446
U. B. OOWEY AXD J. R. SARGENT
cholesterol biosynthesized within the body. The synthesis of cholesterol
in the liver is controlled by (i) dietary intake of cholesterol by feedback
inhibition (see above), (ii) by the dietary caloric intake and (iii) by
hormonal factors (Kritchevsky et al., 1962; Dupont, 1968; Jensen
and Dam, 1966; Muroya et al., 1968). In contrast, the biosynthesis of
cholesterol in the intestine is controlled very largely by the levels of
bile salts secreted into the intestine by the liver. I n trout and goldfish
it has been shown that cholesterol synthesis in the intestinal tract is
controlled by dietary cholesterol (Siperstein and Luby, 1968).
Little if any attention has yet been paid to the optimal level of
cholesterol in fish diets despite the probable importance of this factor
in the well-being of the fbh. A very wide variety of sterols is present
in the marine environment (Bergman, 1962) so that fish may possibly
have a wider variety of sterols in their diet than land animals. Nevertheless cholesterol is the major if not the only sterol present in fish
tissues, with the exception of steroid hormones. The bile salts in teleost
fish are the same as those found in mammals (Haslewood, 1962) so that
it is very likely that cholesterol chemistry is at least qualitatively
similar in mammals and fishes. Cholesterol levels ranging from l - l O %
are present in flesh oils of a wide range of fish species (Shimma and
Taguchi, 19S4b) showing that fish possess the ability to store cholesterol
in considerable quantities in their adiposite tissue. This is entirely
expected since both the cholesterol and triglycerides stored in the
adiposite tissue are derived from the same dietary sources. Both the
cholesterol and the polyenoic acids in the fat depots are structural
reserves as distinct from energy reserves. It is likely that fish in the
natural state will have a, diet approximating in cholesterol content to
that of their own tissueswhichranges from 20-70 mg/100g (Shimma and
Taguchi, 1964) and, in the absence of definite information concerning
optimal dietary cholesterol levels for fish culture, the latter figure maybe
taken as a guide line. It should be noted that problems, such as lipid
accumulation, may well cause altered cholesterol metabolism in fish.
Much of the interest in cholesterol biosynthesis in fish stems from
the fact that the normal trace intermediate in cholesterol metabolism,
squalene, is found in enormous levels in the liver oils of certain sharks
(Tsjirnoto, 1932). This hydrocarbon has been implicated as a buoyancy
material in the livers of certain sharks that lack a swim bladder
(Corner et al., 1969). Acetate injected into sharks is found very largely
as squalene in the liver (Diplock and Haslewood, 1966) though small
amounts are also found in cholesterol (Kayama et al., 1969). Fractions
of the liver of Scyliorhinus are capable of metabolizing radioactive
mevalonic acid readily to squalene, though negligible formation of
U. B. OOWEY AXD J. R. SARGENT
cholesterol biosynthesized within the body. The synthesis of cholesterol
in the liver is controlled by (i) dietary intake of cholesterol by feedback
inhibition (see above), (ii) by the dietary caloric intake and (iii) by
hormonal factors (Kritchevsky et al., 1962; Dupont, 1968; Jensen
and Dam, 1966; Muroya et al., 1968). In contrast, the biosynthesis of
cholesterol in the intestine is controlled very largely by the levels of
bile salts secreted into the intestine by the liver. I n trout and goldfish
it has been shown that cholesterol synthesis in the intestinal tract is
controlled by dietary cholesterol (Siperstein and Luby, 1968).
Little if any attention has yet been paid to the optimal level of
cholesterol in fish diets despite the probable importance of this factor
in the well-being of the fbh. A very wide variety of sterols is present
in the marine environment (Bergman, 1962) so that fish may possibly
have a wider variety of sterols in their diet than land animals. Nevertheless cholesterol is the major if not the only sterol present in fish
tissues, with the exception of steroid hormones. The bile salts in teleost
fish are the same as those found in mammals (Haslewood, 1962) so that
it is very likely that cholesterol chemistry is at least qualitatively
similar in mammals and fishes. Cholesterol levels ranging from l - l O %
are present in flesh oils of a wide range of fish species (Shimma and
Taguchi, 19S4b) showing that fish possess the ability to store cholesterol
in considerable quantities in their adiposite tissue. This is entirely
expected since both the cholesterol and triglycerides stored in the
adiposite tissue are derived from the same dietary sources. Both the
cholesterol and the polyenoic acids in the fat depots are structural
reserves as distinct from energy reserves. It is likely that fish in the
natural state will have a, diet approximating in cholesterol content to
that of their own tissueswhichranges from 20-70 mg/100g (Shimma and
Taguchi, 1964) and, in the absence of definite information concerning
optimal dietary cholesterol levels for fish culture, the latter figure maybe
taken as a guide line. It should be noted that problems, such as lipid
accumulation, may well cause altered cholesterol metabolism in fish.
Much of the interest in cholesterol biosynthesis in fish stems from
the fact that the normal trace intermediate in cholesterol metabolism,
squalene, is found in enormous levels in the liver oils of certain sharks
(Tsjirnoto, 1932). This hydrocarbon has been implicated as a buoyancy
material in the livers of certain sharks that lack a swim bladder
(Corner et al., 1969). Acetate injected into sharks is found very largely
as squalene in the liver (Diplock and Haslewood, 1966) though small
amounts are also found in cholesterol (Kayama et al., 1969). Fractions
of the liver of Scyliorhinus are capable of metabolizing radioactive
mevalonic acid readily to squalene, though negligible formation of