Chemistry of Lipids

Classification of Lipids

• Lipids can be classified into triacylglycerol, phospholipids, and cholesterol.
• Free fatty acids can be found in tissues and are transported in serum bound to the protein albumin.

Functions of Eicosanoids

• Eicosanoids are chemical messengers synthesized in the cells where their physiological activity is expressed.
• Prostaglandins are among the most potent biological substances known, with slight structural differences giving them highly distinct biological effects.
• Prostaglandins can induce smooth muscle contraction, lower blood pressure, and contribute to the inflammatory response.
• Aspirin and other nonsteroidal anti-inflammatory agents inhibit the synthesis of prostaglandins by inhibiting cyclooxygenase.
• Thromboxane stimulates constriction and clotting of platelets, while prostacyclin has the opposite effect, promoting vasodilation and inhibiting platelet aggregation.
• Certain prostaglandins are involved in the induction of labor and other reproductive processes.
• Leukotrienes promote constriction of bronchi associated with asthma.

Lipoproteins

• HDL (good cholesterol) carries cholesterol from tissues to the liver.
• Chylomicrons carry dietary triacylglycerols from the intestine.
• LDL (bad cholesterol) carries cholesterol from the liver to tissues.
• Lipoprotein(a) (Lp(a)) is structurally and metabolically distinct from other lipoproteins, with prothrombotic, proinflammatory, and proatherogenic properties.
• High plasma concentrations of Lp(a) are associated with an increased risk of cardiovascular diseases.

Derived Lipids

• Cholesterol is a derived lipid, formed from acetyl-CoA, with 27 carbon atoms.
• Vitamin D is formed from 7-dehydrocholesterol when exposed to ultraviolet radiation.
• Bile acids are formed in the liver and are composed of 80% cholesterol.
• Adrenocortical hormones and sex hormones are steroid hormones formed from cholesterol.

Cholesterol and Atherosclerosis

• Excess oxidized LDL leads to the formation of fatty streaks (foam cells) in the subintimal space.
• Foam cells separate endothelial cells, exposing them to blood, which leads to plaques and thrombus formation.
• Thrombus formation can lead to complete occlusion of the lumen, eventually causing acute myocardial infarction (AMI) or cerebrovascular accident (CVA).