Generality
Lipases are water-soluble enzymes that catalyze the digestion of food lipids, breaking down the ester bond that binds the hydroxyl groups of glycerol to long chain fatty acids.
In the absence or lack of lipase, the absorption of fats does not occur correctly and part of the food lipids pass into the feces causing steatorrhea (abundant emission of pasty droppings, with a shiny and shiny appearance).
Synthesis
Unlike amylases, which in the upper tract of the digestive tract are secreted only by the salivary glands, lipases are released both in the oral cavity and in the gastric gland.
Furthermore, the lingual lipase, which is secreted in the posterior region of the tongue, is active in a wide spectrum of pH (2-6) and can therefore continue its activity even in the acid pH of the stomach (as opposed to the ptyalin which operates preferentially to pH between 6.7 and 7).
Fat digestion
Gastric and lingual lipases attack triglycerides (which represent about 90-98% of food lipids), detaching a fatty acid and producing, therefore, diacylglycerols (glycerol esterified with 2 fatty acids) and free fatty acids. In the two or three hours that food remains in the stomach, oral and gastric lipases are able to split about 30% of food lipids.
However, the most important source of lipase remains the pancreatic one, which is why the aforementioned steatorrhea is typical of all those conditions that decrease pancreatic function.
The final products deriving from the action of pancreatic lipase are monoglycerides (2-acylglycerols) and free fatty acids; unlike salivary lipase, which detaches only one fatty acid, in fact, pancreatic lipase can detach both fatty acids from the primary hydroxyls (carbon 1 and 3) of glycerol. The 2-acylglycerol, thus obtained, spontaneously isomerizes in the alpha (3-acylglycerol) form and can therefore be again attacked by a lipase which breaks it down into glycerol plus a free fatty acid.
The activity of pancreatic lipases is aided by colipase enzymes secreted by the pancreas, which favor their adhesion to fat droplets. Not only that, in order for optimal fat digestion to take place, the intervention of bile produced by the liver is required, which - in synergy with the peristaltic movements - leads to the emulsion of fats, breaking down the lipidic aggregates into very fine and easily attackable droplets from lipase.
What happens in the small intestine is a fundamental step in the digestive process of fats, since only monoglycerides and free fatty acids can be absorbed by the intestinal mucosa.
Accordingly, it is possible to have steatorrhea even in the presence of liver diseases or extensive intestinal resections.
In addition to lipase, the pancreas also produces a phospholipase (called phospholipase A2) and a carboxylesterase. The first preferentially removes the fatty acid in position two of the phospholipids, producing free fatty acids and lysophospholipids, while the carboxylesterase splits cholesterol esters, liposoluble vitamins, triglycerides, diglycerides and monoglycerides.
Other lipases are produced by the liver, vasal endothelium and within cells, such as lysosomal and hormone-dependent lipases.
Absorption and Distribution of Fat
Once absorbed, the fatty acids and other digestive products are converted back into triacylglycerols and aggregated to specific transport proteins, giving rise to small lipoprotein clusters called chylomicrons. These are poured into the lymphatic circulation and then into the bloodstream, then transported to the muscle and adipose tissue. In the capillaries of these tissues, the extracellular enzyme lipoprotein-lipase hydrolyzes the triacylglycerols to fatty acids and glycerol, which enter the target cells. In those of muscular type, fatty acids are oxidized to produce energy, while in the target cells of adipose tissue they are re-esterified to triacylglycerols to be stored as reserve fats.
High lipases »
