2. STEROLS: STRUCTURE AND DISTRIBUTION
121
(62). Von Behring interpreted his negative findings as demonstrating
"that sterols are not fundamentally important compounds but make
their appearance only in the course of phylogenetic development," and
also "that sterols are connected with functions which are acquired
later."
Equally unsuccessful have been attempts to detect sterols in certain
primitive algae of the class of Myxophyceae (63). Such observations
are of considerable interest for the Myxophyceae and bacteria are
organisms without a well-formed nucleus. Such simultaneous absence
of sterols and a well-developed nucleus suggests that sterols first appear in the course of evolution with the formation of the cell nucleus.
Attractive as it is, this hypothesis is in need of broader experimental
support. There are for example some indications that certain bacteria
such as Azotobacter chroöcoccum (64) and Escherichia coli (65) contain small but definite amounts of sterols. These organisms should be
reinvestigated under the most rigid exclusion of extraneous sterols. It
should also be borne in mind that the absence of sterols in bacterial
lipids has been shown only by negative color reactions and the lack of
formation of insoluble digitonides. Saturated sterols, however, would
not be detectable by conventional color reactions, and only slight structural modification of the sterol molecule, such as an inversion of the 3hydroxyl group, leads to compounds not precipitated by digitonin. Until
more reliable data have been gathered in this most attractive twilight
zone of sterol production, we can state with certainty only that a
number of bacteria and algae are devoid of the type of sterols normally
met with in plants and animals.
Our present knowledge of the sterols of Protozoa, summarized in
Table II, is quite fragmentary. Such paucity of information is mainly
due to the extraordinary difficulties encountered in securing uniform
material in quantities large enough for careful chemical studies. The
few protozoans, whose sterols have been isolated, are relatively rich in
lipids. These in turn contain the high percentage of unsaponifiable
material which is so characteristic of fats of lower invertebrates (S).
Cholesterol has been isolated from the fish parasite, Eimeria gadi (66)
and the mycetozoan, Labyrinthula minuta (67). In the former it may
well be of exogenous origin, but in the latter which has been grown on
a sterol-free medium, its endogenous origin appears certain. Cholesterol
is probably also the principal sterol of the malaria organism, Tlasmodium
knowlesi (68). Unidentified sterols have been isolated from the familiar,
pelagic flagellate, Noctiluca miliaris (69), and the ciliate, Tetrahymena
geleii (70).
It has been shown with certainty that some, but not all, protozoans
121
(62). Von Behring interpreted his negative findings as demonstrating
"that sterols are not fundamentally important compounds but make
their appearance only in the course of phylogenetic development," and
also "that sterols are connected with functions which are acquired
later."
Equally unsuccessful have been attempts to detect sterols in certain
primitive algae of the class of Myxophyceae (63). Such observations
are of considerable interest for the Myxophyceae and bacteria are
organisms without a well-formed nucleus. Such simultaneous absence
of sterols and a well-developed nucleus suggests that sterols first appear in the course of evolution with the formation of the cell nucleus.
Attractive as it is, this hypothesis is in need of broader experimental
support. There are for example some indications that certain bacteria
such as Azotobacter chroöcoccum (64) and Escherichia coli (65) contain small but definite amounts of sterols. These organisms should be
reinvestigated under the most rigid exclusion of extraneous sterols. It
should also be borne in mind that the absence of sterols in bacterial
lipids has been shown only by negative color reactions and the lack of
formation of insoluble digitonides. Saturated sterols, however, would
not be detectable by conventional color reactions, and only slight structural modification of the sterol molecule, such as an inversion of the 3hydroxyl group, leads to compounds not precipitated by digitonin. Until
more reliable data have been gathered in this most attractive twilight
zone of sterol production, we can state with certainty only that a
number of bacteria and algae are devoid of the type of sterols normally
met with in plants and animals.
Our present knowledge of the sterols of Protozoa, summarized in
Table II, is quite fragmentary. Such paucity of information is mainly
due to the extraordinary difficulties encountered in securing uniform
material in quantities large enough for careful chemical studies. The
few protozoans, whose sterols have been isolated, are relatively rich in
lipids. These in turn contain the high percentage of unsaponifiable
material which is so characteristic of fats of lower invertebrates (S).
Cholesterol has been isolated from the fish parasite, Eimeria gadi (66)
and the mycetozoan, Labyrinthula minuta (67). In the former it may
well be of exogenous origin, but in the latter which has been grown on
a sterol-free medium, its endogenous origin appears certain. Cholesterol
is probably also the principal sterol of the malaria organism, Tlasmodium
knowlesi (68). Unidentified sterols have been isolated from the familiar,
pelagic flagellate, Noctiluca miliaris (69), and the ciliate, Tetrahymena
geleii (70).
It has been shown with certainty that some, but not all, protozoans