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8 Focus on Health & Medicine: Cholesterol, the Most Prominent Steroid
Cholesterol is obtained in the diet from a variety of sources, including meat, cheese, butter, and
eggs. Table 19.4 lists the cholesterol content in some foods. While the American Heart Association currently recommends that the daily intake of cholesterol should be less than 300 mg, the
average American diet includes 400–500 mg of cholesterol each day.
Why is cholesterol classified as a lipid?
(a) Label the rings of the steroid nucleus in cholesterol. (b) Give the number of the carbon to
which the OH group is bonded. (c) Between which two carbons is the double bond located?
(d) Label the polar bonds in cholesterol and explain why it is insoluble in water.
Cholesterol Content in Some Foods
While health experts agree that the amount of cholesterol in the diet should be limited, it is also
now clear that elevated blood cholesterol (serum cholesterol) can lead to coronary artery disease.
It is estimated that only 25% of the cholesterol in the blood comes from dietary sources, with the
remainder synthesized in the liver. High blood cholesterol levels are associated with an increased
risk of developing coronary artery disease, heart attack, and stroke. To understand the relationship between cholesterol and heart disease, we must learn about how cholesterol is transported
through the bloodstream.
Like other lipids, cholesterol is insoluble in the aqueous medium of the blood, since it has only
one polar OH group and many nonpolar C C and C H bonds. In order for it to be transported
from the liver where it is synthesized, to the tissues, cholesterol combines with phospholipids and
proteins to form small water-soluble spherical particles called lipoproteins.
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FOCUS ON HEALTH & MEDICINE: CHOLESTEROL, THE MOST PROMINENT STEROID
ester of cholesterol
The surface of the lipoprotein particle
contains polar or ionic groups.
Cholesterol is embedded in the
phospholipid surface, with the nonpolar
carbon skeleton in the interior and the
polar OH group pointing outward.
The interior of the lipoprotein
particle is hydrophobic.
In a lipoprotein, the polar heads of phospholipids and the polar portions of protein molecules are
arranged on the surface. The nonpolar molecules are buried in the interior of the particle. In this
way, the nonpolar material is “dissolved” in an aqueous environment.
Lipoproteins are classified on the basis of their density, with two types being especially important
in determining serum cholesterol levels.
• Low-density lipoproteins (LDLs) transport cholesterol from the liver to the tissues.
• High-density lipoproteins (HDLs) transport cholesterol from the tissues back to the liver.
LDL particles transport cholesterol to tissues where it is incorporated in cell membranes. When
LDLs supply more cholesterol than is needed, LDLs deposit cholesterol on the wall of arteries,
forming plaque (Figure 19.8). Atherosclerosis is a disease that results from the buildup of these
fatty deposits, restricting the flow of blood, increasing blood pressure, and increasing the likelihood of a heart attack or stroke. As a result, LDL cholesterol is often called “bad” cholesterol.
Plaque Formation in an Artery
a. Open artery
b. Blocked artery
a. Cross-section of a clear artery with no buildup of plaque
b. Artery almost completely blocked by the buildup of plaque
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HDL particles transport excess cholesterol from the tissues back to the liver, where it is converted
to other substances or eliminated. Thus, HDLs reduce the level of serum cholesterol, so HDL
cholesterol is often called “good” cholesterol.
Thus, a physical examination by a physician includes blood work that measures three quantities:
total serum cholesterol, HDL cholesterol, and LDL cholesterol. Current recommendations for
these values and the role of HDLs and LDLs in determining serum cholesterol levels are shown
in Figure 19.9.
Several drugs called statins are now available to reduce the level of cholesterol in the bloodstream. These compounds act by blocking the synthesis of cholesterol at its very early stages.
Two examples include atorvastatin (Lipitor) and simvastatin (Zocor).
Generic name: atorvastatin
Trade name: Lipitor
Generic name: simvastatin
Trade bame: Zocor
FIGURE 19.9 HDLs, LDLs, and Cholesterol Level
Current recommendations by the National Cholesterol
• Total serum cholesterol: < 200 mg/dL
• HDL cholesterol: > 40 mg/dL
• LDL cholesterol: < 100 mg/dL
Cholesterol is synthesized in the liver. High HDL levels are
considered desirable since HDL transports cholesterol back
to the liver. Low LDLs are considered desirable to avoid the
buildup of plaque in the arteries.
Would you expect triacylglycerols to be contained in the interior of a lipoprotein particle or on
the surface with the phospholipids? Explain your choice.
Identify the functional groups in (a) atorvastatin; (b) simvastatin.
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19.9 STEROID HORMONES
Many biologically active steroids are hormones secreted by the endocrine glands. A hormone is
a molecule that is synthesized in one part of an organism, which then elicits a response at a
different site. Two important classes of steroid hormones are the sex hormones and the adrenal
There are two types of female sex hormones, estrogens and progestins.
