrogens can also be formed in peripheral tissues.
Steroid hormone are usually metabolised to more polar components to facilitate elimination. The most frequently occurring reactions are hydrogenation of the double bonds, hydroxylation, predominantly at C2, C6 and C16, and conjugation of the oxygenated functional groups, mainly with glucuronic or sulphuric acid. Metabolism of steroids shows sexual differences which are mostly attributed to differences in hepatic metabolising enzymes. Hence, different profiles of conjugated and non-conjugated steroids can be observed in excreta.
Plants and microorganisms do not possess animal-like endocrine glands. However, as they also synthesise and metabolise steroids, biosynthetic pathways might exist leading to compounds which show the same structure as the animal hormones.. Many plants are even said to possess a direct route for the synthesis of steroid hormones, e.g. by conversion of sistosterol to pregnenolone and progesterone. Microorganisms have been shown to convert sterols to C19-17-ketosteroids.
9. Effects of Steroid hormones in mammals and humans
Effects of steroid hormones become visible after a time lag of a few hours, which is due to their mode of action. After having passed the cell membrane the steroid have to bind to a cytoplasmatic receptor. The hormone-receptor complex binds to a specific promoter sequence of the DNA in the nucleus to influence the transcription of mRNA. The mRNA codes the synthesis of proteins which may function as enzymes that control metabolic processes. In muscle tissue androgen, estrogen, and glucocorticoid receptors have been detected.Sex hormones are organ-specific rather than sex specific. They influence not only sex organs but also any other tissue or organ of the body. The androgen receptors molecule, for example, is different effects (androgenic, anabolic) are produced by organ-specific patterns of steroid metabolism enzymes or by interaction with receptors of other steroid classes. Thus, androgens can be aromatized to estrogens in peripheral tissues or androgen metabolism can interact with estrogen receptors.The activity of steroid hoemones is also regulated by the proportion of free circulating hormones and hormones which are bound to steroid hormone-binding proteins of the blood. For example, estrogens can elevate the level of progesterone- and glucocorticoid –binding globulin (CBG), which reduces the availability of these hormones.
Progesterone and pregnenolone
The basic precursor pregnenolone is considered a neurosteroid. The progestogen progesterone has a known function in female organisms (preparation for and maintenance of pregnancy). In male organisms it can be considered as just a metabolic intermediate. An exogenous administration of progesterone can reduce the estrogenic effects of simultaneously administered estrogens, probably by suppressing the estrigen receptor concentration. Progesterone is also said to reduce testosterone-induced aggressive behaviour by inhabiting the formation of testosterone metabolites.The main androgen is testosterone. Its androgenic effects can be observed in the animal kingdom, eg in the development of the cock´s crest or the deer´s antlers. Additionally testosterone exhibits protein anabolic effects. In some peripherals tissues (male assessory sex organs, liver, lung, brain, skin, bone and fat cells).
10. Distribution of glycoalkaloids in the plants and Potatoes
Glycoalkaloids are formed in all parts of the plant. The biosynthesis of glycoalkaloids occurs in places with the highest metabolic activity such s young leaves, fruit, flowers period. Ultimately the highest amounts of glycoalkaloids are found in the tubers and roots. Generally, glycoalkaloids are not transported between different parts of plants. Each part is responsible for its own synthesis and degradatioin of glycoalkaloids. The majority of glycoalkaloids in commercial potatoes are found in the first 1.5-2.0 mm of the peel but this can vary from cultivar to cultivar.
Although the nature and relative concentrations of glycoalkaloids are genetically controlled, the total concentrations are certainly influenced by other factors in the pre and post harvest period. During the pre harvest period environmental factors have strong influence on the biosynthesis of glycoalkaloids, especially climatic and seasonal variations. To a lesser extent agricultural practice and soil composition play a role in the biosynthesis. Seasonal factor such as low or high temperature excessive or too little rains, and layk of or too much sunshine raise the levels of glycoalkaloids. It seems that an glycoalkaloid production is the plants reaction to stress.
Long after harvest glycoalkaloids levels can still rise in potato tubers since the biosynthesis goes on for some time. Factors of influence are light, storage conditions and mechanical injury. Some other factors are also of influence but to a much lesser degree. Exposure of the potato to light has a dramatic effect on the amount of glycoalkaloids.
