1. FATTY ACID OCCURRENCE AND DISTRIBUTION
15
III. Formation of Fatty Acids and Lipids in the Living Organism
A. GENERAL CONSIDERATIONS
A similar mechanism of fatty acid synthesis involving the successive
addition of active acetate or acetyl coenzyme A (acetyl Co A) units to
form the long-chain fatty acids, as outlined in the Lynen fatty acid cycle
(80), operates in both plants and animals. It may be expected, therefore, that similar types of fatty acids, including particularly palmitic,
oleic, and, to a lesser extent, stearic acids will be found in both.
The presence in plants of a wider range and a greater diversity of
fatty acids than in animals suggests that plants are provided with more
varied or less specific enzymes. An explanation has still to be found
for the formation of linoleic and linolenic acids in plants, but not in
animals. Explanations are similarly required for the formation of specific
hydroxy acids, cyclic acids, and acids containing acetylenic bonds. It
has been suggested that lactobacillic acid with a cyclopropane ring is
formed from ds-vaccenic acid by the addition of a methylenic group
across the double bond (24). If this occurs in lactobacillus species why
does it not occur generally in plants and animals?
B. FORMATION OF FATS IN ANIMALS
The fats of animals are derived: (a) endogenously by synthesis from
the nonfatty constituents of the diet; and (b) exogenously from the
fats in the diet, with or without modification.
1. Endogenous Fat
It has long been known that animals on low fat-content diets deposit more fat than is present in the diet and that when fat-free diets
are used, carbohydrates or proteins give rise to similar fats, as shown
in Table I.
It will be seen from Table I that endogenous fat consists essentially
of Ci 6 and Ci 8 acids. Palmitic acid is fairly constant at 25-30%, and
the unsaturated acids are largely monoethenoid. The traces of highly
unsaturated C 20 and C 22 acids originate from the dietary linoleic and
linolenic acids by the addition of acetate to the carboxyl end followed
by desaturation. Linoleic acid gives rise to acids with the first double
bond at carbon 6 from the methyl group end (linoleic type), and linolenic acid forms the linolenic type with the unsaturation commencing at
carbon 3 from the methyl group end (84, 85).
The mechanism of the formation of endogenous fat has only recently
been satisfactorily elucidated.
15
III. Formation of Fatty Acids and Lipids in the Living Organism
A. GENERAL CONSIDERATIONS
A similar mechanism of fatty acid synthesis involving the successive
addition of active acetate or acetyl coenzyme A (acetyl Co A) units to
form the long-chain fatty acids, as outlined in the Lynen fatty acid cycle
(80), operates in both plants and animals. It may be expected, therefore, that similar types of fatty acids, including particularly palmitic,
oleic, and, to a lesser extent, stearic acids will be found in both.
The presence in plants of a wider range and a greater diversity of
fatty acids than in animals suggests that plants are provided with more
varied or less specific enzymes. An explanation has still to be found
for the formation of linoleic and linolenic acids in plants, but not in
animals. Explanations are similarly required for the formation of specific
hydroxy acids, cyclic acids, and acids containing acetylenic bonds. It
has been suggested that lactobacillic acid with a cyclopropane ring is
formed from ds-vaccenic acid by the addition of a methylenic group
across the double bond (24). If this occurs in lactobacillus species why
does it not occur generally in plants and animals?
B. FORMATION OF FATS IN ANIMALS
The fats of animals are derived: (a) endogenously by synthesis from
the nonfatty constituents of the diet; and (b) exogenously from the
fats in the diet, with or without modification.
1. Endogenous Fat
It has long been known that animals on low fat-content diets deposit more fat than is present in the diet and that when fat-free diets
are used, carbohydrates or proteins give rise to similar fats, as shown
in Table I.
It will be seen from Table I that endogenous fat consists essentially
of Ci 6 and Ci 8 acids. Palmitic acid is fairly constant at 25-30%, and
the unsaturated acids are largely monoethenoid. The traces of highly
unsaturated C 20 and C 22 acids originate from the dietary linoleic and
linolenic acids by the addition of acetate to the carboxyl end followed
by desaturation. Linoleic acid gives rise to acids with the first double
bond at carbon 6 from the methyl group end (linoleic type), and linolenic acid forms the linolenic type with the unsaturation commencing at
carbon 3 from the methyl group end (84, 85).
The mechanism of the formation of endogenous fat has only recently
been satisfactorily elucidated.