Bile acids are detergent substances, that is capable of dispersing lipids insoluble in water in aqueous solution. For this reason, bile acids play a leading role in the digestion and absorption processes of lipids.
Bile acids are produced by the liver from cholesterol and - together with their conjugates and respective salts - are the main constituents of bile.
Primary bile acids (produced by the liver)
The "7-α-hydroxylase enzyme initiates that series of biochemical transformations which, starting from cholesterol, lead to the synthesis of primary bile acids: l"cholic acid and from "chenodeoxycholic acid (or simply chenico).
7-α-hydroxylase represents the limiting enzyme in the synthesis of bile acids.
Conjugated bile acids
In bile, chenodeoxycholic and chenodeoxycholic acids are largely conjugated with two amino acids, glycine and taurine (with a ratio of about 3: 1), and as such they are called glycolic acids, taurocholics (more abundant), glycochenodeoxycholic And taurochenodeoxycholic. This conjugation increases the water solubility of the bile acids.
Bile salts
Since bile is an alkaline liquid rich in sodium and potassium, it is believed that primary bile acids and their conjugates are largely present in the form of salts (mainly sodium).
Functions of bile
In the interdigestive phase, the bile - synthesized by the liver - is concentrated in the gallbladder. Once poured into the intestine as needed, thanks to the primary bile salts and other amphipathic substances (phospholipids and lecithins), the bile facilitates digestion and the absorption of fats and fat-soluble vitamins. With its alkalinity, bile neutralizes the frankly acid pH of gastric secretions (HCl); it also stimulates intestinal peristalsis and exerts an antiseptic action against the bacterial flora, inhibiting putrefactive phenomena. Through the bile the products deriving from the degradation of hemoglobin (bilirubin), substances with a toxic or pharmacological action and others of an endogenous nature (thyroid hormones, estrogens, etc.).
Secondary bile acids (produced by the intestinal bacterial flora)
In the intestine, the bile acids are partly deconjugated and dehydroxylated by the enzyme 7-α-dehydroxylase produced by the bacterial flora of the intestine. The products of these reactions are called secondary bile acids and are mainly represented by the "deoxycholic acid and from "lithocolic acid, respectively derived from cholic and chenodeoxycholic acid.
In total, most (94-98%) of the bile acids present in the intestine are reabsorbed and returned to the liver via the portal circulation. In the small intestine and in the colon there is a passive reabsorption that becomes active only in the terminal ileum (the final portion of the small intestine). Only a small part of the bile acids is eliminated with the faeces; this amount is mostly represented by lithocolic acid, poorly reabsorbed .
Once reabsorbed, bile acids reach the liver where they are recycled and secreted again in the bile (enterohepatic circulation of bile acids). Furthermore, their concentration influences the ex-novo synthesis of bile acids, which is all the more stimulated the lower the amount of recyclable bile acids (secondary ones reabsorbed in the intestine), and vice versa.
Resins sequestering bile acids (see cholestyramine)
As stated in the previous paragraph, a drug capable of limiting intestinal reabsorption of bile acids stimulates their synthesis from scratch. Since this process uses the cholesterol present in the body, these drugs lower cholesterol.
Bile acids in the blood, high bile acids
The bile salts escaped the hepatic uptake determine the concentrations present in the blood; for this reason hepatocellular damage prematurely reduces the hepatic uptake of bile acids (in particular from the blood coming from the intestine). High blood levels of bile acids, and in particular secondary ones, are therefore recorded in the presence of hepatitis A, hepatitis B, infectious mononucleosis, cirrhosis, liver tumors and drug- or alcohol-induced liver disease.
The levels of bile acids in the blood, and in particular the primary ones, typically increase in cholestasis, as when - for example - a stone prevents the outflow of bile into the intestine. The same condition occurs in some women during pregnancy, a because of the characteristic hormonal changes that accompany it.
