Absorption

The products of hydrolysis (A) reach the brush border membrane as mixed micelles where they are passively absorbed into the mucosa cells. The micelle uptake mechanism into the cell is not fully understood to date. In particular, it is questionable whether the micelles enter the cells whole or whether they just deliver their contents to them. The transition to the brush border membrane can be imagined as a massage process across the UWL (see below). Reduced intestinal motility may severely impede fat absorption since lipids are unable to passively cross the UWL.

After secretion into the intestinal lumen, the bile acids, which are essential for fat absorption, reach the lower small intestine. There, they are reabsorbed in part and transported back to the liver through the portal vein. From there, they return into the gall bladder, completing the cycle of hepatic circulation (see p. 42). The subsequent intracellular metabolism produces lipoproteins, which pass into the lymphatic system as chylo-microns.

The contents of the intestine are in constant motion due to peristalsis. Only the interface immediately adjacent to the intestinal wall is completely still. It is, therefore, called unstirred water layer (UWL). Diffusion through the UWL and subsequent permeation through the membrane depend on a concentration gradient (C2 > C3 > C4) (B). This gradient is achieved through concentration in the micelles (step A, C1 > C2) on the one hand, and through quick intracellular removal (step D) on the other hand. The last step requires fatty acid binding protein (FABP), which ensures fat removal and thereby a low concentration (C4) of free fatty acids on the inside of the brush border membrane. FABP has greater affinity to unsaturated fatty acids.

FABP also increases fatty acid activation (Acyl-SCoA synthesis), which represents the first step towards intracellu-lar resynthesis (C). The esterification of monoglycerides and fatty acids, as well as phospholipid and cholesterol ester resynthesis, occurs in the endoplasmic reticulum (ER). The latter is catalyzed by an enzyme identical to the pancreatic cholesterol esterase that operates in the intestinal lumen. At the pH range (6.6-8) that prevails in the intestine, the enzyme favors hydrolytic activity, whereas it catalyzes esterification at the pH range (5-6.2) that prevails inside cells.

Subsequently, lipids migrate through the cisterns of the smooth ER to the rough ER, where apoproteins are added, forming prechylomicrons. The pre-chylomicrons are transported to the Golgi body for final assembly. Glyco-sylation initiated inside the rough ER is completed here, providing them with a glycoprotein surface. After fusion of the chylomicrons with the lateral plasma membrane, they are released into the intercellular space by exocytosis and from there enter the lymphatic vessels.

- A. Fat Absorption-

Emulsification O Hydrolysis O

X Bile

Mixed micelles O salts Diffusion through UWL O-

Lumen

Unstirred water layer

Membrane ^

Basal

permeation y

membrane

Metabolism O

Formation of JL

lipoproteins y

*--».Q

Extrusion as

chylomicrons

Cytoplasm

Removal by lymphatic fluid

Removal by lymphatic fluid

B. Fat Absorption -

Lumen

Micelle

Lumen

Micelle

Micelle Chylomicrons

FABP

Cytoplasm

FABP

Cytoplasm

C. Fat Absorption

Cholesterol

Cholesterol ester

Fatty acid glycerol Mixed /

Cholesterol

Cholesterol ester

Fatty acid glycerol Mixed /

Mixed Micelles Formation Digestion

Brush border

Monoacyl |

Mucosa cell

Brush border

Monoacyl |

: Acyl-SCoA

Diacyl

Diacyl a-Glycerophosphate pathway

Fatty acid storage a-Glycerophosphate pathway

Phospholipids

Triacylglycerol

Ribosomal protein synthesis

Triacylglycerol

Ribosomal protein synthesis

Phospholipids

Chylomicrons

Mucosa cell

Storage free cholesterol

Lipoproteins.

Cholesterol esterase bile salts

Cholesterol ester

Lymph

Cholesterol esterase bile salts

Cholesterol ester

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