Steroid hormone
Steroid hormones are steroids that act as hormones . They include the sex hormones of the gonads and the corticosteroids of the adrenal cortex . Mammalian steroid hormones can be divided into five groups according to the steroid receptors to which they bind: glucocorticoids , mineralocorticoids , androgens , estrogens and gestagens .
Substance group
Steroid hormones are derived from cholesterol and are therefore readily soluble in fat and poorly soluble in water. In contrast to the other hormone groups, this allows them to enter the cell directly and do not need a second messenger . There they bind to their corresponding receptors and can thus develop their effect. The steroid-receptor complex then reaches the cell nucleus , where it can influence the cell's metabolism. In the cell nucleus, it effects the transcription of specific DNA segments, so that, for example, other structural proteins are produced, B. can permanently change the membrane properties of the cell. Due to their lipophilicity, steroid hormones are able to cross the blood-brain barrier . When transported via the bloodstream, steroid hormones are dependent on plasma proteins and special transport proteins because of their water-repellent properties .
metabolism
Pregnenolone , which is the central starting compound for steroid hormone synthesis, is produced from cholesterol .
Cholesterol provides the sterile framework from which the steroids owe their name. Mineralocorticoids , glucocorticoids and sex hormones are produced from cholesterol in three different ways . This happens in the adrenal cortex and in the male and female gonads ( testicles and ovaries ). Testosterone (i.e. male sex hormone ) is first produced in the ovary , which is then converted into estradiol using an aromatase (enzyme that makes a carbon ring aromatic) .
They last from a few hours to days, after which they are broken down again in the liver .
The enzymes that catalyze the individual steps from cholesterol to steroid hormones can be disrupted by genetic defects. The 21-hydroxylase deficiency is relatively common . This leads to an overproduction of sex hormones, as the path to cortisol and aldosterone is disturbed. The disease is called adrenogenital syndrome .
Control mechanism
The release of steroid hormones is controlled by the hormones of the pituitary gland , which in turn is controlled by neurons of the hypothalamus (hypothalamo-pituitary control circuit). There are so-called negative feedback mechanisms, i.e. H. when from the body periphery, e.g. B. the adrenal cortex, if too much is produced, the hypothalamus and pituitary gland register this, they release fewer trophic hormones (hormones without their own direct effect, but other endocrine glands that stimulate hormone release) of the glycoprotein and peptide type (e.g. ACTH) which means that the peripheral organ also synthesizes less.
physiology
In humans, the steroid hormones can be categorized as follows:
- Glucocorticoids : their most important physiological representative is cortisol . They have metabolic regulatory tasks in the sense of energy supply (e.g. raise the carbohydrate level in the blood; hence the name) and organ physiological tasks: they prevent e.g. B. an excess of immune and inflammatory reactions of the body, they are therefore used in medical therapy for immunosuppression and have regulatory functions in growth (promote bone loss → osteoporosis ), cardiovascular system (increase in blood pressure), day-night rhythm and induce stress reactions. That is why cortisol is often referred to colloquially as the stress hormone.
- Mineral corticoids : the most important physiological representative is aldosterone . The tasks are to regulate the water and electrolyte balance. In the complex regulation of the water and electrolyte balance, salt and water retention in the body play a role (see also: Renin-Angiotensin-Aldosterone System )
- Sex hormones : a distinction must be made here between female and male sex hormones, whereby physiologically both female and male individuals have both. The sex hormones are mainly, but not exclusively, synthesized by the gonads.
- Androgens : The male sex hormones are also known as androgens . The most important physiological representative of androgens is testosterone , or the more active dihydrotestosterone, which is only formed from other androgens in the target cell. There are also less active forms, mainly from the adrenal cortex. Testosterone causes the development of the male phenotype (appearance) and male behavior. It is therefore increasingly released during embryogenesis (origin of the living being in the uterus) and during puberty. Androgens cause the phenotypic differentiation between male and female already in embryogenesis, which depends exclusively on the presence or absence of the effect of male sex hormones. For example, the development of the external genitals: in the presence of the androgens, the penis and scrotum, into which the testicles immigrate, develop from the system for the sexual organs. In their absence, the same appendix creates the clitoris and labia without testicle (or ovarian) descent together with an external vaginal appendix. The assumption that the presence of a Y chromosome is decisive for the external appearance and nature of an individual is incorrect. This is only relevant in the development of the testicle, which is naturally the main producer of androgens. (See also Disorders of Sexual Differentiation .)
- Female sex hormones: a distinction is made between estrogens (estrogens) and gestagens . Estrogens : the most important physiological representatives are estradiol and estratriol. Together with progesterone, they are responsible for regulating the female cycle and pregnancy . They also regulate the maturation and growth of mainly the internal female genital organs and the maturation of the secondary female sexual characteristics during puberty. Furthermore, they are responsible for the termination of bone growth (closure of the epiphyseal plate) in both sexes. The only physiological progestin is progesterone . Besides cycle regulation, it is the most important pregnancy-maintaining hormone. Both estrogens and progesterone are produced in the ovaries, and to a lesser extent by the adrenal cortex. The fact that these hormones control the female cycle in addition to the pituitary hormones (FSH and LH) is used in oral contraception ( birth control pills ), whereby the woman takes man-made hormones that are chemically derived from estradiol and progesterone.
Medicinal use
Steroid hormones are also used as medicinal substances . Examples of this are countless others, the birth control pill and corticoid-containing (“cortisone” -containing) preparations.
Web links
Individual evidence
- ↑ Lois Jovanovic, Genell J. Subak-Sharpe: Hormones. The medical manual for women. (Original edition: Hormones. The Woman's Answerbook. Atheneum, New York 1987) From the American by Margaret Auer, Kabel, Hamburg 1989, ISBN 3-8225-0100-X , p. 386.