Page 65 - Color Atlas of Biochemistry

56 Biomolecules

Steroids: overview Aand Bare in cis position relative to each
other (seep. 54).One to threehydroxyl
The three most important groups of steroids groups (in α position) are found in the steroid
are the sterols, bile acids,and steroid hor- core at positions 3,7,and 12.Bileacids keep
mones. Particularly in plants, compounds bile cholesterol in a soluble state as micelles
with steroid structures are also found that and promote the digestion of lipids in the
are notable for their pharmacological ef- intestine (see p. 270). Cholic acid and cheno-
fects—steroid alkaloids, digitalis glycosides, deoxycholic acid are primary bile acids that
and saponins. are formed by the liver. Their dehydroxylation
at C-7 by microorganisms from the intestinal
flora gives rise to the secondary bile acids
A. Sterols
lithocholic acid and deoxycholic acid.
Sterols are steroid alcohols.Theyhave a
β-positioned hydroxyl group at C-3 and one
or more double bonds inringB andinthe side C. Steroid hormones
chain. There are no further oxygen functions, The conversion of cholesterol to steroid
as in the carbonyl and carboxyl groups. hormones (see p. 376) is of minor importance
The most important sterol in animals is quantitatively, but of major importance in
cholesterol. Plants and microorganisms have terms of physiology. The steroid hormones
a wide variety of closely related sterols in- are a group of lipophilic signal substances
stead of cholesterol—e. g., ergosterol, β-sitos- that regulate metabolism, growth, and repro-
terol,and stigmasterol. duction (see p. 374).
Cholesterolis present in allanimaltissues, Humans have six steroid hormones:
and particularly in neural tissue. It is a major progesterone, cortisol, aldosterone, testos-
constituent of cellular membranes, in which it terone, estradiol,and calcitriol.With the ex-
regulates fluidity (see p. 216). The storage and ception of calcitriol, these steroids have either
transport forms of cholesterol are its esters no side chain or only a short side one consist-
with fatty acids. In lipoproteins, cholesterol ing of two carbons. Characteristic for most of
and its fatty acid esters are associated with them is an oxo group at C-3, conjugated with
other lipids (see p. 278). Cholesterol is a con- a double bond between C-4 and C-5 of ring A.
stituent of the bile and is therefore found in Differences occur in rings C and D. Estradiol is
many gallstones. Its biosynthesis, metabo- aromatic in ring A, and its hydroxyl group at
lism, and transport are discussed elsewhere C-3 is therefore phenolic. Calcitriol differs
(see pp.172, 312). from other vertebrate steroid hormones; it
Cholesterol-rich lipoproteins of the LDL still contains the complete carbon framework
type are particularly important in the devel- of cholesterol, but lightdependent opening of
opment of arteriosclerosis, in which the arte- ring B turns it into what is termed a “secoste-
rial walls are altered in connection with an roid” (a steroid with an open ring).
excess plasma cholesterol level. In terms of Ecdysone is the steroid hormone of the
dietary physiology, it is important that plant arthropods. It can be regarded as an early
foodstuffs are low in cholesterol. By contrast, form of the steroid hormones. Steroid hor-
animal foods can contain large amounts of mones with signaling functions also occur in
cholesterol—particularly butter, egg yolk, plants.
meat,liver,and brain.

B. Bile acids

Bile acids are synthesized from cholesterol in
the liver (see p. 314). Their structures can
therefore be derived from that of cholesterol.
Characteristic for the bile acids is a side chain
shortened by three C atoms in which the last
carbon atom is oxidized to a carboxyl group.
Thedoublebond in ringB is reduced and rings

Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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