Estradiol, progesterone, and oral
contraceptives were discussed in
• Estradiol and estrone are estrogens synthesized in the ovaries. They control the
development of secondary sex characteristics in females and regulate the menstrual
• Progesterone is a progestin often called the “pregnancy hormone.” It is responsible for
the preparation of the uterus for implantation of a fertilized egg.
The male sex hormones are called androgens.
• Testosterone and androsterone are androgens synthesized in the testes. They control the
development of secondary sex characteristics in males—growth of facial hair, increase in
muscle mass, and deepening of the voice.
Some body builders use anabolic
steroids to increase muscle mass.
Long-term or excessive use can
cause many health problems,
including high blood pressure,
liver damage, and cardiovascular
Synthetic androgen analogues, called anabolic steroids, promote muscle growth. They were first
developed to help individuals whose muscles had atrophied from lack of use following surgery.
They have since come to be used by athletes and body builders, although their use is not permitted
in competitive sports. Many physical and psychological problems result from their prolonged use.
Anabolic steroids, such as stanozolol, nandrolone, and tetrahydrogestrinone have the same effect
on the body as testosterone, but they are more stable, so they are not metabolized as quickly.
Tetrahydrogestrinone (also called THG or The Clear), the performance-enhancing drug used by
track star Marion Jones during the 2000 Sydney Olympics, was considered a “designer steroid”
because it was initially undetected in urine tests for doping. After its chemical structure and properties were determined, it was added to the list of banned anabolic steroids in 2004.
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A second group of steroid hormones includes the adrenal cortical steroids. Three examples of
these hormones are aldosterone, cortisone, and cortisol. All of these compounds are synthesized
in the outer layer of the adrenal gland. Aldosterone regulates blood pressure and volume by
controlling the concentration of Na+ and K+ in body fluids. Cortisone and cortisol serve as antiinflammatory agents and they regulate carbohydrate metabolism.
Cortisone and related compounds are used to suppress organ rejection after transplant surgery
and to treat many allergic and autoimmune disorders. Prolonged use of these steroids can have
undesired side effects, including bone loss and high blood pressure. Prednisone, a widely used
synthetic alternative, has similar anti-inflammatory properties but can be taken orally.
Compare the structures of estrone and progesterone. (a) Identify the differences in the A ring of
these hormones. (b) How do these hormones differ in functionality at C17?
Point out three structural differences between the female sex hormone estrone and the male sex
Identify the functional groups in aldosterone. Classify each alcohol as 1°, 2°, or 3°.
The male sex hormone testosterone and the anabolic steroid nandrolone have very similar
structures and, as you might expect, similar biological activity. Point out the single structural
difference in these two compounds.
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FOCUS ON HEALTH & MEDICINE: FAT-SOLUBLE VITAMINS
19.10 FOCUS ON HEALTH & MEDICINE
Vitamins are organic compounds required in small quantities for normal metabolism
(Section 11.7). Since our cells cannot synthesize these compounds, they must be obtained in the
diet. Vitamins can be categorized as fat soluble or water soluble. The fat-soluble vitamins are
The four fat-soluble vitamins—A, D, E, and K—are found in fruits and vegetables, fish, liver,
and dairy products. Although fat-soluble vitamins must be obtained from the diet, they do not
have to be ingested every day. Excess vitamins are stored in adipose cells, and then used when
needed. Table 19.5 summarizes the dietary sources and recommended daily intake of the fatsoluble vitamins.
Recommended Daily Intake
Liver, kidney, oily fish, dairy products, eggs,
fortified breakfast cereals
900 µg (men)
700 µg (women)
Fortified milk and breakfast cereals
Sunflower and safflower oils, nuts, beans,
whole grains, leafy greens
Cauliflower, soybeans, broccoli, leafy greens,
120 µg (men)
90 µg (women)
Source: Data from Harvard School of Public Health.
Vitamin A (Section 11.7) is obtained from liver, oily fish, and dairy products, and is synthesized
from β-carotene, the orange pigment in carrots. In the body, vitamin A is converted to 11-cisretinal, the light-sensitive compound responsible for vision in all vertebrates (Section 16.7). It is
also needed for healthy mucous membranes. A deficiency of vitamin A causes night blindness,
as well as dry eyes and skin.
Vitamin D, strictly speaking, is not a vitamin because it can be synthesized in the body from
cholesterol. Nevertheless, it is classified as such, and many foods (particularly milk) are fortified
with vitamin D so that we get enough of this vital nutrient. Vitamin D helps regulate both calcium
and phosphorus metabolism. A deficiency of vitamin D causes rickets, a bone disease characterized by knock-knees, spinal curvature, and other skeletal deformities.
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Vitamin E is an antioxidant, and in this way it protects unsaturated side chains in fatty acids from
unwanted oxidation (Section 13.12). A deficiency of vitamin E causes numerous neurological
problems, although it is rare for vitamin E deficiency to occur.
Vitamin K regulates the synthesis of prothrombin and other proteins needed for blood to clot.
A severe deficiency of vitamin K leads to excessive and sometimes fatal bleeding because of
inadequate blood clotting.
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