Food Lipids: Chemistry, Nutrition, and Biotechnology

Catabolism of heptanoyl CoA as well as ricinoleoyl CoA to propionyl CoA and
acetyl CoA has been demonstrated. Following NADH formation during degradation
of 1 nmol of ricinoleate, a stoichiometry of 9:1 is observed when the NADH formation
has ceased. This stoichiometry corresponds to that expected for complete
degradation of ricinoleate to acetyl CoA [17].
3. Propionate
Propionyl CoA can be generated at certain metabolic processes such as the catabolism
of ricinoleic acid, branched chain 2-oxo acids, and odd-numbered, straight chain
fatty acids by the �-oxidation pathway. In mammalian mitochondria, propionyl CoA
is carboxylated to methylmalonyl CoA in a biotin-dependent reaction, and the methylmalonyl
CoA is subsequently rearranged to succinyl CoA in a coenzyme B12 dependent mutase reaction. However, the propionate metabolism pathway in plants
obviously differs from that in animals. Employing intact tissues from different plant
species as well as cell-free systems, Stumpf and coworkers found that 3-hydroxypropionate
was an intermediate of propionate catabolism; specifically, the 14 CO2 was
released in decreasing rates from propionic-1- 14 C, propionic-3- 14 C, and propionic-2-
14 14
C [53]. No Krebs cycle acids could be isolated when propionic-1- C was fed,
whereas these acids were readily isolated when propionic-2- 14 CO2 and propionic-3-
14
C were fed. The C-2 and C-3 of propionate became C-2 and C-1, respectively, of
acetyl CoA, and CO2 was not required for the degradation. On the basis of these
results, the pathway of propionate catabolism called ‘‘modified �-oxidation’’ (Fig.
5) was proposed. This pathway avoids both a biotin-dependent carboxylation and a
coenzyme B12-dependent mutase reaction and is similar to oxidation of propionate
in bacteria.
Recently, catabolism of propionyl CoA to acetyl CoA via modified �-oxidation
pathway was further confirmed in work that used peroxisomes from mung bean
hypocotyls as the enzyme source. Gerbling and Gerhardt [49,54] used high-performance
liquid chromatography (HPLC) to identify the acyl-CoA thioester intermediates,
the intermediates not esterified to CoA-SH, and the end product acetyl CoA.
Oxidation of propionyl CoA to acrylyl CoA resulted in concomitant H2O2 formation,
indicating that the reaction was catalyzed by acyl-CoA oxidase. In the absence of
NAD � , 3-hydroxypropionate accumulated as an end product. Malonic semialdehyde
accumulated when CoA-SH was omitted from the reaction mixture. Catabolism of
propionate via 3-hydroxypropionate to acetate was also reported in the lima bean
(Phaseolus limensis) by Halarnkar et al. [55].
4. Branched Fatty Acids
Figure 6 shows modified �-oxidation of methyl branched fatty acids generated by
the catabolism of branched chain amino acids. The �-oxidation of long chain fatty
acids with methyl groups at odd- or even-numbered carbons is discussed in Sec. III.
Catabolism of leucine, isoleucine, and valine is initiated by an aminotransferase
reaction yielding the branched chain 2-oxo acids, 2-oxo-4-methylpentanoic acid (2oxo-isocaproic
acid), 2-oxo-3-methylpentanoic acid (2-oxo-3-methyl-valeric acid),
and 2-oxo-3-methylbutanoic acid (2-oxo-isovaleric acid), respectively. The branched
chain 2-oxo acids are activated by the oxidative decarboxylation and then catabolized
to propionyl CoA and subsequently to acetyl CoA in peroxisomes from mung bean
hypocotyls. Pathways of the catabolism of the branched chain 2-oxo acids are pro-
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