154
WERNER BERGMANN
we do not known which invertebrates depend on an exogenous source
of sterols like the insects and which are capable of sterol synthesis like
the vertebrates. Certain phytophagous insects have been found capable
of dealkylating the "plant sterols" of their diet and converting them to
cholesterol, the principal sterol found in the adult animal. It is probable that other invertebrates which might depend on exogenous sterols
will also modify them to the compounds which are so characteristic of
certain classes, orders, and families of sponges, mollusks, and echinoderms regardless of their feeding habits or their geographic origin.
The greatest diversity of sterols is encountered among the most
primitive animals, such as sponges and coelenterates. Cholesterol may
be found in such primitive species, be they herbivorous sponges feeding
on phytoplankton or carnivorous sea anemones. It is, however, neither
the most typical nor the predominant sterol of lower invertebrates. At
higher levels of evolution other sterols disappear and cholesterol rises
to an increasingly dominant position until it has become the only principal sterol of the vertebrate. Once this evolution in the direction of the
almost exclusive use of cholesterol had become apparent, the writer
(8) suggested "that the reduction of a multitude of closely related compounds which seemingly perform closely related functions, to a few
compounds if not only one, is a general phenomenon of biochemical
evolution." In this evolution cholesterol may be viewed as representing
the survival of the fittest sterol {192, 193).
The diverse sterols of invertebrates performing similar functions may
be regarded as homologous in the biological sense. Many of them
are also homologs in the chemical sense since they differ from each
other only in the presence of one or more CH 2 -units. Thus ß-sitosterol
is a homolog of cholesterol in both the biological and chemical sense.*
In recent years the theory of the biochemical formation of sterols from
squalene (VIII-XIII) has been found to be in complete accord with
the experimental facts. The familiar homologs of cholesterol must be
formed by the attachment of one or two carbon atoms to the final
product or one of the intermediates in this transformation sequence. In
the case of the biosynthesis of ergosterol from yeast it has already been
shown that the addition of a single carbon unit to C-24 constitutes the
final step (194).
From the viewpoint of biosynthesis, therefore, cholesterol is the
primary and simpler sterol, and the alkyl groups at C-24 are secondary
features of as yet unknown significance. In primitive invertebrates these
additional features are nearly as common as the unsubstituted side chain
* An analogous connection between homology in the biological and chemical
sense is encountered among the fatty acids.
WERNER BERGMANN
we do not known which invertebrates depend on an exogenous source
of sterols like the insects and which are capable of sterol synthesis like
the vertebrates. Certain phytophagous insects have been found capable
of dealkylating the "plant sterols" of their diet and converting them to
cholesterol, the principal sterol found in the adult animal. It is probable that other invertebrates which might depend on exogenous sterols
will also modify them to the compounds which are so characteristic of
certain classes, orders, and families of sponges, mollusks, and echinoderms regardless of their feeding habits or their geographic origin.
The greatest diversity of sterols is encountered among the most
primitive animals, such as sponges and coelenterates. Cholesterol may
be found in such primitive species, be they herbivorous sponges feeding
on phytoplankton or carnivorous sea anemones. It is, however, neither
the most typical nor the predominant sterol of lower invertebrates. At
higher levels of evolution other sterols disappear and cholesterol rises
to an increasingly dominant position until it has become the only principal sterol of the vertebrate. Once this evolution in the direction of the
almost exclusive use of cholesterol had become apparent, the writer
(8) suggested "that the reduction of a multitude of closely related compounds which seemingly perform closely related functions, to a few
compounds if not only one, is a general phenomenon of biochemical
evolution." In this evolution cholesterol may be viewed as representing
the survival of the fittest sterol {192, 193).
The diverse sterols of invertebrates performing similar functions may
be regarded as homologous in the biological sense. Many of them
are also homologs in the chemical sense since they differ from each
other only in the presence of one or more CH 2 -units. Thus ß-sitosterol
is a homolog of cholesterol in both the biological and chemical sense.*
In recent years the theory of the biochemical formation of sterols from
squalene (VIII-XIII) has been found to be in complete accord with
the experimental facts. The familiar homologs of cholesterol must be
formed by the attachment of one or two carbon atoms to the final
product or one of the intermediates in this transformation sequence. In
the case of the biosynthesis of ergosterol from yeast it has already been
shown that the addition of a single carbon unit to C-24 constitutes the
final step (194).
From the viewpoint of biosynthesis, therefore, cholesterol is the
primary and simpler sterol, and the alkyl groups at C-24 are secondary
features of as yet unknown significance. In primitive invertebrates these
additional features are nearly as common as the unsubstituted side chain
* An analogous connection between homology in the biological and chemical
sense is encountered among the fatty acids.