The major energy store of the body is a hydrophobic compound, triacylglycerol, for reasons discussed in earlier chapters. Other hydrophobic molecules play important roles in cellular function, particularly the sterol cholesterol and its esters (cholesteryl esters). Mechanisms for transporting these non-water soluble lipid species in the blood have therefore evolved.
Non-esterified fatty acids are carried in the plasma bound to albumin. Some fat-soluble micronutrients and regulators of metabolism - e.g. fat-soluble vitamins and steroid hormones - are transported in the plasma by specific carrier proteins such as the cortisol-binding globulin, which carries cortisol. The transport of both triacylglycerol and cholesterol occurs in specialised macromolecular structures known as lipoproteins. Because triacylglycerol and cholesterol are carried by the same system, the metabolism of these two types of lipid in the plasma is closely interrelated.
The lipoproteins are particles with a lipid, highly hydrophobic core and a relatively hydrophilic outer surface. A typical lipoprotein particle (Fig. 9.1) consists of a core of triacylglycerol and cholesteryl ester, with an outer surface monolayer of phospholipid and free cholesterol. (As discussed in Section 184.108.40.206, cholesteryl esters are highly hydrophobic. By comparison, free cholesterol - i.e. unesterified cholesterol - has amphipathic properties because of its hydroxyl group.) The amphipathic phospholipids and cholesterol stabilise the particle in the aqueous environment of the plasma: their hydrophobic (polar) heads face outwards, and their hydrophilic tails protrude into the particle. The term 'particle' is a technical one: these are not solid particles, but more like small lipid droplets. In fact they are emulsion particles. They enable fat to be stably incorporated into plasma in the same way as the phospholipids
«O— Free cholesterol Phospholipid
«O— Free cholesterol Phospholipid
Cholesteryl ester Triacylglycerol
Fig. 9.1 A typical lipoprotein particle. From Hyperlipidaemia: diagnosis and management, 2nd edition, by P.N. Durrington (1995). Reprinted by permission of Elsevier Science present in egg yolk (mainly phosphatidylcholine) stabilise olive oil in vinegar when making mayonnaise, and in the same way that droplets of triacylglycerol are stabilised by phospholipids in milk.
Each lipoprotein particle has associated with it one or more protein molecules, the apolipoproteins. These proteins have hydrophobic domains, which 'dip into' the core and anchor the protein to the particle, and also hydrophilic domains that are exposed at the surface.
The lipoproteins consist of a heterogeneous group of particles with different lipid and protein compositions, and different sizes. They have traditionally been separated into groups, or fractions, based on either electrophoretic mobility or flotation in an ultracentrifuge. The latter technique has given rise to a much-used classification system, which will be used here. It could almost be viewed as a coincidence that the fractions isolated in the ultracentrifuge also have some functional distinction. However, the distinctions are not absolute, and each ultracentrifugal fraction may consist of a range of particles with somewhat different metabolic functions.
The characteristics of the major lipoprotein fractions are listed in Table 9.1. The chylomicron and VLDL particles are relatively rich in triacylglycerol and are often referred to together as the triacylglycerol-rich lipoproteins; they are mainly concerned with delivery of triacylglycerol to tissues. The smaller LDL and HDL particles, on the other hand, are more involved with transport of cholesterol to and from cells. The major apolipoproteins involved in lipoprotein metabolism are listed in Box 9.1, and some important enzymes involved with the lipoproteins in the plasma are listed in Box 9.2.
Table 9.1 Characteristics of the major lipoprotein classes.
Fraction Density range. Diameter, Major lipids Major Composition (percentage by weight)
g/ml nm apolipoproteins
Chylomicrons <0.950 80-1000
Very low density lipoproteins 0.950-1.006 30-80 (VLDL)
Low density lipoproteins (LDL) 1.019-1.063 20-25
High density lipoproteins 1.063-1.210 9-15 (HDL)
Endogenous TG B100, C, E (from liver)
Cholesterol and B100 cholesteryl ester Cholesteryl ester Al, Al I, C, E and PL
Protein 1 10
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