|1. Life / Biology / Human Anatomy|
They are right inside you, taking care of your body. Steroid hormones are small molecules responsible for the controlling of many reactions. First their formulas:
Steroid Hormones, Chemical Formulas
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HC (_) CH
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R1: -CH(CH3)-(CH2)3-CH(CH3) 2
R1: -OH (-H Desoxycortisol)
R2: -CO-CH2OH R3: -OH
R1: -H R2: -H
R1: -H R2: -CO-CH3
R: -OH (-H)
How Do Steroid Hormones Work?
In a very simplified description hormones are chemical substances that control the biochemistry of an organism by signalling. Each hormone has a very distinct function affecting a very definite region of an organism. For that reason every hormone is synthesised by specialised cells commonly found in various glands (eg, the pineal gland in the brain, the testicles or ovaries, etc.), from where the hormones are injected into the body fluids (eg, lymph, blood, etc.).
Hormones are responsible for many different effects: growth, immune response, hunger, stress and sexual appetite. From a biological or medicinal point of view it is probably more sensible to group the hormones according to their function. From a chemical point of view the hormones are classified according to their chemical resemblance. This means that the hormones of a certain family can have different effects, but their working mechanism is similar. One special family of hormones are the steroid hormones (except for retinoic acid), for which the basic precursor is cholesterol, a molecule infamous for causing cardio-vascular problems.
All of the steroid hormones work inside of the target cells. That means that they must pass through the cell membrane, for which reason they must be actively transported. That is, most steroid hormones cannot be found swirling around the blood in their free form (although some actually are - 10% of cortisol is free in the blood, for example). During transport they are bound by complicated proteins. These proteins work like a mail envelope; it is this envelope that contains the cellular post code, and that will ensure that the hormone gets to the correct destination.
At the membrane the envelope is opened and the hormone 'message' is released into the cells, where it will be recognised by other proteins1, which in their turn will initiate a signal cascade ending in the cell nucleus. There, the signal will stimulate the activation of a specific gene, which will finally lead to the production of another enzyme or a protein. It is this rather complicated mechanism that makes steroid hormones work in a time period of days to weeks.
Questionably Important Details on Some Steroid Hormones
The common names of the steroid hormones are widely recognised and must be learned by heart. Systematic nomenclature as suggested by IUPAC2 is slowly gaining acceptance, but it's still way too complicated to pronounce. Generally steroids with 21 carbon atoms are called pregnanes; those with 19 are androstanes; and those with 18 are estranes. Cholesterol (27 carbon atoms) is a derivative of squalene, which is a substance of the family of the triterpenes. In the table above some important mammalian steroid hormones are shown.
Some Steroid Hormones
Pregnenolone is produced directly from cholesterol, by modification of the R1 chain. Pregnolone is the precursor molecule for all C-18, C-19 and C-21 steroids.
Progesterone is produced directly from pregnenolone and mainly secreted from the corpus luteum (which is a yellow follicle, literally 'yellow body') in the ovary. It is essential for the successful implantation of a fertilised egg and prepares the breast for milk production. The absence of a fertilised egg means that the corpus luteum will shrivel, turning into a bit of white scar tissue (Corpus albicans), thus decreasing the production of progesterone. This decrease initiates menstruation. Modifications of progesterone are included in oral contraceptives along with estrogen. As a precursor for other hormones, like testosterone, it is also produced in other organs, but it is not secreted from there.
Testosterone is an androgen, or male sex hormone. It is secreted by the testes and is responsible for secondary male sex characteristics (low voice, facial hair). Increased testosterone levels during adolescence is responsible for the annoying behaviour of teenagers. Testosterone also stimulates metabolism and accounts for increased muscular development. A higher dosage of testosterone has serious negative effects on our health. In spite of that, some athletes (for example Maradonna or Ben Johnson or the East German women's water polo team) use these hormones to improve performance.
Aldosterone (not depicted) is a mineralocorticoid, ultimately produced from progesterone in the zona glomerulosa of the adrenal gland, situated atop each kidney. It raises blood pressure and fluid volume and increases sodium ion uptake.
Cortisol is also produced in the adrenal cortex; it is synthesised from progesterone. It is involved in stress adaptation and elevates blood pressure and sodium ion uptake. Cortisol has numerous effects on the immune system, which is the reason why it is found in many preparations.
Estradiol3 is the main female sex hormone, produced in the ovary and responsible for secondary female sex characteristics (pubic hair and breasts). Males also produce estradiol in smaller amounts in the adrenal cortex. Estrogen, among other things, prepares the vagina for the menstrual cycle. Ovulation is not actually inhibited by oestrogen; it is stimulated by it. A different hormone, luteal hormone (or LH for short), via a process we don't know about, turns from inhibiting oestrogen release to stimulating it. This turns into a positive feedback loop, where the oestrogen stimulates more LH and vice versa. It all gets to a point where ovulation occurs, and the corpus luteum is formed, at about the 12-14th day in the menstrual cycle. Then LH and oestrogen return to the negative feedback loop.
The contraceptive pill is oestrogen with progesterone-induced bleeds. They fool the body into thinking that it is pregnant, not to prevent formation of the corpus luteum. Of course, when women are pregnant, they don't ovulate, hence why they take not oestrogen, but stuff which is broken down to oestrogen in the body. The induced bleeds are to prevent endometrial cancer - oestrogen stimulates endometrial growth, and the progesterone induces the bleed to prevent it.
1 These are called 'hormone response elements', or HREs.
2 IUPAC is the 'International Union for Pure and Applied Chemistry'; these guys are responsible for stuff like nomenclature.
3 Estrodiol is o/estrogen. Oestrogen stimulates thickening of the endometrium and the formation of endometrial glands, within the uterus (not the vagina; the vagina doesn't have an endometrium which does this).
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