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