Biology of Disease

X]VeiZg&)/ DISORDERS OF THE CARDIOVASCULAR SYSTEM

)%' W^dad\nd[Y^hZVhZ

Cholesterol and Lipoproteins

Aside from any inherited tendency to develop hypertension (Section 14.17),

there are a number of nongenetic factors that correlate with an increased

risk of cardiovascular disease. These include smoking, lack of exercise, a

diet containing too much and inappropriate fat types (Chapter 10); all lead

to an abnormal concentration of cholesterol in the blood. Homocysteine is

known to be raised in certain genetic conditions and coronary heart disease

and is suspected to have a role in increasing cholesterol levels. Homocysteine

concentrations can be lowered by treatments with folate and vitamin B 12 with

a subsequent reduction in blood cholesterol.

Cholesterol is essential because it forms part of the plasma membrane of cells

and is used in the biosynthesis of bile salts and steroid hormones. However,

atherosclerosis is largely due to problems with cholesterol. The body can

synthesize it but it is also obtained from the diet. Cholesterol is practically

insoluble in plasma and, like triacylglycerols, is transported in the blood

inlipoprotein particles. It is an imbalance between the different types of

lipoprotein particles that leads to clinical problems.

Plasma lipoprotein particles consist of a core of triacylglycerols and cholesterol

esters surrounded by phospholipids, proteins and free cholesterol (Figure

14.14). They are classified by their densities, the greater the proportion of

triacylglycerol the lower the density (Table 14.1). There are several different

types of lipoproteins in lipoprotein particles, called apolipoprotein-A and –B,

-C, -D and -E, usually abbreviated to apoA, apoB, apoC, apoD and apoE.

The transport of cholesterol round the body is a complicated process (Figure

14.15). There is some cholesterol in the chylomicrons derived from the diet,

but the liver exports the cholesterol obtained from the diet or synthesized,

together with triacylglycerols it synthesized from dietary carbohydrate,

X]VeiZg&)/ DISORDERS OF THE CARDIOVASCULAR SYSTEM

W^dad\nd[Y^hZVhZ

Femoral vein

Short

saphenous

vein

Long

saphenous

vein

Figure 14.13 Schematic to show the saphenous

veins of the leg.

Apolipoprotein

Cholesterol

Cholesterol

esters

Triacylglycerols

Phospholipids

Figure 14.14 Schematic of a generalized

lipoprotein particle.