Cholesterol (also known as cholesterol, cholest-5-en-3ß-ol, 5-cholesten-3ß-ol) is a white, almost odourless solid that occurs in all animal cells. The term is composed of the Greek “chole” = “bile” and “stereos” = “solid”, as it was already found in gallstones in the 18th century.
Cholesterol is a vital sterol and a very important component of the plasma membrane in cells, as it contributes to their stability together with other proteins. Furthermore, it serves to transport signal substances into and out of the cells. Since cholesterol is not water-soluble (lipophilic and hydrophobic), it is mainly found in the cells.
The human body contains about 140g of cholesterol. To be able to transport it in the blood, cholesterol is bound to lipoproteins. These are divided into 5 groups according to their different density: In the body, cholesterol serves as a precursor of steroid hormones and bile acids, among other things.
In the process, the cholesterol is converted via precursors into the sex hormones and the adrenal hormones cortisol and aldosterone. The two bile acids cholic acid and glycocholic acid also originate from cholesterol. Furthermore, an intermediate product of the cholesterol biosynthesis, 7-dehydrocholesterol, is used to form vitamin D3 from UV light. – Chylomicrons,
- Very low density lipoproteins (VLDL),
- Intermediate density lipoproteins (IDL),
- Low density lipoproteins (LDL) and
- High density lipoproteins (HDL). – Testosterone,
- Oestradiol and
Uptake and dismantling
Since cholesterol is a vital sterol for humans, over 90% of it is produced by the body itself. In an adult this means a production of 1-2g per day. Cholesterol absorption from the intestine is about 0.1-0.3g per day and can be increased to a maximum of 0.5g.
The basic substance for cholesterol synthesis is “activated acetic acid” (acetyl CoA). After a few steps, isopentenyl diphosphate is produced from this via mevalonic acid (mevalonate). After a ring closure from squalene to lanosterol, the cholesterol is produced after numerous enzymatic reactions.
The cholesterol is excreted via the liver by releasing the cholesterol in the form of bile acids into the gallbladder, from where it reaches the intestines. The body needs bile acids to absorb water-insoluble substances from the intestines. This therefore also includes the absorption of cholesterol.
However, since about 90% of the bile acids are reabsorbed from the intestines, the excretion of cholesterol is relatively ineffective. Some drugs that bind bile acids can increase the excretion of cholesterol. However, this leads to a counter-regulation mechanism in the liver and thus to a relative increase in the synthesis of cholesterol sinus, or increased absorption of cholesterol from the blood.
There are numerous mechanisms that affect the balance between The main mechanism is the HMG-CoA reductase inhibition. This is the most important enzyme in cholesterol biosynthesis and is inhibited by feedback from cholesterol itself. Thus, cholesterol or its precursors inhibit the cholesterol-forming enzyme and the cholesterol synthesis is stopped.
This is the most direct way of cholesterol regulation. In addition, there are numerous other pathways that function at the level of gene regulation. For example, the proteins activate
- Self-produced and
- Required cholesterol. – SCAP,
- Insig-1 and
- Insig-2 in the presence of cholesterol the proteolysis of SREBPs, which then regulate genes that control the synthesis of cholesterol.