3. LIPIDS: STEROID METABOLISM
171
^
CH 2 OH
Phytol-C 20
(4 isoprene units)
Acetate
Vitamins E, K
Chlorophyll
0'
0H|
"Active" isoprene
unit
Squalene -C 30
(6 Isoprene units)
Tetra, pentacyclic terpenes
Lanosterol
Cholesterol
Vitamin D
β -Carotene -C 40
(8 Isoprene units)
Vitamin A
FIG. 2. Formation of cholesterol as part of a general biosynthetic scheme involving mevalonic acid ("active" isoprene unit) as a general precursor. Adapted
from Isler and Zeller (39).
not synthesize sterols becomes less remarkable. Presumably mevalonic
acid is required for the biosynthesis of some essential polyisoprenoid
constituent of the organism.
The possibility of alternative pathways cannot, however, be excluded. Although similarities in the biogenesis of ß-carotene and
squalene exist, the results of Zabin (40) might be interpreted as indicating that the mechanism of formation of the typical carotenoid
lycopene may be quite unrelated to that of polyisoprenoid substances
in animals or that formation of the polymerizing units may differ in
171
^
CH 2 OH
Phytol-C 20
(4 isoprene units)
Acetate
Vitamins E, K
Chlorophyll
0'
0H|
"Active" isoprene
unit
Squalene -C 30
(6 Isoprene units)
Tetra, pentacyclic terpenes
Lanosterol
Cholesterol
Vitamin D
β -Carotene -C 40
(8 Isoprene units)
Vitamin A
FIG. 2. Formation of cholesterol as part of a general biosynthetic scheme involving mevalonic acid ("active" isoprene unit) as a general precursor. Adapted
from Isler and Zeller (39).
not synthesize sterols becomes less remarkable. Presumably mevalonic
acid is required for the biosynthesis of some essential polyisoprenoid
constituent of the organism.
The possibility of alternative pathways cannot, however, be excluded. Although similarities in the biogenesis of ß-carotene and
squalene exist, the results of Zabin (40) might be interpreted as indicating that the mechanism of formation of the typical carotenoid
lycopene may be quite unrelated to that of polyisoprenoid substances
in animals or that formation of the polymerizing units may differ in