84
F. B. SHORLAND
condensation of acetyl CoA units as outlined in the Lynen fatty acid
cycle (80). However, there appears to be some variation in the operation of the acetate condensation process. In plants (128) and some
microorganisms (138) linoleic and linolenic acids, which do not
occur in the endogenous fats of animals, are also formed by the
successive condensation of acetate units, indicating the presence of
mechanisms in plants (123, 128, 129) that are absent in animals. There
is no substantial evidence that these more highly unsaturated acids are
formed by the desaturation of saturated acids or of oleic acid. On the
other hand, there is in animals a mechanism for the synthesis of the
highly unsaturated C 20 and C 22 unsaturated acids from the dietary
linoleic and linolenic acids by the addition of acetate followed by desaturation (84, 85). Such a mechanism is absent from land plants which
do not elaborate the highly unsaturated C 20 and C 22 acids. On the
other hand, the presence of such acids in aquatic plants suggests that
such a mechanism must also operate here.
The many different types of acids in plant life, some of which are
specific, such as the branched-chain acids of tubercle bacilli, petroselinic acid of the Umbelliferae, chaulmoogric acid of the Flacourtiaceae, to name a few, must surely be the result of the presence of diverse
enzyme systems, the nature of which remains obscure.
It is not possible to make completely satisfactory comparisons between the total fatty acids in animals and plants, as only the depot fats,
such as perinephric fat or seed fat, have been examined. The organ
lipids such as those of the liver differ in composition from their depot
fats, particularly in land mammals, and leaf fats differ from seed fats.
Nevertheless, allowing for these» defects it is useful to compare the distribution and nature of the fatty acids on the basis of the depot fats in
different organisms.
B. THE FATTY ACIDS OF DEPOT FATS OF ORGANISMS
In comparing the fatty acids in the depot fats of different organisms
it is convenient to consider the types of fatty acids separately. In the
discussion below the amounts of fatty acids are expressed as weight
percentages of the total fatty acids.
1. n-Saturated Acids
n-Saturated acids, usually mainly palmitic acid, are found in the
lipids of nearly all living organisms. The lowest recorded value is 0.5%
found in the fatty acids of the nettle (Urtica dioica) (231). The saturated acids in the seed fats range from 2% in the seed fats of the
Canadian nasturtiums (347) to over 90% in some of the Palmae (3).
F. B. SHORLAND
condensation of acetyl CoA units as outlined in the Lynen fatty acid
cycle (80). However, there appears to be some variation in the operation of the acetate condensation process. In plants (128) and some
microorganisms (138) linoleic and linolenic acids, which do not
occur in the endogenous fats of animals, are also formed by the
successive condensation of acetate units, indicating the presence of
mechanisms in plants (123, 128, 129) that are absent in animals. There
is no substantial evidence that these more highly unsaturated acids are
formed by the desaturation of saturated acids or of oleic acid. On the
other hand, there is in animals a mechanism for the synthesis of the
highly unsaturated C 20 and C 22 unsaturated acids from the dietary
linoleic and linolenic acids by the addition of acetate followed by desaturation (84, 85). Such a mechanism is absent from land plants which
do not elaborate the highly unsaturated C 20 and C 22 acids. On the
other hand, the presence of such acids in aquatic plants suggests that
such a mechanism must also operate here.
The many different types of acids in plant life, some of which are
specific, such as the branched-chain acids of tubercle bacilli, petroselinic acid of the Umbelliferae, chaulmoogric acid of the Flacourtiaceae, to name a few, must surely be the result of the presence of diverse
enzyme systems, the nature of which remains obscure.
It is not possible to make completely satisfactory comparisons between the total fatty acids in animals and plants, as only the depot fats,
such as perinephric fat or seed fat, have been examined. The organ
lipids such as those of the liver differ in composition from their depot
fats, particularly in land mammals, and leaf fats differ from seed fats.
Nevertheless, allowing for these» defects it is useful to compare the distribution and nature of the fatty acids on the basis of the depot fats in
different organisms.
B. THE FATTY ACIDS OF DEPOT FATS OF ORGANISMS
In comparing the fatty acids in the depot fats of different organisms
it is convenient to consider the types of fatty acids separately. In the
discussion below the amounts of fatty acids are expressed as weight
percentages of the total fatty acids.
1. n-Saturated Acids
n-Saturated acids, usually mainly palmitic acid, are found in the
lipids of nearly all living organisms. The lowest recorded value is 0.5%
found in the fatty acids of the nettle (Urtica dioica) (231). The saturated acids in the seed fats range from 2% in the seed fats of the
Canadian nasturtiums (347) to over 90% in some of the Palmae (3).