Prostaglandins have many biochemical functions, which are different in different tissues. PGs also bring about opposing actions in the same tissue.

Prostaglandins are classified mainly into two groups shown in Table 4.5: Prostanoids are further subdivided into three groups: Structure of prostaglandins: Figure 4.8 Structure and Biosynthetic Pathway of Prostaglandins Table 4.5 Structure of Prostaglandin Prostaglandin Substituent Group Structure PGA Double bond C10—C11keto group C9 PGB Double bond C8—C12keto group C9 PGC Double bond C11—C12keto group C9 PGD —OH…

Prostaglandins are derivatives of a hypothetical 20 carbon fatty acid, namely prostanoic acid and hence known as prostaglandins or eicosanoids. The structure and biosynthetic pathway of prostaglandins are shown in Figure 4.8.

INTRODUCTION Prostaglandins (PG) are a group of biologically active lipids. They are widely present in mammalian tissues and body fluids. Though they were initially found in the seminal fluid, it is now known that they are present in all tissues. PGs are considered as hormones since their action appears to be confined to the cells…

The protein components of lipoproteins are known as apolipoproteins or, simply, apoproteins. They perform the following functions:

The comparative characteristic features of different lipoproteins with regard to electrophoretic patterns, size composition, etc., are given in Table 4.4.   Table 4.4 Characteristics of Human Plasma Lipoproteins

The principal function of HDL is transport of cholesterol from peripheral cells and transporting it back to the liver from where most of the cholesterol excreted from the body is removed.

HDL is synthesised and secreted from both liver and intestinal mucosa. It contains the maximum amount of protein cholesterol, and phospholipids are the main lipids present in HDL. HDL from intestine contains apo-A and that synthesised in liver contains apo-C.

The principal function of LDL is transport of cholesterol to extrahepatic tissues.

They are formed from VLDL in the blood circulation. They are particularly rich in cholesterol. Approximately, 70% of the plasma cholesterol is present in the LDL fraction. The main apoprotein of LDL is apo-B100. The LDL level in plasma is clinically important as high level is indicative of susceptibility to ischemic heart disease (IHD).