2. STEROLS: STRUCTURE AND DISTRIBUTION
105
same time, and for the same reasons, by those of Welsch (7). They were
published, however, in form of a dissertation only, and they consequently never received quite the attention they deserve. As early as fifty
years ago, therefore, sufficient evidence had been accumulated to
demonstrate clearly that there exists a diversity of sterols, at least among
the invertebrates.
For twenty-five years thereafter the field of comparative biochemistry
of sterols remained dormant. During this period numerous papers appeared with incidental references to the occurrence of cholesterol in
vertebrates, merely adding further testimony to the ubiquity of this
sterol in that phylum. Occasional references to the sterols of invertebrates were often of little value because of the a priori assumption that
cholesterol is the sterol of all animals. Only after the elucidation of the
structural skeleton of cholesterol in 1932 had the time become ripe
for a resumption of studies which had had such a promising beginning.
Due to the efforts of groups of investigators chiefly located in the United
States and Japan, data had become available by 1949 to permit the
author to survey this field from the viewpoint of comparative biochemistry, to point toward some interesting connections between the
sterol content of an animal and its classification, and to speculate on
the evolutionary aspects of the sterols (8).
II. The Sterols
A. GENERAL PROPERTIES
Sterols are admirably suited for comparative biochemical research.
They appear to be present in all animals and in a diversity so desirable
for comparative studies. In addition, they offer considerable practical
advantages. They are remarkably stable compounds particularly when
embedded in other organic material. Only the A
5>7
-sterols (I), minor
components of many sterol mixtures, are prone to undergo air oxidation.
The relative stability of other sterols permits the collection and storage
of biological material, air-dried or preserved by liquids, without fear of
substantial alterations of its sterol content. It also makes possible the
utilization of carefully selected museum specimens occasionally needed
to complete a comparative study. Sterols have also been isolated from
soils, and there exists good evidence for their fossil occurrence (9).
The isolation of sterols from biological material is generally accomplished with ease. Recrystallization of the unsaponifiable fraction of the
fat is frequently sufficient to obtain a crystalline sterol mixture, free
from other alcohols and from hydrocarbons. Where sterols constitute
only a minor fraction of the unsaponifiable matter, removal of the non-
105
same time, and for the same reasons, by those of Welsch (7). They were
published, however, in form of a dissertation only, and they consequently never received quite the attention they deserve. As early as fifty
years ago, therefore, sufficient evidence had been accumulated to
demonstrate clearly that there exists a diversity of sterols, at least among
the invertebrates.
For twenty-five years thereafter the field of comparative biochemistry
of sterols remained dormant. During this period numerous papers appeared with incidental references to the occurrence of cholesterol in
vertebrates, merely adding further testimony to the ubiquity of this
sterol in that phylum. Occasional references to the sterols of invertebrates were often of little value because of the a priori assumption that
cholesterol is the sterol of all animals. Only after the elucidation of the
structural skeleton of cholesterol in 1932 had the time become ripe
for a resumption of studies which had had such a promising beginning.
Due to the efforts of groups of investigators chiefly located in the United
States and Japan, data had become available by 1949 to permit the
author to survey this field from the viewpoint of comparative biochemistry, to point toward some interesting connections between the
sterol content of an animal and its classification, and to speculate on
the evolutionary aspects of the sterols (8).
II. The Sterols
A. GENERAL PROPERTIES
Sterols are admirably suited for comparative biochemical research.
They appear to be present in all animals and in a diversity so desirable
for comparative studies. In addition, they offer considerable practical
advantages. They are remarkably stable compounds particularly when
embedded in other organic material. Only the A
5>7
-sterols (I), minor
components of many sterol mixtures, are prone to undergo air oxidation.
The relative stability of other sterols permits the collection and storage
of biological material, air-dried or preserved by liquids, without fear of
substantial alterations of its sterol content. It also makes possible the
utilization of carefully selected museum specimens occasionally needed
to complete a comparative study. Sterols have also been isolated from
soils, and there exists good evidence for their fossil occurrence (9).
The isolation of sterols from biological material is generally accomplished with ease. Recrystallization of the unsaponifiable fraction of the
fat is frequently sufficient to obtain a crystalline sterol mixture, free
from other alcohols and from hydrocarbons. Where sterols constitute
only a minor fraction of the unsaponifiable matter, removal of the non-