3. LIPIDS: STEROID METABOLISM
197
pregnant mare shows a curious change in urinary excretion of estrogens
during pregnancy. Estrone and estradiol-17/3 are predominant up to
about mid-pregnancy. In the later stages estradiol-17ß is progressively
replaced by equilin to equilenin. It is not known whether this is a true
metabolic change or a change in the method of excretion of these
steroids. The total amount of estrogenic material, as judged by bioassay,
also falls after mid-pregnancy. The cow in contrast with the mare excretes estrogenic material in the urine at a steadily increasing rate as
pregnancy advances. The significance of these changes would be easier
to interpret if the actual substances concerned were determined by
reliable chemical methods.
One of the remarkable features of the cattle urines is the low concentration of neutral steroids. Thus the major neutral steroid of pregnant cow's urine is 5/?-androstane-3a,17a-diol present in amounts of
about 0.2 mg. per liter. From 20 to 40 mg. "apparent ketosteroid" were
found per liter of cattle urine, but of this only about 0.3 mg. per liter
is true ketosteroid (159). The high values have been attributed to interference from ionone compounds in the diet. Of the species investigated
so far, appreciable amounts of ketosteroids have been found only in the
urine of man and guinea pigs (160-164).
F. CONCLUSIONS
The principal known pathways of cholesterol metabolism in higher
animals are the formation of steroid hormones and of bile acids. The
latter is probably quantitatively the more important. Nothing is known
of sterol metabolism in lower animals or in plants or microorganisms.
The principal known metabolic transformations of steroid hormones are
reductions, oxidations, and conjugations. The last results in the formation of more water-soluble substances which appear in the urine. There
are suggestions that fecal excretion of steroids may be of more importance in some species than others.
It is unfortunate that knowledge of steroid metabolism is restricted
almost exclusively to higher animals, and despite numerous recorded
species differences no body of facts exists which could be called a comparative biochemistry of steroid metabolism. This situation has almost
certainly been brought about by the influence of medical thinking and
the demands for the solution of the problems of endocrine diseases in
man. It is particularly striking that after the unprecedented expenditure
of time and effort on the production and study of cortisone and hydrocortisone it is now realized that these substances cure no disease and
that their prolonged administration for the relief of symptoms is accompanied by the risk of unpleasant and even dangerous side effects.
197
pregnant mare shows a curious change in urinary excretion of estrogens
during pregnancy. Estrone and estradiol-17/3 are predominant up to
about mid-pregnancy. In the later stages estradiol-17ß is progressively
replaced by equilin to equilenin. It is not known whether this is a true
metabolic change or a change in the method of excretion of these
steroids. The total amount of estrogenic material, as judged by bioassay,
also falls after mid-pregnancy. The cow in contrast with the mare excretes estrogenic material in the urine at a steadily increasing rate as
pregnancy advances. The significance of these changes would be easier
to interpret if the actual substances concerned were determined by
reliable chemical methods.
One of the remarkable features of the cattle urines is the low concentration of neutral steroids. Thus the major neutral steroid of pregnant cow's urine is 5/?-androstane-3a,17a-diol present in amounts of
about 0.2 mg. per liter. From 20 to 40 mg. "apparent ketosteroid" were
found per liter of cattle urine, but of this only about 0.3 mg. per liter
is true ketosteroid (159). The high values have been attributed to interference from ionone compounds in the diet. Of the species investigated
so far, appreciable amounts of ketosteroids have been found only in the
urine of man and guinea pigs (160-164).
F. CONCLUSIONS
The principal known pathways of cholesterol metabolism in higher
animals are the formation of steroid hormones and of bile acids. The
latter is probably quantitatively the more important. Nothing is known
of sterol metabolism in lower animals or in plants or microorganisms.
The principal known metabolic transformations of steroid hormones are
reductions, oxidations, and conjugations. The last results in the formation of more water-soluble substances which appear in the urine. There
are suggestions that fecal excretion of steroids may be of more importance in some species than others.
It is unfortunate that knowledge of steroid metabolism is restricted
almost exclusively to higher animals, and despite numerous recorded
species differences no body of facts exists which could be called a comparative biochemistry of steroid metabolism. This situation has almost
certainly been brought about by the influence of medical thinking and
the demands for the solution of the problems of endocrine diseases in
man. It is particularly striking that after the unprecedented expenditure
of time and effort on the production and study of cortisone and hydrocortisone it is now realized that these substances cure no disease and
that their prolonged administration for the relief of symptoms is accompanied by the risk of unpleasant and even dangerous side effects.