440
C. B. COWEY AND J. R. SARQENT
residual 2-monoglyceride can be analysed to define the fatty acyl
moieties at the 2 position of the original triglycerides. In recent
years, however, a method of stereospecific analysis has been established
to enable individual fatty acids from all three positions of triglycerides to be examined separately (Fig. 10). This method was developed by Brockerhoff (1967) and subsequently applied by him and
his co-workers to a variety of fish triglycerides. It consists essentially
of cleaving the fatty acid from position 2 of a triglyceride chemically
with methyl magnesium bromide. The resulting 1,3-diglyceride is then
substituted, again chemically, at position 2 with a phosphatidyl phenol
group so as to resemble a phospholipid. The 1,3 diacyl, 2 phenylphosphatidyl glycerol can then be cleaved with phospholipase A to
release fatty acids from the 1 position. Fatty acid from the 3 position
is then recovered from the remaining 3 acyl, 2 phenylphosphatidyl
glycerol.
Before considering the results from these analyses it is to be
emphasized that analyses are carried out on mixed samples of triglycerides and phospholipids. The results consequently represent a
statistical distribution of fatty acids on different positions of populations of mixed molecules. Furthermore it has been repeatedly emphasized (Brockerhoff, 1966) that the results so far represent trends.
These trends cannot be considered as yet to represent hard and fast
rules, although relatively clear cut results have been obtained in many
cases.
Table XVII summarizes data on the complete stereospecific analysis
of triglycerides from a number of fishes. It may be noted that the
saturated acids are much more common in positions 1 and 3 than they
are in position 2. Position 3, however, differs from position 1 in
containing a higher proportion of longer chain acids. The polyunsaturated fatty acids, 20 : 5, 22 : 5 and 22 : 6 are consistently concentrated on position 2 and this position also contains the bulk of the
palmitic acid (16:O). Thus the fatty acids of major interest to
nutritionists are concentrated on position 2 of the triglycerides.
Although less work has been done with phospholipids Table XVIII
shows that in this case also polyunsaturated acids are concentrated on
position 2. Similarly it is well established that position 2 contains the
bulk of polyunsaturated, essential fatty acids in phospholipids from
land animals (Menzell and Olcott, 1964 ; Carroll, 1965). In contrast to
triglycerides, however, fish phospholipids contain the bulk of palmitic
acid in position 1.
Such data, especially those on fish triglycerides, may be considered
in terms of the concept of a metabolically stable 2-monoglyceride
C. B. COWEY AND J. R. SARQENT
residual 2-monoglyceride can be analysed to define the fatty acyl
moieties at the 2 position of the original triglycerides. In recent
years, however, a method of stereospecific analysis has been established
to enable individual fatty acids from all three positions of triglycerides to be examined separately (Fig. 10). This method was developed by Brockerhoff (1967) and subsequently applied by him and
his co-workers to a variety of fish triglycerides. It consists essentially
of cleaving the fatty acid from position 2 of a triglyceride chemically
with methyl magnesium bromide. The resulting 1,3-diglyceride is then
substituted, again chemically, at position 2 with a phosphatidyl phenol
group so as to resemble a phospholipid. The 1,3 diacyl, 2 phenylphosphatidyl glycerol can then be cleaved with phospholipase A to
release fatty acids from the 1 position. Fatty acid from the 3 position
is then recovered from the remaining 3 acyl, 2 phenylphosphatidyl
glycerol.
Before considering the results from these analyses it is to be
emphasized that analyses are carried out on mixed samples of triglycerides and phospholipids. The results consequently represent a
statistical distribution of fatty acids on different positions of populations of mixed molecules. Furthermore it has been repeatedly emphasized (Brockerhoff, 1966) that the results so far represent trends.
These trends cannot be considered as yet to represent hard and fast
rules, although relatively clear cut results have been obtained in many
cases.
Table XVII summarizes data on the complete stereospecific analysis
of triglycerides from a number of fishes. It may be noted that the
saturated acids are much more common in positions 1 and 3 than they
are in position 2. Position 3, however, differs from position 1 in
containing a higher proportion of longer chain acids. The polyunsaturated fatty acids, 20 : 5, 22 : 5 and 22 : 6 are consistently concentrated on position 2 and this position also contains the bulk of the
palmitic acid (16:O). Thus the fatty acids of major interest to
nutritionists are concentrated on position 2 of the triglycerides.
Although less work has been done with phospholipids Table XVIII
shows that in this case also polyunsaturated acids are concentrated on
position 2. Similarly it is well established that position 2 contains the
bulk of polyunsaturated, essential fatty acids in phospholipids from
land animals (Menzell and Olcott, 1964 ; Carroll, 1965). In contrast to
triglycerides, however, fish phospholipids contain the bulk of palmitic
acid in position 1.
Such data, especially those on fish triglycerides, may be considered
in terms of the concept of a metabolically stable 2-monoglyceride