3. MECHANISMS FOR FATTY ACID OXIDATION
91
It is found in many plants, and there have been reports of its existence
in animal tissues (49). Soybean lipoxidase has been obtained in crystalline form and has a molecular weight of 102,000. The amino acid composition of the crystalline protein shows no unusual amino acids; nor
does it contain any metal or non-amino structure in significant concentration (50). However, recently urd bean and mung bean lipoxidase
were shown to have essential sulfhydryl groups although all other
lipoxidases do not (51). Tappel has proposed a mechanism of lipoxidase-catalyzed linoleate oxidation which is in harmony with free-radical
mechanisms and with the established theory of the oxidation of 1,4diolefins (49). Franke has collected a thorough review of the information on this enzyme and should be consulted for further information
(52).
The role of lipoxidase in the plant cell is at present poorly understood. There is some indirect evidence that the protein, serving as a
temporary electron sink, holds momentarily an electron derived from
the double bond flanked methylene group of linoleate or linolenate. In
an in vitro system consisting of crystalline lipoxidase and substrate, the
electron which is abstracted temporarily must of necessity be returned
to the free radical substrate to yield the hydroperoxide. Perhaps in the
cell this electron may be channeled to conventional electron transport
systems, while the highly reactive free radical that has been formed
may then initiate reactions in which free radicals participate.
B. FATTY ACID PEROXIDASE
This enzyme has been found in extracts of cotyledons from germinated peanut seedlings and catalyzes the reaction (53):
RCH 2 CH 2 COOH + 2 H 2 0 2 -> RCH 2 CHO + C0 2 + 3 H 2 0
(28)
There is good presumptive evidence that the main reaction products
are carbon dioxide and a long-chain aldehyde with a chain length one
carbon less than its precursor fatty acid. The reaction requires the generation of hydrogen peroxide by such systems as the glycolic acidglycolic oxidase system. The system is sensitive to azide in low concentrations and cyanide at higher concentration, which would suggest
the involvement of a porphyrin-metal complex in the catalytic reaction.
It will remove only one carbon, namely the carboxyl carbon of either
a long-chain mono unsaturated or a saturated fatty acid. Further oxidation in the hydrocarbon chain does not occur. However, a microsomal
system from peanuts in the presence of DPN
+ can release C-2 and C-3
carbons as C0 2 at decreasing rates, thereby suggesting a stepwise
α-oxidation. Recent work with extracts of acetone powders of peanut
91
It is found in many plants, and there have been reports of its existence
in animal tissues (49). Soybean lipoxidase has been obtained in crystalline form and has a molecular weight of 102,000. The amino acid composition of the crystalline protein shows no unusual amino acids; nor
does it contain any metal or non-amino structure in significant concentration (50). However, recently urd bean and mung bean lipoxidase
were shown to have essential sulfhydryl groups although all other
lipoxidases do not (51). Tappel has proposed a mechanism of lipoxidase-catalyzed linoleate oxidation which is in harmony with free-radical
mechanisms and with the established theory of the oxidation of 1,4diolefins (49). Franke has collected a thorough review of the information on this enzyme and should be consulted for further information
(52).
The role of lipoxidase in the plant cell is at present poorly understood. There is some indirect evidence that the protein, serving as a
temporary electron sink, holds momentarily an electron derived from
the double bond flanked methylene group of linoleate or linolenate. In
an in vitro system consisting of crystalline lipoxidase and substrate, the
electron which is abstracted temporarily must of necessity be returned
to the free radical substrate to yield the hydroperoxide. Perhaps in the
cell this electron may be channeled to conventional electron transport
systems, while the highly reactive free radical that has been formed
may then initiate reactions in which free radicals participate.
B. FATTY ACID PEROXIDASE
This enzyme has been found in extracts of cotyledons from germinated peanut seedlings and catalyzes the reaction (53):
RCH 2 CH 2 COOH + 2 H 2 0 2 -> RCH 2 CHO + C0 2 + 3 H 2 0
(28)
There is good presumptive evidence that the main reaction products
are carbon dioxide and a long-chain aldehyde with a chain length one
carbon less than its precursor fatty acid. The reaction requires the generation of hydrogen peroxide by such systems as the glycolic acidglycolic oxidase system. The system is sensitive to azide in low concentrations and cyanide at higher concentration, which would suggest
the involvement of a porphyrin-metal complex in the catalytic reaction.
It will remove only one carbon, namely the carboxyl carbon of either
a long-chain mono unsaturated or a saturated fatty acid. Further oxidation in the hydrocarbon chain does not occur. However, a microsomal
system from peanuts in the presence of DPN
+ can release C-2 and C-3
carbons as C0 2 at decreasing rates, thereby suggesting a stepwise
α-oxidation. Recent work with extracts of acetone powders of peanut