Post-absorption Processing of Lipids Flashcards
Other than production of energy, what is another key role of fats?
They are incorporated into cell membranes
This is particularly phospholipids and cholesterol
What are the essential fatty acids?
What are they precursors for?
Omega-3 and omega-6
They are precursors for eicosanoids
What are eicosanoids?
A family of oxygenated derivatives of 20-carbon polyunsaturated fatty acids
They are signalling molecules
Including prostaglandins, leukotrienes and thromboxanes
What is cholesterol used for?
- cell membranes
- synthesis of bile salts
- synthesis of steroid hormones
- production of Vitamin D
What is the pathway leading from linoleic acid (omega-6) to arachidonic acid?
- linoleic acid (omega-6)
- gamma-linoleic acid (GLA)
- dihomo-gamma-lineolic acid (DGLA)
- arachidonic acid
What is produced from DGLA?
1-series prostaglandins and thromboxanes
What is produced from arachidonic acid?
How is this influenced by aspirin?
- 2-series prostaglandins and thromboxanes
- 4-series leucotrienes
- lipoxins
Aspirin increases lipoxin production
It decreases prostaglandin and thromboxane production
What is produced from DHA/EPA (omega-3)?
How is this influenced by aspirin?
- 3-series prostaglandins and thromboxanes
- protectins and resolvins
- 5-seies leucotrienes
Aspirin increases production of protectins and resolvins
What is linoleic acid?
It is an example of an omega-6 fatty acid
What is the role of lipoxins, resolvins and protectins?
They can orchestrate a resolution to inflammation
What is a chylomicron?
A small fat globule composed of protein and lipid
Where are chylomicrons found?
What is their role?
They are found in the blood and lymph
They transport fat from the intestine, to the liver and adipose tissue
Where are chylomicrons formed?
They are formed in the endoplasmic reticulum of the enterocytes of the small intestine
From here, they supply tissues with fat absorbed from the diet
How do chylomicrons enter the circulation?
They DO NOT pass directly from the GI tract to the liver
They are released into the lymph and enter circulation via the thoracic duct, into the subclavian vein
What happens once the chylomicron has reached adipose, cardiac or skeletal muscle tissue?
Its triglyceride components are hydrolysed by lipoprotein lipase
This leads to the release of free fatty acids which can be absorbed by the tissues
What are the components of a chylomicron?
Triglycerides are within a ball of phospholipids
There are apolipoproteins embedded in the wall
What happens to chylomicron remnants after lipoprotein lipase has taken action?
They travel to the liver for cholesterol synthesis
How is the liver involved in the regulation of cholesterol levels in the body?
- It can synthesise cholesterol for export to other cells
- it can remove cholesterol from the body by converting it to bile salts
It then enters the bile to be eliminated through faeces
What does the liver synthesise for cholesterol transport?
The liver synthesises the various lipoproteins involved in transporting cholesterol and other lipids throughout the body
What is meant by cholesterol synthesis being under negative feedback regulation?
Increased cholesterol in a hepatocyte leads to decreased activity of HMG-CoA reductase
What is contained within a lipoprotein?
- triacylglycerol
- cholesterol
- phospholipids
- apolipoproteins (amphipathic proteins)
How are lipoproteins differentiated?
Based on their density and the types of apolipoprotein they contain
The lower the density of a lipoprotein, the more lipid it contains relative to protein
What are the 4 major types of lipoprotein?
- chylomicrons
- very low-density lipoprotein (VLDL)
- low-density lipoprotein (LDL)
- high-density lipoprotein (HDL)
What is the role of chylomicrons and VLDL?
They deliver triacylglycerols to the cells in the body
Where is VLDL synthesised?
In the liver
What removes the triacylglycerols from the chylomicrons/VLDL?
What does it do to them?
Lipoprotein lipase
It digests the triacylglycerols into fatty acids and monoglycerides
These can diffuse into the cell to be oxidised
They can be resynthesised into triacylglycerols in adipose cells
What is the role of LDL?
It delivers cholesterol to cells in the body
How is LDL formed?
As VLDL particles are stripped of triacylglycerols, they become more dense
They are remodelled in the liver and transformed into LDL
How do cells take up cholesterol via LDL?
Receptor-mediated endocytosis
LDL binds to a specific LDL receptor and it is then internalised in an endocytic vesicle
How is cholesterol prepared for use in the cell after receptor-mediated endocytosis?
Receptors are recycled to the cell surface whilst hydrolysis in an endolysosome releases cholesterol for use in the cell
What is the role of HDL?
It is involved in reverse cholesterol transport
How is excess cholesterol eliminated from the body?
Excess cholesterol is eliminated from the body via the liver
The liver secretes cholesterol in bile or converts it to bile salts
How does the liver remove LDL from the circulation?
The liver removes LDL and other lipoproteins from circulation by receptor mediated endocytosis
What is involved in reverse cholesterol transport?
Excess cholesterol from cells is returned to the liver by HDL
How is HDL synthesised and secreted?
What happens before it returns to the liver?
The HDL precursor is synthesised and secreted by the liver and small intestine
It travels in the circulation and gathers cholesterol to form mature HDL
Mature HDL returns to the liver