3. LIPIDS: STEROID METABOLISM
191
which have not been observed in animal tissues. The discovery that
various microorganisms could bring about hydroxylations at various
positions in the steroid molecule is of importance from the theoretical
as well as the practical point of view since it provides test systems for
the study of mechanisms of hydroxylation about which much has still
to be learned. As with the mammalian steroid hydroxylating systems,
those in microorganisms incorporate oxygen into the hydroxyl group
from molecular oxygen but not from water (136). A second example
of similarity in mechanism of steroid enzymatic hydroxylation, irrespective of the origin of the enzyme system from animal tissues or microorganisms, is provided by the observation that in 11-hydroxylation there
is simple replacement of the hydrogen atom in the position to be
hydroxylated, and not Waiden inversion (137).
Animal organisms do not appear to degrade the ring structure of
steroids to any significant extent; for example C
14 0 2 could not be detected in the expired air of mice treated with 4-C
14 -testosterone (138).
There are, however, numerous examples of the utilization of steroids as
the sole source of carbon by microorganisms, and in the case of
Pseudomonas testosteroni it seems likely that the carbon must be derived
from the ring structure of testosterone (139).
Other bacteria undoubtedly exist which are able to degrade sterols
occurring naturally in their environment. Large amounts of plant and
animal sterols continually returned to the soil do not accumulate, and
the presence of microorganisms in soil which actively degrade cholesterol
has been demonstrated (140). Apart from the isolation of a keto acid
derived from cholesterol by oxidative fission of the A ring nothing is
known of the mechanism of this degradation (141).
Although much has been learned about the biosynthesis of sterols in
microorganisms, nothing is known of their functions or metabolism in
these organisms.
D. ABSORPTION AND METABOLISM OF STEROLS BY ANIMALS
Of all the steroids, only sterols are likely to be present in the diet of
animals in significant amounts. It is known that cholesterol is absorbed
from the intestines in mammals and birds. Interest in this absorption
centers round the fact that feeding cholesterol to certain species of
animals results in atherosclerosis. On a body weight basis, small animals
such as the rat, guinea pig, and rabbit absorb much more cholesterol
than larger animals, including man (108). Among the experimental
animals studied, the rabbit has an unusually high capacity to absorb
cholesterol whereas in man cholesterol absorption is poor even under
the most favorable conditions. The absorption of cholesterol is influenced
191
which have not been observed in animal tissues. The discovery that
various microorganisms could bring about hydroxylations at various
positions in the steroid molecule is of importance from the theoretical
as well as the practical point of view since it provides test systems for
the study of mechanisms of hydroxylation about which much has still
to be learned. As with the mammalian steroid hydroxylating systems,
those in microorganisms incorporate oxygen into the hydroxyl group
from molecular oxygen but not from water (136). A second example
of similarity in mechanism of steroid enzymatic hydroxylation, irrespective of the origin of the enzyme system from animal tissues or microorganisms, is provided by the observation that in 11-hydroxylation there
is simple replacement of the hydrogen atom in the position to be
hydroxylated, and not Waiden inversion (137).
Animal organisms do not appear to degrade the ring structure of
steroids to any significant extent; for example C
14 0 2 could not be detected in the expired air of mice treated with 4-C
14 -testosterone (138).
There are, however, numerous examples of the utilization of steroids as
the sole source of carbon by microorganisms, and in the case of
Pseudomonas testosteroni it seems likely that the carbon must be derived
from the ring structure of testosterone (139).
Other bacteria undoubtedly exist which are able to degrade sterols
occurring naturally in their environment. Large amounts of plant and
animal sterols continually returned to the soil do not accumulate, and
the presence of microorganisms in soil which actively degrade cholesterol
has been demonstrated (140). Apart from the isolation of a keto acid
derived from cholesterol by oxidative fission of the A ring nothing is
known of the mechanism of this degradation (141).
Although much has been learned about the biosynthesis of sterols in
microorganisms, nothing is known of their functions or metabolism in
these organisms.
D. ABSORPTION AND METABOLISM OF STEROLS BY ANIMALS
Of all the steroids, only sterols are likely to be present in the diet of
animals in significant amounts. It is known that cholesterol is absorbed
from the intestines in mammals and birds. Interest in this absorption
centers round the fact that feeding cholesterol to certain species of
animals results in atherosclerosis. On a body weight basis, small animals
such as the rat, guinea pig, and rabbit absorb much more cholesterol
than larger animals, including man (108). Among the experimental
animals studied, the rabbit has an unusually high capacity to absorb
cholesterol whereas in man cholesterol absorption is poor even under
the most favorable conditions. The absorption of cholesterol is influenced