The pancreatic lipases (triacylglycerol acyl hydrolases) are enzymes secreted by the pancreas into the small intestine and are responsible for the digestion of triglycerides in the diet consumed, producing free fatty acids and glycerol.
In other words, they are enzymes that digest fats, especially neutral fats, which are the most abundant in food (triglycerides). These fats consist of a glycerol nucleus to which three fatty acid molecules are esterified.
Graphic diagram in which the process of absorption of triglycerides through the wall of the digestive tract is represented. Pancreatic lipase converts triglycerides into monoglycerides and free fatty acids (Source: Posible2006 Via Wikimedia Commons)
Other enzymes that break down fat are also contained in pancreatic secretions, known as phospholipases A and B, capable of breaking down the fatty acid of lecithin and isolecithin, respectively.
The pancreas is a dual-function organ; On the one hand, it secretes hormones that have to do with the metabolism of carbohydrates (insulin and glucagon) and, on the other, it secretes enzymes for digestive function such as lipases (which digest fats), proteases (which digest proteins) and amylases (which digest carbohydrates).
Unlike proteases, pancreatic lipases are secreted into the small intestine as active proteins and their activity can be increased in the presence of bile acids and other compounds.
Pancreatic juice is not only composed of enzymes, but also contains fluid and other chemical components, such as bicarbonate for example, all synthesized by cells other than the pancreas and under strict regulatory mechanisms.
Some pancreatic diseases are characterized by an enzyme deficiency with normal fluid secretion or vice versa, that is, a deficit of fluid secretion and normal enzyme secretion.
Structure
In humans, pancreatic lipase is an enzyme composed of a single polypeptide chain, with a molecular weight close to 50 kDa, similar to the enzyme in cattle, sheep and pigs.
It is a glycoprotein that has mannose, fucose, galactose, glucose and N-acetyl glucosamine residues in its carbohydrate portion. In humans, two isoenzymes of pancreatic lipase have been proposed with isoelectric points of 5.80 and 5.85, respectively.
According to some studies, this enzyme is encoded by a gene that has about 1,395 nucleotides, whose translational product corresponds to a molecule of about 465 amino acids.
The N-terminal end of the fully processed and mature protein is preceded by a sequence or signal peptide of 16 hydrophobic amino acids, which play an important role in the translocation of this enzyme after its synthesis.
The human enzyme has an active site located at the C-terminal end, where there is a triad of amino acids: Asp-His-Ser, of which serine seems to be the most important catalytically speaking.
Activation and inhibition
This enzyme is secreted in its active form, but its activity increases in the presence of amino acids, calcium ions, and bile salts. Bile salts, specifically, are responsible for reducing the pH of the intestinal lumen from 8.1 to 6, which is the optimal pH for the enzyme.
Some authors point out that if the concentration of bile salts increases too much, pancreatic lipase is inhibited, but this inhibition is counteracted or reversed by another enzyme, colipase, which functions as a cofactor of pancreatic lipase and is encoded by different genes. at first.
However, some authors affirm that pancreatic lipase, as well as phospholipases, are, in fact, synthesized and secreted as inactive “zymogens”, which require proteolytic digestion carried out by the enzyme trypsin, also contained in the pancreatic juices.
The excessive presence of salts of heavy metals such as copper, iron and cobalt has also been shown to be inhibitory for pancreatic lipase. The same as the presence of halogens, iodine, fluorine and bromine.
Features
The main function of the pancreatic lipase enzyme is to promote the intestinal digestion of triglycerides ingested with the diet, a function it achieves by hydrolyzing these compounds and releasing a mixture of diglycerides, monoglycerides, free fatty acids and glycerol molecules.
Pancreatic lipase generally hydrolyzes the bonds at positions 1 and 3 of the triglycerides it attacks; it also catalyzes the digestion of some synthetic esters and, in both cases, it can only do so at the interface between water and fats, so the finer the emulsion is, the greater the lipase activity.
Graphic scheme of the fats ingested through the diet. In the intestine these are emulsified in the duodenum by amphipathic solutions in the form of bile salts and phospholipids, which can then be attacked by pancreatic lipase (Source: Cruithne9 Via Wikimedia Commons)
The first step in the digestion of fats in the small intestine is their "emulsification" in the intestinal fluid, due to the presence of bile salts from the liver and intestinal peristaltic movements.
It is important to note that, in the process of digestion of fats, short chain free fatty acids (between 2 and 10 carbon atoms) and glycerol molecules are rapidly absorbed through the intestinal mucosa.
Triglycerides, usually characterized by the presence of long chain fatty acids (with more than 12 carbon atoms), are digested by pancreatic lipases once they have been "accommodated" in structures known as micelles, the product of emulsification.
Normal values
The pancreas, like every organ in the body, can be subject to different diseases of infectious, inflammatory, tumor, toxic or traumatic origin, which can have serious implications for systemic functioning.
The enzymes amylase and pancreatic lipase are often used as serum indicators of some pathologies related to the digestive system and its accessory glands.
It has been found that usually high levels of lipases in the serum can be due to pancreatitis, and the same has been proposed with respect to another enzyme produced by the pancreas, amylase.
Normal values for pancreatic lipase in humans are between 0 and 160 U / L in plasma, while a figure greater than 200 U / L is a value at which the presence of pancreatitis or another pancreatic condition is suspected..
The level of pancreatic lipase may not only increase in the serum due to chronic or acute inflammation of the pancreas (pancreatitis), but it can also be an indication of pancreatic cancer, severe gastroenteritis, duodenal ulcers, HIV infections, etc.
It can also occur in people with familial lipoprotein lipase deficiency.
References
- Best, CH, & Taylor, NB (1961). The physiological basis of medical practice (Vol. 541). Baltimore: Williams & Wilkins.
- De Caro, A., Figarella, C., Amic, J., Michel, R., & Guy, O. (1977). Human pancreatic lipase: a glycoprotein. Biochimica et Biophysica Acta (BBA) -Protein Structure, 490 (2), 411-419.
- Frank, B., & Gottlieb, K. (1999). Amylase normal, lipase elevated: is it pancreatitis? The American journal of gastroenterology, 94 (2), 463.
- Hall, JE (2015). Guyton and Hall textbook of medical physiology e-Book. Elsevier Health Sciences.
- Marcin, J. (2017). Healthline. Retrieved December 26, 2019, from www.healthline.com/health/amylase-and-lipase-tests
- Ruch, TC, Patton, HD, & Howell, WH (1973). Physiology and biophysics. Saunders.
- Winkler, FK, d'Arcy, A., & Hunziker, W. (1990). Structure of human pancreatic lipase. Nature, 343 (6260), 771.