11. Plant Steroids in Transgenic Plants Safe for Humans
For many years, it was presumed that the regulators of growth and development in plants are distinct from the hormones that regulate growth and development in animals. Yet, many of the pesticides used to control insects and fungi were found to disrupt the endocrine systems of animals.
In 1979, steroid compounds called brassinosteroids (BRs) were discovered in the pollen of Brassica napus (rapeseed). Later studies showed that BRs are ubiquitous among plant species. BRs are similar in structure to animal steroid hormones, including ecdysone, progesterone and testosterone. Steroid hormones regulate transcription through nuclear receptors and by membrane bound cell surface receptors. Trans-membrane BRs receptors have been recognized in plants. Recently, nuclear transcription factors regulated by BRS were recognized. Like animal steroids, BRs act on a number of aspects of growth and development, and function in a broad range of disease resistance as part of an innate immunity system.A number of BR inhibitors and analogues have been produced, and BR analogs with high growth promoting activity have been patented. BR analogues proved active in the control of mosquitoes through their resemblance to the insect molting hormone. Synthetic BR analogues selectively inhibited multiplication of herpes simplex virus by interfering with late viral protein synthesis. BR derivatives were also effective in inhibiting replication of the measles virus. These finding suggest that BRs are active in both invertebrate and vertebrate animal viruses including those infecting humans. Might they not interfere with the synthesis or action of human steroid hormones.
As in the synthesis of animal steroids, synthesis of BR is achieved using certain cytochrome p450 molecules. Researchers from the Salk Institute patented transgenic plants altered to either impair or over-express a gene for cytochrome p450 activity. The resulting transgenic plants could be manipulated to produce insect resistance, dwarfism and darker green foliage compared to wild type plants. Another cytochrome p450 gene was isolated from dwarf plants and that gene and protein patented. Cotton plants with improved fibre characteristics resulted from treatment of cottonseeds with BRs prior to planting, or by producing cotton fibre in liquid cell culture. Transgenic cotton plants also produced fibres with improved characteristics when sprayed with BR analogues.
BRs are versatile and allow the production of greater cell mass if desired, as for example in forage. BRs may also provide dwarfism for grain crops to avoid lodging of the crops. BRs appear to be associated with multiple disease resistance. However, manipulation of BRs may create unexpected toxins as byproducts of the modified p450 genes. BR analogues may provide the happy prospect of making crops grow larger, while warding off insect pests and curing genital herpes. But it is imperative that BRs and transgenic crops producing them should be tested thoroughly for untoward toxicity for human beings and other non-target species.
12. Differentiation and similarities in plants, Humans and mammals
The Steroid stucture is widespread in both the animal and the vegetable kingdom.The general structure of steroids. R1 and R2 are usually methyl groups which are occasionally oxygenated. R2 is absent in steroids which have ring A and/or B aromatic. The side chain on C17 (R3) may comprise two four five eight nine or ten carbon atoms. If it is absent, the position is usually oxygenated. The hydroxyl group on C3 may be oxidised frequently occurring double bond position are C4-C5 or C5-C6.
Well-Known compounds in vertebrates with steroid structure are cholesterol, bile acids (e.g. lithocholic acid.), sex hormones and mineral and glucocorticoids, plants contain a variety of steroid including phytosterols (e.g. stigmasterol, sistosterol), cardenolides (e.g. digitoxigenin), digitanoles, sapogenis (e.g. disogenin) and alkaloids (e.g. solanidine, tomatidine). Ecdysteroids are moulting hormones of insects. Fungi contain mycosterols (e.g. ergosterol). Even single-cell organisms contain steroids, e.g. yeasts (ergosterol), algae (fucosterols) or bacteria.
Steroid hormones are natural constituents of almost all organisms, Microorganisms, Plants, animals, and humans all possess steroid synthesis and metabolism enzyme system. Nutrition regularly provides humans with certain amount of steroid hormones, which is negligible compared to the amount synthesized by humans themselves. Thus, no harm arising from dietry steroid hormones can be expected. Steroid hormones are of paramount importance to the maturity of an organism and are the factors which counteract premature senescence. There is no important function of the human or animal organism which is not affected by the action of some of the steroid hormones.
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