Sources and initial stage of biosynthesis
Absolutely all corticosteroids originate from a single molecule — cholesterol ($C_{27}$). Adrenal cortex cells obtain it from two main sources: taking up cholesterol esters circulating in the blood as part of low-density lipoproteins (LDL), or using their own intracellular stores of esterified cholesterol.
The synthesis process is localized in different compartments of the adrenal cortex cells. It is triggered by corticotropin. This regulatory hormone performs a dual function: first, it stimulates the release of free cholesterol from its esters, and second, it activates the corticosteroid synthesis cascade itself.
The first biochemical step is the conversion of 27-carbon cholesterol into 21-carbon pregnenolone. The mechanism of this reaction involves the oxidation of the twentieth carbon atom ($C_{20}$) and the subsequent cleavage of a six-carbon fragment from the side chain of the cholesterol molecule.
Diverging pathways: progesterone and hydroxylases
The resulting pregnenolone ($C_{21}$) is an intermediate link. It is rapidly converted into progesterone ($C_{21}$), which serves as a sort of biochemical "crossroads". The future fate of progesterone and the final product obtained depend on the set of enzymes in a specific cell and the sequence of reactions.
The key enzymes at this stage are hydroxylases. Their task is to sequentially attach hydroxyl (OH) groups to the steroid nucleus.
Main pathways of progesterone conversion:
- Cortisol synthesis ($C_{21}$): the main glucocorticoid is formed as a result of the sequential action of three enzymes. First, progesterone undergoes hydroxylation with the help of 17-hydroxylase, then 21-hydroxylase comes into play, and the process is completed by the action of 11-hydroxylase.
- Aldosterone synthesis ($C_{21}$): to produce the main mineralocorticoid, progesterone is hydroxylated by only two enzymes: first 21-hydroxylase, and then 11-hydroxylase.
- Androgen synthesis: an alternative pathway producing steroids containing 19 carbon atoms ($C_{19}$). A prominent representative of the end product of this pathway is testosterone.
Regulation, transport, and catabolism
The rate at which cortisol is synthesized and secreted is a tightly regulated parameter in the body. This task is entrusted to the hypothalamic-pituitary system, which operates via a negative feedback mechanism: as soon as hormone levels reach the required mark, their further synthesis is inhibited.
Once produced in the adrenal glands, steroid hormones enter the bloodstream. Due to their lipophilic nature, they cannot freely dissolve in plasma, and are therefore transported exclusively in complexes with specific carrier proteins.
Used hormones are subject to utilization. The main site of their catabolism is the liver. It is there that corticosteroid inactivation reactions take place. The main biochemical reactions at this stage are hydroxylation and oxidation of the molecules, which renders them biologically inactive and prepares them for elimination from the body.
Briefly: answers to topical questions
In which zones of the adrenal cortex are aldosterone, cortisol, and androgens synthesized?
The synthesis of aldosterone, cortisol, and androgens is strictly distributed among the three morphofunctional zones of the adrenal cortex.
- Aldosterone is a mineralocorticoid synthesized in the zona glomerulosa.
- Cortisol is a glucocorticoid produced in the zona fasciculata.
- Androgens are produced in the cells of the zona reticularis.
Which specific carrier proteins bind corticosteroids in blood plasma?
The cited sources specifically name transcortin: it transports glucocorticoids — cortisol and corticosterone, as well as progesterone.
The sources also indicate that albumin carries a number of hormones, including aldosterone, but it is described as a carrier protein rather than a specific protein for corticosteroids.
How is aldosterone secretion regulated as opposed to cortisol?
The regulation of aldosterone and cortisol secretion is carried out by different systems.
| Hormone | Regulatory system | Main mechanism |
|---|---|---|
| Aldosterone | Renin-angiotensin system | Activated upon a decrease in pressure, blood volume, or sodium levels |
| Cortisol | Hypothalamo-pituitary system | Negative feedback (suppression of CRH and ACTH secretion) |
Aside from hydroxylation and oxidation, what conjugation reactions occur during the inactivation of corticosteroids in the liver?
During the inactivation of steroid hormones, a sulfation reaction takes place.
- Sulfation is conjugation with sulfuric acid to form sulfates, where steroid hormones act as substrates.
Which hormones belong to the glucocorticoid class besides cortisol?
Besides cortisol, which is the main representative of this group, corticosterone belongs to the glucocorticoid class. This hormone is also transported by the specific protein transcortin.
To remember the enzymes for cortisol synthesis, remember the code "17-21-11" (in descending order of the last digit). Aldosterone synthesis is shorter — "21-11", skipping the first step (17-hydroxylase).
Class and exam questions
From which substance are all corticosteroids formed?
The sole initial substrate for all corticosteroids is cholesterol ($C_{27}$), which is sourced from LDL or intracellular stores.
What determines which specific hormone is synthesized from progesterone?
The final form of the steroid depends on the set of hydroxylases in a specific cell and the sequence of OH-group attachment reactions.
What role does corticotropin play in this process?
Corticotropin stimulates the release of cholesterol from esters and activates the process of corticosteroid synthesis in the adrenal cortex.
Where and how does corticosteroid inactivation take place?
Catabolism of steroid hormones occurs predominantly in the liver. The main inactivation reactions are hydroxylation and oxidation.
What else to study on this topic
- Mechanism of 6-carbon fragment cleavage from cholesterol
- Specificity of blood plasma carrier proteins
- Differences between adrenal cortex hydroxylases
- Formation of $C_{19}$ steroids and testosterone synthesis
- Negative feedback in the hypothalamic-pituitary system
Master the entire topic in the "Biochemistry" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Corticosteroids" and 3,616 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 115 video lessons on the subject
Other topics in the section "Hormonal Regulation of Metabolism and Body Functions"
Free video lessons: biochemistryShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.