Lymphatic system chylomicrons secreted into

Figure 25-2. The formation and secretion of (A) chylomicrons by an intestinal cell and (B) very low density lipoproteins by a hepatic cell. (RER, rough endoplasmic reticulum SER, smooth endoplasmic reticulum G, Golgi apparatus N, nucleus C, chylomicrons VLDL, very low density lipoproteins E, endothelium SD, space of Disse, containing blood plasma.) Apolipoprotein B, synthesized in the RER, is incorporated into lipoproteins in the SER, the main site of synthesis of triacylglycerol. After addition of carbohydrate residues in G, they are released from the cell by reverse pinocytosis. Chylomicrons pass into the lymphatic system. VLDL are secreted into the space of Disse and then into the hepatic sinusoids through fenestrae in the endothelial lining.

The overall metabolism of vitamin A in the body is regulated by esterases. Dietary retinyl esters are hydrolyzed enzymatically in the intestinal lumen, and free retinol enters the enterocyte, where it is re-esterified. The resulting esters are then packed into chylomicrons delivered via the lymphatic system to the liver, where they are again hydrolyzed and re-esterified for storage. Prior to mobilization from the liver, the retinyl esters are hydrolyzed, and free retinol is complexed with the retinol-binding protein for secretion from the liver [101]. Different esterases are involved in this sequence. Hydrolysis of dietary retinyl esters in the lumen is catalyzed by pancreatic sterol esterase (steryl-ester acylhydrolase, cholesterol esterase, EC 3.1.1.13) [102], A bile salt independent retinyl-palmitate esterase (EC 3.1.1.21) located in the liver cell plasma hydrolyzes retinyl esters delivered to the liver by chylomicrons. Another neutral retinyl ester hydrolase has been found in the nuclear and cytosolic fractions of liver homogenates. This enzyme is stimulated by bile salts and has properties nearly identical to those observed for... 

Figure 4.13 Uptake of bile acids in the jejunum. Bile adds (BA) and cholesterol (C) are secreted from the liver, via the bile, into the duodenum. Cholesterol is transported back into the blood, from the enterocyte, within chylomicrons. The latter enter the lymphatic system (i.e. the lacteals). Bile acids are absorbed from the jejunum into the hepatic portal vein for re-uptake into the liver.

Transport to the liver Retinol esters present in the diet are hydrolyzed in the intestinal mucosa, releasing retinol and free fatty acids (Figure 28.19). Retinol derived from esters and from the cleavage and reduction of carotenes is reesterified to long-chain fatty acids in the intestinal mucosa and secreted as a component of chylomicrons into the lymphatic system (see Figure 28.19). Retinol esters contained in chylomicrons are taken up by, and stored in, the liver. 

The mixture of lipids moves to the endoplasmic reticulum, where fatty acyl CoA synthetase converts free fatty acids into their activated CoA derivatives. Fatty acyl CoAs are then used to produce triacylglycerols, cholesteryl esters, and phospholipids. These, together with the fat-soluble vitamins (A, D, E, and K) and a single protein (apolipoprotein B-48), form a chylomicron, which is secreted into the lymphatic system and carried to the blood. 

These lipids are then packaged into spherical lipoproteins, particles of lipids and proteins, known as chylomicrons, which are secreted into lymphatic vessels and subsequently enter the blood stream. Once in the circulatory system the triacylglycerol components of the chylomicrons are degraded to fatty acids and glycerol by the enzyme lipoprotein lipase, which is attached to the luminal (inner) side of capillary vessels in heart, muscle, adipose (commonly... 

The well-fed, or postabsorptive, state. After we consume and digest an evening meal, glucose and amino acids are transported from the intestine to the blood. The dietary lipids are packaged into chylomicrons and transported to the blood by the lymphatic system. This fed condition leads to the secretion of insulin, which is one of the two most... 

Retinol is esterified to palmitic acid in the intestinal mucosa and secreted as components of chylomicrons into the lymphatic system and through blood stored in liver. 

Lipoproteins are assembled in two organs, the small intestine and the liver. The lipoproteins assembled in the intestine contain the lipids assimilated from the diet. These lipoproteins, called chylomicrons, leave the enterocyte and enter the bloodstream via the Lymphatic system. The lipoproteins assembled in the liver contain lipids originating from the bloodstream and from de novo synthesis in the liver. The term de novo simply means "newly made from simple components" as opposed to "acquired from the diet" or "recycled from preexisting complex components." These lipoproteins, called very-low-dcnslty lipoproteins (VLDLs), are secreted from the liver into the bloodstream. The liver also synthesizes and secretes other Lipoproteins called high-density Lipoproteins (HDLs), which interact with the chylomicrons and VLDLs in the bloodstream and promote their maturation and function. The data in Table 6-4 show that chylomicrons contain a small proportion of protein, whereas HDLs have a relatively high protein content. Of greater interest is the identity and function of the proteins that constitute these particles. These proteins confer specific properties to lipoprotein particles, as detailed later in this chapter. 

By the process of exocytosis, chylomicrons are secreted by the intestinal epithelial cells into the chyle of the lymphatic system and enter the blood through the thoracic duct. Chylomicrons begin to enter the blood within 1 to 2 hours after the start of a meal as the meal is digested and absorbed, they continue to enter the blood for many hours. Initially, the particles are called nascent (newborn) chylomicrons. As they accept proteins from HDL within the lymph and the blood, they become mature chylomicrons. 

Together with dietary fat, both tocopherols and tocotrienols are absorbed in the digestive tract, incorporated into chylomicrons, and transported in the lymphatic system. However, following chylomicron clearance, tocotrienols disappear from circulating plasma, whereas tocopherol, especially a-tocopherol, is preferentially secreted into plasma. a-Tocopherol transfer protein, which controls plasma a-tocopherol levels, has an affinity of only 12% for a-tocotrienol (compared with 100% for a-tocopherol). The reason for the low plasma tocotrienol concentrations compared with tocopherols is the very fast clearance, as demonstrated by Yap et al. ... 

Figure 7. Mixed micelles carry the solubilized hydrophobic lipids through the aqueous digest medium into close proximity with the intestinal mucosal cells (enterocytes). Micelles are not absorbed intact, but the various components are taken up by the enterocytes at independent rates. Intracellular esterification of cholesterol by acyl-coenzyme A prevents its transport back to the intestinal lumen. After esterification, cholesterol esters are assembled into chylomicrons and secreted into the lymphatic system and, finally, to the blood circulation.

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