3. LIPIDS: STEROID METABOLISM
169
detected in pig liver (26), and desmosterol (XIV) found in chicks
(27).
3. Sites of Cholesterol Biosynthesis in Animals
Although much of the work on cholesterol biosynthesis in mammals
has involved the liver, many other tissues in a variety of mammals and
birds have been found to synthesize this sterol. These include the
gonads and adrenals, aorta, blood, intestines, skeletal muscle, and skin.
It might be concluded that synthesis of cholesterol is a function of all
cells. Nervous tissue, however, is able to synthesize cholesterol only
during myelinization.
Investigations with liver cell "fractions" have indicated that the
endoplasmic reticulum (microsomes) and the soluble portion of the
cell sap are required for cholesterol synthesis (28a,b). Nuclei and mitochondria do not appear to be necessary in vitro although they may well
influence sterol synthesis in vivo.
4. Sterol Biosynthesis in Yeasts and Other Organisms
Although there is a considerable body of information on the distribution of sterols in living organisms, almost nothing is known of the biosynthesis of these substances in cells other than those of a few species
of animals and of yeasts.
Ergosterol, the characteristic of C 28 sterol of yeast, has been shown
to be formed from acetate (29), mevalonic acid (30), or squalene
(31). The additional carbon atom, the methyl group at C-24, is derived
from formate, which was shown not to contribute carbon atoms to any
other part of the structure (32).
An interesting group of 4,4',14-trimethyl steroids related to lanosterol
has been found to have antibiotic properties (33). These include
helvolic acid (XV) from the flora of sea water obtained near a sewage
outlet in Sardinia, eburicoic acid (XVI), and polyporenic acid (XVII)
found among products of wood-rotting fungi of the Basidiomycetes.
The fungus Polyporus sulfureus can also produce eburicoic acid, making
use of formate for carbon atom at C-28 (34). The absence of an oxygen
substituent at C-3 in helvolic acid is remarkable unless the organism by
which it is produced has the ability to remove the C-3 oxygen after the
steroid is synthesized.
The sterols of marine invertebrates have been extensively studied
with particular reference to the occurrence of these substances in the
different phyla and classes. Nothing is known of the details of sterol
biosynthesis in these animals, but substances postulated as intermediates
in cholesterol biosynthesis in higher animals have occasionally been
169
detected in pig liver (26), and desmosterol (XIV) found in chicks
(27).
3. Sites of Cholesterol Biosynthesis in Animals
Although much of the work on cholesterol biosynthesis in mammals
has involved the liver, many other tissues in a variety of mammals and
birds have been found to synthesize this sterol. These include the
gonads and adrenals, aorta, blood, intestines, skeletal muscle, and skin.
It might be concluded that synthesis of cholesterol is a function of all
cells. Nervous tissue, however, is able to synthesize cholesterol only
during myelinization.
Investigations with liver cell "fractions" have indicated that the
endoplasmic reticulum (microsomes) and the soluble portion of the
cell sap are required for cholesterol synthesis (28a,b). Nuclei and mitochondria do not appear to be necessary in vitro although they may well
influence sterol synthesis in vivo.
4. Sterol Biosynthesis in Yeasts and Other Organisms
Although there is a considerable body of information on the distribution of sterols in living organisms, almost nothing is known of the biosynthesis of these substances in cells other than those of a few species
of animals and of yeasts.
Ergosterol, the characteristic of C 28 sterol of yeast, has been shown
to be formed from acetate (29), mevalonic acid (30), or squalene
(31). The additional carbon atom, the methyl group at C-24, is derived
from formate, which was shown not to contribute carbon atoms to any
other part of the structure (32).
An interesting group of 4,4',14-trimethyl steroids related to lanosterol
has been found to have antibiotic properties (33). These include
helvolic acid (XV) from the flora of sea water obtained near a sewage
outlet in Sardinia, eburicoic acid (XVI), and polyporenic acid (XVII)
found among products of wood-rotting fungi of the Basidiomycetes.
The fungus Polyporus sulfureus can also produce eburicoic acid, making
use of formate for carbon atom at C-28 (34). The absence of an oxygen
substituent at C-3 in helvolic acid is remarkable unless the organism by
which it is produced has the ability to remove the C-3 oxygen after the
steroid is synthesized.
The sterols of marine invertebrates have been extensively studied
with particular reference to the occurrence of these substances in the
different phyla and classes. Nothing is known of the details of sterol
biosynthesis in these animals, but substances postulated as intermediates
in cholesterol biosynthesis in higher animals have occasionally been