442
0. B. UOWEY AND J. R. SAROENT
present in fish, must also be specific for position 1 and 3 of triglycerides.
It should be emphasized, however, that the above data show that the
2-monoglyceride structure is only partially retained. Subsequent work,
while supporting this conclusion, clearly shows that the greater part of
the dietary fat of fish is completely broken down before deposition
(Brockerhoff and Hoyle, 1967). Thus the larger part of depot triglyceride is likely to be synthesized by the de novo pathway of Weiss
and Kennedy (1956). Evidence exists that such a de novo biosynthesis
of triglyceride occurs readily in the livers of the dogfish, Squalus
acanthias L. (Malins and Sargent, 1971).
At f i s t sight it is difficult to understand why depot triglycerides
should contain a relatively stable backbone, because the only known
roles of triglycerides in cells are to provide fatty acids for energy or for
the biosynthesis of membrane lipid. Consequently it is in the very
nature of triglycerides that they must ultimately be completely broken
down. It is possible that the relative stability of the backbone structure
may be more apparent than real in fish triglycerides due to the very
high levels of polyenoic acids in the latter and the lower rates of turnover of triglycerides in fish than in land animals. It is interesting to note
that Brockerhoff (1966) has suggested that the 2-polyenoyl structure
derives basically from phospholipids. Thus, lecithin is known to be
readily cleaved enzymically to lysolecithin which can then be selectively
reacylated with unsaturated and short chain saturated fatty acids on
position 2 in vitro. Consequently a 2-polyenoyl glyceride structure is
inherent in phospholipids as was evident in Table XVIII. Lecithins
are likely t o be hydrolysed in the gut to Iysolecithins which are then
absorbed and reconverted to lecithins in the intestinal mucosa;
selective re-acylation of lysophosphatides with polyunsaturated acids
on position 2 will ensure an efficient retention of the 2-polyenoyl
structure in phospholipids of the food chain. Reversal of lecithin biosynthesis to produce a 1,a-diglyoeride and re-acylation of the latter on
position 3 could result in appearance of the 2-polyenoyl structure
in triglycerides. Consequently the origin of the 2-polyenoyl structure
in marine lipids in the phospholipids becomes an attractive possibility. It has been pointed out by Brockerhoff (1966) that polyenoic
and short chain saturated fatty acids are relatively polar so that
marine phospholipids will exhibit a pronounced polarity gradient from
position 3 to position 1. Such a gradient may well be fundamental to
the important role of phospholipids a t phase boundaries (membranes)
and may be especially important in biomembranes of animals in an
aquatic environment.
0. B. UOWEY AND J. R. SAROENT
present in fish, must also be specific for position 1 and 3 of triglycerides.
It should be emphasized, however, that the above data show that the
2-monoglyceride structure is only partially retained. Subsequent work,
while supporting this conclusion, clearly shows that the greater part of
the dietary fat of fish is completely broken down before deposition
(Brockerhoff and Hoyle, 1967). Thus the larger part of depot triglyceride is likely to be synthesized by the de novo pathway of Weiss
and Kennedy (1956). Evidence exists that such a de novo biosynthesis
of triglyceride occurs readily in the livers of the dogfish, Squalus
acanthias L. (Malins and Sargent, 1971).
At f i s t sight it is difficult to understand why depot triglycerides
should contain a relatively stable backbone, because the only known
roles of triglycerides in cells are to provide fatty acids for energy or for
the biosynthesis of membrane lipid. Consequently it is in the very
nature of triglycerides that they must ultimately be completely broken
down. It is possible that the relative stability of the backbone structure
may be more apparent than real in fish triglycerides due to the very
high levels of polyenoic acids in the latter and the lower rates of turnover of triglycerides in fish than in land animals. It is interesting to note
that Brockerhoff (1966) has suggested that the 2-polyenoyl structure
derives basically from phospholipids. Thus, lecithin is known to be
readily cleaved enzymically to lysolecithin which can then be selectively
reacylated with unsaturated and short chain saturated fatty acids on
position 2 in vitro. Consequently a 2-polyenoyl glyceride structure is
inherent in phospholipids as was evident in Table XVIII. Lecithins
are likely t o be hydrolysed in the gut to Iysolecithins which are then
absorbed and reconverted to lecithins in the intestinal mucosa;
selective re-acylation of lysophosphatides with polyunsaturated acids
on position 2 will ensure an efficient retention of the 2-polyenoyl
structure in phospholipids of the food chain. Reversal of lecithin biosynthesis to produce a 1,a-diglyoeride and re-acylation of the latter on
position 3 could result in appearance of the 2-polyenoyl structure
in triglycerides. Consequently the origin of the 2-polyenoyl structure
in marine lipids in the phospholipids becomes an attractive possibility. It has been pointed out by Brockerhoff (1966) that polyenoic
and short chain saturated fatty acids are relatively polar so that
marine phospholipids will exhibit a pronounced polarity gradient from
position 3 to position 1. Such a gradient may well be fundamental to
the important role of phospholipids a t phase boundaries (membranes)
and may be especially important in biomembranes of animals in an
aquatic environment.