Chemical and enzymes in small intestine

Small intestine is a long tube, with a small diameter (about 1 inch), extending from pyloric sphincter to the ileocecal valve. Small intestine can be divided into Duodenum, Jejunum, and ileum.

Food moves through it relatively slowly, over a period of hours, allowing time for the actions of digestion and absorption for which this part of the GI tract is designed. The main classes of human digestive enzymes include proteases, lipases and carbohydrases, which respectively break down the macronutrients protein, fats and carbohydrates.

Up to 3L of intestinal juice is secreted daily by cells in the walls of the duodenum, and to a lesser extent the jejunum and ileum.

Pancreatic and intestinal enzymes that finish the digestion of proteins into amino acids. Proteolytic enzymes, including trypsin and chymotrypsin, are secreted by the pancreas. Carboxypeptidase, a pancreatic brush border enzyme, splits one amino acid at a time.

Proteases - break down proteins at optimum pH 7.9-9.7
• Trypsin and chymotrypsin – break down protein polypeptides in dipeptides
• Carboxypolypeptidase – splits peptides into individual amino acids
• Aminopeptidase and dipeptidase free the end amino acid products

Lipids (fats) are degraded into fatty acids and glycerol. Lipases - break down majority of dietary fats at optimum pH 8.0
• Lipase – hydrolyses triglycerides into free fatty acids and 2-monoglyceride, with the present of bile salt.
• Phospholipase – splits the fatty acids of phospholipids
• Esterase – hydrolyses cholesterol esters

All three are serine proteases, but with different cleavage specificities. Their action is complemented by exopeptidases.

Some carbohydrates are degraded into simple sugars, or monosaccharides (e.g., glucose, galactose) and are absorbed by the small intestine. Pancreatic juice supplies a cocktail of enzymes for the digestion of nearly all major nutrients. α-Amylase is secreted in large amounts. This enzyme is different from the salivary α-amylase, which has a slightly different structure (94% amino acid identity) and is encoded by a different gene.

Pancreatic amylase breaks down some carbohydrates (notably starch) into oligosaccharides. Disaccharidases and oligosaccharidases hydrolyze sucrose and lactose, as well as the maltose, maltotriose, and α-limit dextrins that are formed by the action of α-amylase on starch. Other carbohydrates pass undigested into the large intestine, where they are digested by intestinal bacteria.

Carbohydrases - break down carbohydrates at optimum pH 6.7-7.2
• Amylase – breaks down starch, glycogen and other carbohydrates polysaccharides into disaccharides
Chemical and enzymes in small intestine

Small intestines – Main functions

The intestine (bowel) is a winding muscular tube and it extends from the stomach to the anus. Its main purpose is to digest food. The small intestine is longest part of the digestive system where 90% of the digestion and absorption of food occurs, the other 10% taking place in the stomach and large intestine.

The major purpose of the small intestine is digestion and absorption of nutrients. In the small intestine, enzymes (produced by the salivary glands in the mouth, in the pancreas and in the intestinal cells) break down nutrients such as carbohydrates, proteins or fats into their building blocks.

For example, proteins, peptides and amino acids are acted upon by enzymes such as trypsin and chymotrypsin, which are produced by the pancreas. Pancreas also produced lipases and this enzyme break-up triglycerides into free fatty acids and monoglycerides.

The intestinal cells assume the roles of absorbing the building blocks (for example sugar, amino acids or fatty acids) together with vitamins, salts and water which pass into the bloodstream to be used by the body.

The small intestinal cells also produce countless intestinal hormones. These hormones associated and stimulus the production of bile or pancreatic juice. For example, enzymes will enter the small intestine in response to the hormone cholecystokinin, which is produced in response to the presence of nutrients.

The other hormone, secretin activate bicarbonate to be released into the small intestine from the pancreas to neutralize the potentially harmful acid coming from the stomach.
Small intestines – Main functions 

  

Cholesterol absorption

High cholesterol intake generally increases the serum levels of total (X) and low-density lipoprotein cholesterol (LDL-C). If dietary fat is kept constant, the increase in serum LDL-C after physiologically augmented cholesterol intake varies from 4 to 58%.

The amount of absorbed cholesterol depends on cholesterol intake, biliary cholesterol secretion, and cholesterol absorption efficiency, which in turn is affected by
(a) the amount and the condition of absorptive intestinal mucosa and
(b) micellar solubilization of cholesterol with the aid of bile acids

When foods are consumed, cholesterol arrives in the small intestine from both the diet and bile. Dietary cholesterol accounts for approximately 300 mg/d, whereas biliary cholesterol is estimated to contribute 800∼1,400 mg/d.

Biliary cholesterol enters the small intestine unesterified, along with the other major components of bile (phosphatidylcholine and bile acids). As the components of bile mix with dietary lipids, micelles form spontaneously. Micelles are created by interactions between bile salts and the plasma membrane of hepatocytes. Micelles are formed when a critical concentration of lipid from bile mixes with lipids entering the small intestine from the diet.

Cholesterol is absorbed from micelles into the intestinal wall through protein channel, Niemann-Pick C1 Like 1 protein (NPC1L1) on the enterocyte plasma membrane.
Cholesterol absorption