Food Lipids: Chemistry, Nutrition, and Biotechnology

onstrated that �-oxidations require both oxygen and a source of reducing power.
Hydroxy fatty acid was not an intermediate of �-oxidaton spiral (Fig. 7).
Since the work by Shine and Stumpf, the mechanism of �-oxidation in plants
has not been specifically studied, although Galliard and Matthew [60] noted that �oxidation
in cucumber fruits was not stimulated by flavoproteins and required a metal
enzyme.
The �-oxidation complex is generally thought to be membrane-bound, probably
to the endoplasmic reticulum [56], and has not been purified. Specificity studies
[57,60,61] show that myristic and 16:0 and unsaturated C 18 fatty acids are the most
effective substrates (C 12 C 16 >> C 18; 14:0 = 18:1 = 18:2 = 18:3). Neither
methyl 16:0 nor � 2 16:1 is an active substrate, and 2-L-hydroxy 16:0 inhibits �oxidation.
The physiological significance of the �-oxidation pathway remains to be carefully
evaluated. Although the �-oxidation pathway produces NADH, the present
consensus is that �-oxidation is not linked to ATP production.
The �-oxidation pathway may be a very effective mechanism for bypassing
blocking groups in a hydrocarbon chain of a fatty acid by shifting the �-oxidation
reading frame by a C1 unit, thereby allowing effective �-oxidation to occur. The
involvement of �- and �-oxidation in the metabolism of phytanic acid was postulated
by Steinberg and colleagues [62] based on the studies on animal tissues. In patients
with Refsum’s disease, an inheritable condition affecting the nervous system, large
amounts of phytanic acid accumulate in tissue with a marked decrease in �-oxidation
capacity, while in tissues of normal individuals, phytanic acid is rapidly metabolized,
although �-oxidation of pristanic acid is normal in both types of tissue. Phytanic
acid is 3,7,11,15-tetramethylhexadecanoic acid. The 3-methyl group completely
blocks normal �-oxidation. If the reading frame is moved by one carbon, �-oxidation
converts phytanic acid to 2,6,10,14-tetramethylpentadecanoic acid or pristanic acid.
The �-methyl group now is readily bypassed by the �-oxidation enzyme, and propionyl
CoA is eliminated by the first turn of the spiral instead of acetyl CoA. Moreover,
the two remaining methyl branches of the molecule also reside on �-carbon
atoms as the �-oxidation spiral continues and they, too, are bypassed, with production
of propionyl CoA.
2-D-Hydroxy fatty acids, a product of an offshoot of �-oxidation, are the major
acyl components of cerebrosides in higher plants. �-Oxidation of even chain length
acids could be the source of the odd-numbered fatty acids in some plant lipids. When
an odd chain fatty acid undergoes �-oxidation, the terminal product will be propionyl
CoA, which can then enter the modified �-oxidation pathway. An intermediate of
the latter pathway is malonic semialdehyde, a precursor of �-alanine [63]. Thus, �oxidation
is indirectly involved in synthesis of a component of CoA and ACP.
A common �-oxidation process exists in seeds and in leaves. Laties et al. [64]
found that the respiration induced by cutting potato tuber tissue was mainly due to
�-oxidation of fatty acids, with carbohydrate-based respiration becoming important
only at later stages of the aging process [65]. In young leaf tissue, added fatty acids
were more rapidly oxidized by �-oxidation than by �-oxidation [66]. Long chain
aldehydes are components of volatile products formed by plants, particularly in response
to damage. The profile of the long chain aldehydes obtained from cucumber
fruits was explained by the specificity of the �-oxidation system in this tissues [60].
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