444
C. B. COWEY AND J. R. SAROENT
oleic acids (Nevenzel, 1970). That is waxes are frequently deficient in
long chain polyunsaturated acids and consequently must be regarded
as being useful for energy provision only and not for structural purposes.
Biosynthesis of wax esters is known to occur by reduction of the
corresponding fatty acid (Sand and Schlenk, 1969) and esterification
of the alcohol so formed with another fatty acid. It is claimed that
wax esters can be synthesized in Squalus by a biochemical process not
requiring metabolic energy, i.e. reversal of a hydrolytic reaction
(Friedberg and Greene, 1967). Such a finding implies that species which
elaborate large quantities of wax esters (e.g. the mullet which deposits
considerable quantities in roe) need not expend metabolic energy during
this process. The requirement for metabolic energy in the form of ATP
in the biosynthesis of wax esters, triglycerides and diacyl glyceryl ethers
has more recently been shown in Squalus liver (Sargent et al., 1971).
Fatty alcohols are now known to be the direct precursors of glyceryl
ether structures (alkyl and alk-l-enyl glyceryls) in both mammals
(Snyder et al., 1970a, b) and in Squalus (Sargent and Malins, 1971).
Alkyl diacyl glycerols are present in large amounts in some
marine fish including elasmobranchs and the raffish (Chimaera) (Malins
and Wekell, 1970), but are unlikely to be present in more than traces
in commercially important species. Nonetheless the phospholipid
analogues of alk-l-enyl glycerols are very probably important constituents of membranes. Recent evidence demonstrates that the latter
compounds are synthesized by routes analogous to those established for
lecithins and cephalins (WyMe et al., 1971). Biosynthetic provision of
fatty alcohol should present no problems in the elaboration of these
structures.
With regard to the possible use of dogfish liver oil as a dietary
supplement in fish culture it is worth noting that enzymes capable of
cleaving the ether linkage in glyceryl ethers are active in the intestine of
the trout (Salmo gairdneri) (Malins and Wekell, 1970). Dogfish liver
oil has already been used successfully in the culture of Salmo salar L.
(Bergstrom, 1968). In view of the extensive cleavage of glyceryl ethers
in trout intestine and almost certainly in other fish there would appear
to be little if any need to supplement artificial diets with these particular
ether structures.
C . Cholesterol
There are three roles for cholesterol. First, cholesterol is an important constituent of biomembrane systems in all eukaryotic species.
Indeed the appearance of membrane bounded organelles in the eukaryotes is accompanied by the appearance of cholesterol which is not
C. B. COWEY AND J. R. SAROENT
oleic acids (Nevenzel, 1970). That is waxes are frequently deficient in
long chain polyunsaturated acids and consequently must be regarded
as being useful for energy provision only and not for structural purposes.
Biosynthesis of wax esters is known to occur by reduction of the
corresponding fatty acid (Sand and Schlenk, 1969) and esterification
of the alcohol so formed with another fatty acid. It is claimed that
wax esters can be synthesized in Squalus by a biochemical process not
requiring metabolic energy, i.e. reversal of a hydrolytic reaction
(Friedberg and Greene, 1967). Such a finding implies that species which
elaborate large quantities of wax esters (e.g. the mullet which deposits
considerable quantities in roe) need not expend metabolic energy during
this process. The requirement for metabolic energy in the form of ATP
in the biosynthesis of wax esters, triglycerides and diacyl glyceryl ethers
has more recently been shown in Squalus liver (Sargent et al., 1971).
Fatty alcohols are now known to be the direct precursors of glyceryl
ether structures (alkyl and alk-l-enyl glyceryls) in both mammals
(Snyder et al., 1970a, b) and in Squalus (Sargent and Malins, 1971).
Alkyl diacyl glycerols are present in large amounts in some
marine fish including elasmobranchs and the raffish (Chimaera) (Malins
and Wekell, 1970), but are unlikely to be present in more than traces
in commercially important species. Nonetheless the phospholipid
analogues of alk-l-enyl glycerols are very probably important constituents of membranes. Recent evidence demonstrates that the latter
compounds are synthesized by routes analogous to those established for
lecithins and cephalins (WyMe et al., 1971). Biosynthetic provision of
fatty alcohol should present no problems in the elaboration of these
structures.
With regard to the possible use of dogfish liver oil as a dietary
supplement in fish culture it is worth noting that enzymes capable of
cleaving the ether linkage in glyceryl ethers are active in the intestine of
the trout (Salmo gairdneri) (Malins and Wekell, 1970). Dogfish liver
oil has already been used successfully in the culture of Salmo salar L.
(Bergstrom, 1968). In view of the extensive cleavage of glyceryl ethers
in trout intestine and almost certainly in other fish there would appear
to be little if any need to supplement artificial diets with these particular
ether structures.
C . Cholesterol
There are three roles for cholesterol. First, cholesterol is an important constituent of biomembrane systems in all eukaryotic species.
Indeed the appearance of membrane bounded organelles in the eukaryotes is accompanied by the appearance of cholesterol which is not