24
F. B. SHORLAND
into esterified long-chain fatty acids. ATP, Co A, and Mn
2+ are essential
components of the system, and palmitic and oleic acids were the only
labeled acids that accumulate. It appears that fatty acid formation in
the avocado is by condensation of CoA-activitated acetate units. The
evidence supports the concept that as in animals the biosynthesis of
fatty acids in the avocado is through a mechanism essentially the reverse of ^-oxidation, which is known to occur in plants. Stumpf and
Barber (130) found, for example, that the mitochondria from peanut
cotyledons possessed a fatty acid oxidase system similar to that present
in animal tissues. The acetyl coenzyme A thus produced is similarly
oxidized via the Krebs cycle and some accumulation of acetoacetate also
occurs. However, in addition, in higher plants, other mechanisms are
present for the oxidation of fatty acids. One of these is associated with
the microsomal particles and catalyzes the oxidation of palmitate along
the carbon chain with the aid of DPN (131). Another, present in the
soluble fraction after removal of mitochondria and microsomes, oxidizes
only the carboxyl group of palmitic acid (132).
The mechanism for the interconversion of fat and carbohydrates in
plants has not been generally established. However, Beevers (133) has
shown that cell-free extracts of endosperm and cotyledons of castor
bean seedlings bring about the production of malate from isocitrate and
acetate, thereby providing a mechanism for the interconversion of fat
and carbohydrates.
D. SYNTHESIS OF FATS IN MICROORGANISMS
It is well known that yeasts and molds when grown on a glucose or
sucrose medium produce much fat, thus showing that, as in animals, a
mechanism exists for the conversion of carbohydrates to fats. The possibility of the participation of acetate units in this conversion is indicated
from early experiments with timothy grass bacillus (My cob act er turn phlei)
in which it was shown that the addition of acetate to the medium increased the amount of lipid formed (134). The first definite proof, however, of the participation of acetate in the synthesis of the lower fatty
acids in microorganisms was that of Wood et al. (135), who showed in
1945 that Clostridium acetobutylicum synthesizes butyric acid from two
molecules of acetate. Subsequently studies on C. kluyveri shed much
light on the mechanism of fatty acid synthesis. This anaerobic organism
requires for food only ethanol and acetate, which it converts almost
quantitatively to butyric and caproic acids. The building up of the
higher fatty acids does not take place, as the coenzyme A transphorase
system will not catalyze the activation of fatty acids with more than
eight carbon atoms (136).
F. B. SHORLAND
into esterified long-chain fatty acids. ATP, Co A, and Mn
2+ are essential
components of the system, and palmitic and oleic acids were the only
labeled acids that accumulate. It appears that fatty acid formation in
the avocado is by condensation of CoA-activitated acetate units. The
evidence supports the concept that as in animals the biosynthesis of
fatty acids in the avocado is through a mechanism essentially the reverse of ^-oxidation, which is known to occur in plants. Stumpf and
Barber (130) found, for example, that the mitochondria from peanut
cotyledons possessed a fatty acid oxidase system similar to that present
in animal tissues. The acetyl coenzyme A thus produced is similarly
oxidized via the Krebs cycle and some accumulation of acetoacetate also
occurs. However, in addition, in higher plants, other mechanisms are
present for the oxidation of fatty acids. One of these is associated with
the microsomal particles and catalyzes the oxidation of palmitate along
the carbon chain with the aid of DPN (131). Another, present in the
soluble fraction after removal of mitochondria and microsomes, oxidizes
only the carboxyl group of palmitic acid (132).
The mechanism for the interconversion of fat and carbohydrates in
plants has not been generally established. However, Beevers (133) has
shown that cell-free extracts of endosperm and cotyledons of castor
bean seedlings bring about the production of malate from isocitrate and
acetate, thereby providing a mechanism for the interconversion of fat
and carbohydrates.
D. SYNTHESIS OF FATS IN MICROORGANISMS
It is well known that yeasts and molds when grown on a glucose or
sucrose medium produce much fat, thus showing that, as in animals, a
mechanism exists for the conversion of carbohydrates to fats. The possibility of the participation of acetate units in this conversion is indicated
from early experiments with timothy grass bacillus (My cob act er turn phlei)
in which it was shown that the addition of acetate to the medium increased the amount of lipid formed (134). The first definite proof, however, of the participation of acetate in the synthesis of the lower fatty
acids in microorganisms was that of Wood et al. (135), who showed in
1945 that Clostridium acetobutylicum synthesizes butyric acid from two
molecules of acetate. Subsequently studies on C. kluyveri shed much
light on the mechanism of fatty acid synthesis. This anaerobic organism
requires for food only ethanol and acetate, which it converts almost
quantitatively to butyric and caproic acids. The building up of the
higher fatty acids does not take place, as the coenzyme A transphorase
system will not catalyze the activation of fatty acids with more than
eight carbon atoms (136).