Lipids are heterogeneous group of compounds and are two types, namely, exogenous lipids and endogenous lipids. These are related to fatty acids and are insoluble in water. However, these are soluble in ether and chloroform. These are of three types:
a. Simple lipids: These are esters of alcohol and fatty acids. The fatty acids, present in diet, may be:
• Saturated fatty acids such as lauric and palmitic.
• Monounsaturated fatty acids such as oleic and palmitoleic.
• Polyunsaturated fatty acids such as linolenic and arachidonic; abundant in corn oil and safflower oil; have hypolipidaemic effect.
b. Compound lipids: These are also esters of alcohol and fatty acids but certain other groups are also present in their structure, e.g. sulfolipids and phospholipids.
c. Neutral lipids: These are non-polar lipids such as glycerides, cholesterol and its esters. Triglycerides are composed of three fatty acids which are linked to 3-atom containing alcohol or glycerol. It is a lipid that chemically is a sterol but not a fat.
Transport of exogenous (dietary) lipids
In the intestinal mucosa, dietary triglycerides and cholesterol are incorporated into the core of chylomicrons. The surface coat of chylomicrons is composed of phospholipids, free cholesterol and various apoproteins. The chylomicron is then transported to the circulation via the thoracic duct.
Finally dietary triglycerides are delivered to fat and muscle cells and dietary cholesterol is taken up by the liver. The fatty acids leave the cells, ionize in plasma (pH 7.4), combine with albumin and are known as free fatty acids (FFAs). These then reach other cells where they are oxidized by n-oxidation to provide energy.
Cholesterol is not used for energy. It serves as a structural component of all cell membranes, as a precursor for adrenal and gonadal steroids and as precursor of hepatic bile acids. In liver, it may be stored, oxidized to bile acids or alternatively incorporated into HDL or VLDL and resecreted as lipoprotein back into circulation.
Dietary cholesterol regulates endogenous hepatic cholesterol synthesis. In liver, it is synthesized from mevalonic acid which is obtained from reduction of acetyl coenzyme-A by 3-hydroxy-3-methyglutaryl- coenzyme-A reductase (HMC-CoA reductase)
A small amount of it is also absorbed from the gut which is also taken up by hepatocytes. The total intracellular cholesterol in hepatocytes provide a negative feedback for cholesterol synthesis by inhibiting HMG-CoA reductase. Due to this, synthesis of cholesterol by the liver is decreased.
Transport of endogenous Lipids
Lipids (triglycerides, cholesterol, and its esters) are transported from liver to the peripheral tissues and back to the liver by the endogenous lipid transport system. The atherogenic Apo-B 100 lipoprotein guided system transports VLDL, IDL and LDL while antiatherogenic Apo-AT governed lipoprotein system transports HDL.
When concentrations of cholesterol or triglycerides or both in plasma are very high, the condition is called hyperlipidaemia or hyperlipoproteinemias. Medical attention is required, if the level of it is above 250 mg/100 ml and triglycerides is above 150 mg/100 ml.
Atherosclerosis (hardening of arteries due to deposition of lipoid material in the intimal layers of the arteries) occurs due to elevated level of plasma lipo-proteins and can lead to coronary thrombosis and infarction.