Bile and Plasma Lipids

Question 1

What are lipids?

Answer 1

Lipids are a structure with a carboxyl group on one end and a methyl group on the other.

Question 2

What are the different classifications of lipids?

Answer 2

Simple lipids-> triglycerides and Mono glycerol
Complex lipids-> phospholipids
Ampithiatic lipids -> cholesterol

Question 3

How is bile secreted?

Answer 3

When chyme containing fat enters the duodenum, it stimulates enteroendocrine cells in the duodenum to release cholecystokinin. This hormone stimulates the gall bladder to contract and release bile and the relaxation of the sphincter of Oddi.

Question 4

What stimulates secretin release?

Answer 4

Secretin is released by enteroendocrine cells in response to the entry of acidic chyme in the duodenum. It stimulates pancreatic secretion containing pancreatic amylase and lipase for digestion and biliary bicarbonate secretion for neutralisation of excess stomach acid in the duodenum.

Question 5

What is the composition of bile?

Answer 5

Bile is composed mainly of water, excess cholesterol and the bile pigments bilirubin and biliverdin.

Question 6

What is the function of bile?

Answer 6

Bile is responsible for the emulsification of fat, neutralising stomach acid in the duodenum and the formation of micelles for transfer of fat in the body.

Question 7

What are the enzymes involved in lipid digestion?

Answer 7

Pancreatic lipase breaks down fat. It is stabilised by colipase which anchors it to prevent neutralisation from bile salts. Cholesterol ester hydrolase breaks down cholesterol ester into cholesterol and fatty acid
Phospholipase A2 breaks down phosphatidylcholine into phospholipid

Question 8

What is the pH of bile?

Answer 8

Bile is between 7-8 pH

Question 9

How do micelles enter enterocytes?

Answer 9

They use the channel NCRP1 to cross the lumen of intestinal enterocyte cells

Question 10

How do chylomicrons form?

Answer 10

When micelles enter, their shell degrades. Triglycerides and the product of lipid digestion form the lipoprotein chylomicron. Enterohepatic circulation recycles the bile salt back to the liver.

Question 11

What is the composition of lipoproteins?

Answer 11

Lipoproteins are formed of triglycerides, protein, cholesterol and a unique apolipoprotein which differentiates it.

Question 12

‘What are the types of apolipoproteins?

Answer 12

Chylomicron
LDL and VLDL: same apo group but LDL have the greatest cholesterol levels of all apolipoproteins and high protein levels
HDL: High cholesterol levels, highest protein levels. It removes excess cholesterol back to circulation.

Question 13

What is the circulation of exogenous and endogenous lipids in the body?

Answer 13

Dietary lipids are absorbed in the enterocytes as micelles. They are repackaged into chylomicrons and enter the lacteal ducts for lymphatic circulation in the thoracic duct and then deposited into the subclavian vein. In the capillaries, they are digested by lipoprotein lipase, or are taken up by muscle cells for utilisation or adipose tissue for storage. The remnants of lipoprotein lipase digestion are circulated to the liver and endogenously forms VLDL which enters capillaries to be distributed to muscles or adipose tissue. Remaining VLDL is broken down by lipoprotein lipase to form IDL which are distributed to the peripheral tissues.

Question 14

How is nascent HDL formed?

Answer 14

Nascent (early) HDLL is formed from the small intestine as chylomicron which is then synthesised in the liver.

Question 15

How does nascent HDL mature?

Answer 15

Free cholesterol from macrophages and the peripheral tissues are added to nascent HDL. This nascent cholesterol is converted byt LCAT enzyme to form cholesterol ester hydrolase. Mature HDL is now formed containing cholesterol ester hydrolase.

Question 16

Where and why does reverse cholesterol transport take place? What are the 2 methods for reverse cholesterol transport?

Answer 16

Reverse cholesterol transport occurs when there is too much cholesterol in the body. Mature HDL and chylomicrons bind directly to SCBR1 receptor on the liver. Or the CETP enzyme can transfer cholesterol from mature HDL to LDL or IDL which will bind to the LDLR receptor.

Question 17

Why are lipids important in the body?

Answer 17

Lipids are used for forming components of membranes such as phospholipids and cholestero. It is used in insulation and as a source of energy and a transfer, such as prostacyclin.

Question 18

What is the pathway for cholesterol synthesis and where does it occur?

Answer 18

Cholesterol synthesis occurs in the liver on the endoplasmic reticulum..
Acetyl coA is converted by HMG-coa reductase to isopentylyl, pentynyl transferase. Six of these molecules are added to squalene by condensation reaction and squalene forms cholesterol.

Question 19

How can we treat high cholesterol?

Answer 19

Statins are drugs which inhibit HMG coA reductase.

Question 20

How does the body regulate high cholesterol?

Answer 20

When cholesterol is high, HMG coA reductase is degraded and expression of genes for enzymes that catalyse cholesterol formation are not transcribed.

Question 21

How does the body regulate low cholesterol?

Answer 21

When cholesterol is low, HMG coA is a functioning shape and expression for enzymes for cholesterol synthesis are transcribed.

Question 22

How is cholesterol removed from the body?

Answer 22

Cholesterol cannot be degraded and must either be directly excreted, or it has to be converted into a different form. 1g of cholesterol is excreted from the body directly. Half of this is converted into bile acids to leave in the faeces. Cholesterol can be directly excreted or by bacterial action in the large intestine be converted into coprastinol.

Question 23

What are the tributaries of cholesterol?

Answer 23

Cholesterol forms Vitamin D, steroid hormones like cortisol, aldosterone and progesterone.