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Metabolism of Dietary Lipids
Definition: Lipids are heterogeneous
gp of water-insoluble (hydrophobic)
organic molecules stored as
triacylglycerol in adipose tissues.
Major source of energy for body
Importance
Lipids are major components of cell
membranes, and are responsible for most of
the permeability filter functions of
membranes.
Membranes act as barriers to separate
compartments within eukaryotic cells, and to
separate all cells from their surroundings.
Importance
Lipids are also used as metabolic fuels.
Adipose tissue fat stores contain ~9 kcal/gram
(= ~37 kJ/g) of dry weight lipid. This compares
to ~4 kcal/g for protein or carbohydrate.
Lipids include the vitamins A, D, E, and K, and
include some non-vitamin enzyme cofactors.
Importance
Lipids include a number of hormones, of which the
most important are steroids and prostaglandins
Lipids act as insulation, and play a role in physical
appearance of animals.
Finally, lipids play roles in a variety of pathological
conditions, including heart disease and some
genetic disorders related to lipid metabolism.
Chemistry of Lipid Lipids are a variety of organic molecules
grouped together on the basis of their solubility in nonpolar solvents.
Their varied biological functions include 1-energy source 2-energy storage 3-cell membrane structural components, hormones, vitamins, vitamin adsorption, protection, and insulation.
Chemistry of Lipid The four main groups of lipids include:
1. Fatty acids (saturated and unsaturated) 2. Glycerides (glycerol-containing lipids) 3. Nonglyceride lipids (sphingolipids, steroids, waxes) 4. Complex lipids (lipoproteins, glycolipids)
Lipids can also be broadly subdivided as either saponifiable or nonsaponifiable.
Classes of Lipids
All biological lipids are amphipathic Fatty acids Triacylglycerols Glycerophospholipids Sphingolipids Waxes Cholesterol Prostaglandins &Leukotrienes (eicosanoids) Fat soluble vitamins (A D E K)
Triacylglycerol (TAG)
FA on C1 is saturated FA on C2 is unsat. FA on C3 is either Slightly soluble Stored in adipocytes
Digestion
Digestion begins in the stomach
Acid-stable lipase (lingual lipase)
cleaves triacylglycerols (TAG) with
short or medium chain FA (less
than 12 C) found in milk fat
Gastric lipase secreted by stomach
Digestion
Emulsification of dietary lipid in small
intestine (duodenum) to increase surface
area of lipids by bile salts (made in liver &
stored in gallbladder)
Pancreatic enzymes ( lipase and colipase)
which are hormonally controlled
Digestion
Hormones that regulate the
utilization of nutrients Gut hormones:
A- Gastrin from the gastric antrum and duodenum
stimulates gastric acid and pepsin secretion.
B- Cholecytokinine from duodenum and jejunum
stimulates contraction of gallbladder and secretion
of pancreatic enzymes
C- Secretin from duodenum and jejunum
stimulates of secretion of biocarbonate by the
pancreas
Control of Lipid Digestion
Cholecystokinin
Secretin
y
Absorption of Lipids by Intestinal cells
Pancreatic enzymes ( lipase and colipase) digests
the TAG to 2-monoacylglycerol and free fatty
acids, which are packaged into micelles
The micelles, which are micro droplets
emulsified by bile salts, also contain other dietary
lipids such as cholesterol ad fat-soluble vitamins.
Absorption of Lipids by Intestinal cells
The micelles travel to the microvilli of
the intestinal epithelial cells, which
absorb 2-monoacylglycerol and free
fatty acids and other dietary lipid
The bile salts are reabsorbed , recycled
by the liver and secreted into the gut
during subsequent digestive cycle.
Absorption of Lipids by Intestinal cells (enterocytes)
Free FA, cholesterol,2-monoacylglycerol &bile salts form mixedMicelles to be absorbedby brusher border mem--brane Short & medium FADo not require micelles &Go to liver via serum albumin
Synthesis of chylomicrons
In the intestinal epithelial cells, the fatty acids
from micelles are activated by fatty-acyl CoA
sythetase (thiokinase) to form fatty acyl CoA
Fatty acyl CoA reacts with a 2-mono-
acylglycerol. Then form triacylglycerol.
The triacylglycerol pass into the lymph
packaged in nascent (newborn) chylomicrons,
which enter the blood.
Resynthesis of TAG & cholesterol esters
Use of Dietary Lipids by Tissues
TAG in chylomicrons is broken to FFA + glycerol by
lipoprotein lipase in adipocytes, muscle cells &
peripheral tissues.
Fate of FFA adipocytes + muscle cells associated
with albumin for energy production or stored as
TAG
Fate of glycerol liver glycerol-3-P glycolysis
or gluconeogenesis
Fate of chylomicron remnants liver