8. Lipids and Essential Fatty Acids in Aquatic Food Webs
171
one out of three will tend to be polyunsaturated in TAGs (Sargent et aI., 1993b).
This enzyme mechanism, along with other differences in enzyme specificity, will
secure some degree of genetic control of the composition of the membranes,
which is critical from a functional point of view. The fatty acid composition of the
membrane affects the activity of the enzymes, and a certain fraction of PUFA, or
more specifically DHA, is needed to ensure optimum activity of the enzymes
associated with the cell membranes (Dratz and Holte, 1992; Stubbs, 1992).
Enzymes of PL synthesis seem to exhibit low specificity for individual PUFA
(Mourente et aI., 1991). Even though the enzymes show higher affinity to PUFA
than to monounsaturated and saturated fatty acids, the enzymes may still incorporate enhanced levels of non-EFA (polyunsaturated, monounsaturated, and saturated fatty acids) in the membrane PLs under conditions of EFA deficiency,
resulting in a reduced membrane fraction of PUFA (Sargent et aI., 1993b). We
may anticipate that a reduced fraction of PUFA, which is species-specific, in
the membranes will, at some point, result in gradually reduced activity of all
membrane-bound enzymes. The full impact of a suboptimal membrane composition can hardly be imagined, but the physiological capacity or the general health
of deficient specimens will surely be reduced. This explains why inadequate
dietary EFA supplies may affect animal health in a general fashion.
8.2.3.4. Regulation of Metabolism
From lipid studies of humans and other warm-blooded animals, we have learned
that the C20 fatty acids (EPA and AA) of the membrane PLs are precursors in
prostaglandin synthesis (Fig. 8.5). These prostaglandins are themselves precursors for a number of compounds known as tissue hormones. Older literature
describing the formation and interactions of prostaglandins may be confusing, but
a useful description is given by Weber (1989). He states that prostaglandin G J ,
which is synthesized from EPA, is a regulatory antagonist to prostaglandin G z ,
which is synthesized from AA. Both AA and EPA are derived from the membrane
PLs (see further speculations in Sargent et aI., 1993b). The GiG z ratio, which is
believed to affect or control many cellular processes (Weber, 1989), will then
depend on the ratio of EPA to AA in the membranes and finally on the ratio of the
specific ro3 to ro6 in the diet. Marine fish may be less able to elongate EFA than
humans, but we can most likely apply our knowledge from human research to fish
and other animals. Keep in mind, though, that the values of typical and critical
ratios of fatty acids EPNAA and prostaglandins GzlG J are most likely different in
fish and humans.
The relatively poor enzymatic control (i.e., genetic control) of PL composition
(i.e., EPNAA or ro3/ro6 ratios) and ofthe successive prostaglandin synthesis (Gi
G z ratio) implies that the dietary fatty acid composition of animals will more or
less directly affect their regulatory hormonal processes (review by Bell et aI.,
1991; Sargent et aI., 1993b; Weber, 1989). The current recommended dietary ratio
ro3/ro6 for humans is roughly in the range of 0.1-1, but this range is still partly
based on an educated guess. The human diet during the Stone Age period is
171
one out of three will tend to be polyunsaturated in TAGs (Sargent et aI., 1993b).
This enzyme mechanism, along with other differences in enzyme specificity, will
secure some degree of genetic control of the composition of the membranes,
which is critical from a functional point of view. The fatty acid composition of the
membrane affects the activity of the enzymes, and a certain fraction of PUFA, or
more specifically DHA, is needed to ensure optimum activity of the enzymes
associated with the cell membranes (Dratz and Holte, 1992; Stubbs, 1992).
Enzymes of PL synthesis seem to exhibit low specificity for individual PUFA
(Mourente et aI., 1991). Even though the enzymes show higher affinity to PUFA
than to monounsaturated and saturated fatty acids, the enzymes may still incorporate enhanced levels of non-EFA (polyunsaturated, monounsaturated, and saturated fatty acids) in the membrane PLs under conditions of EFA deficiency,
resulting in a reduced membrane fraction of PUFA (Sargent et aI., 1993b). We
may anticipate that a reduced fraction of PUFA, which is species-specific, in
the membranes will, at some point, result in gradually reduced activity of all
membrane-bound enzymes. The full impact of a suboptimal membrane composition can hardly be imagined, but the physiological capacity or the general health
of deficient specimens will surely be reduced. This explains why inadequate
dietary EFA supplies may affect animal health in a general fashion.
8.2.3.4. Regulation of Metabolism
From lipid studies of humans and other warm-blooded animals, we have learned
that the C20 fatty acids (EPA and AA) of the membrane PLs are precursors in
prostaglandin synthesis (Fig. 8.5). These prostaglandins are themselves precursors for a number of compounds known as tissue hormones. Older literature
describing the formation and interactions of prostaglandins may be confusing, but
a useful description is given by Weber (1989). He states that prostaglandin G J ,
which is synthesized from EPA, is a regulatory antagonist to prostaglandin G z ,
which is synthesized from AA. Both AA and EPA are derived from the membrane
PLs (see further speculations in Sargent et aI., 1993b). The GiG z ratio, which is
believed to affect or control many cellular processes (Weber, 1989), will then
depend on the ratio of EPA to AA in the membranes and finally on the ratio of the
specific ro3 to ro6 in the diet. Marine fish may be less able to elongate EFA than
humans, but we can most likely apply our knowledge from human research to fish
and other animals. Keep in mind, though, that the values of typical and critical
ratios of fatty acids EPNAA and prostaglandins GzlG J are most likely different in
fish and humans.
The relatively poor enzymatic control (i.e., genetic control) of PL composition
(i.e., EPNAA or ro3/ro6 ratios) and ofthe successive prostaglandin synthesis (Gi
G z ratio) implies that the dietary fatty acid composition of animals will more or
less directly affect their regulatory hormonal processes (review by Bell et aI.,
1991; Sargent et aI., 1993b; Weber, 1989). The current recommended dietary ratio
ro3/ro6 for humans is roughly in the range of 0.1-1, but this range is still partly
based on an educated guess. The human diet during the Stone Age period is