Food Lipids: Chemistry, Nutrition, and Biotechnology

chain intermediate accumulated as an end product along with acetyl CoA. Under
steady state conditions, 18:2 degradation by peroxisomes from cucumber cotyledons
led to intermediate accumulation at the C4 level as well [29]. Thus, fatty acid catabolism
generally appears to slow down, at least at the C4 intermediate level. This is
basically in accordance with the known kinetic properties of acyl-CoA oxidase.
Two isoforms of both acyl-CoA oxidase (AOx) and thiolase, recently identified
in the cucumber cotyledon glyoxysomes, exhibit substrate specificity toward long
chain (AOx I, thiolase I) and short chain (AOx II, thiolase II) substrates, respectively
[30]. When palmitoyl CoA was employed, it was found to inhibit AOx II but not
AOx I. However, neither thiolase I nor thiolase II is affected by palmitoyl CoA. The
metabolism of short chain substrates by the multifunctional protein exhibited 50%
inhibition at palmitoyl-CoA concentrations of about 2 �M; but an inhibition of the
metabolism of long chain substrates by the multifunctional protein became evident
only at palmitoyl-CoA concentrations exceeding 10 �M. It is suggested that, in vivo,
limited amounts of glyoxysomal NAD and CoA are available at first for the degradation
of long chain (C12 or more) acyl CoAs; the corresponding �-oxidation systems
permit cells to avoid accumulation of long chain acyl CoAs.
3. �-Oxidation Functions
The �-oxidation pathway in animals is a major source of energy, whereas in plants
�-oxidation functions in carbon mobilization during postgerminative growth (Fig.
2). During germination, fatty acids hydrolyzed from the triglyceride stored in the
spherosomes (oil bodies) in the fat-storing cells of the oilseeds pass into the glyoxysomes
and are catabolized by the �-oxidation pathway. The resulting acetyl CoA
is then converted into succinate by the glyoxylate cycle. The succinate then passes
out of the glyoxysome and is converted into carbohydrates, which are required for
the growth of the seedling until it becomes fully photosynthetic. In this case, the �oxidation
pathway is participating in gluconeogenesis.
Recent studies further demonstrate that �-oxidation is involved in the regulation
of distribution of carbon pools in plant tissues. Expression of lauric acid (12:0)-ACP
thioesterase (MCTE) in leaves of Brassica napus led to enhanced 12:0-acyl carrier
protein (ACP) thioesterase activity and production of 12:0 in isolated chloroplasts,
but no 12:0 accumulated in leaf glycerolipids [31]. Further study showed that 12:0-
CoA oxidase activity, but not 16:0-CoA oxidase, was increased severalfold in developing
seeds of B. napus expressing high levels of MCTE. In addition, isocitrate
lyase levels were sevenfold higher in high-12:0 developing seeds. Control B. napus
seed incubated with [ 14 C]acetate incorporated almost all label into fatty acids,
whereas MCTE-expressing seeds incorporated only 50% of the label into lipids, the
remainder lying in range of water-soluble components, including sucrose and malate.
These results suggest that a substantial portion of the 12:0 produced in high-12:0 B.
napus is recycled to acetyl CoA and sucrose through �-oxidation and glyoxylate
cycle pathways [32].
In addition, �-oxidation has been presumed to be involved in biosynthesis of
jasmonic acid, a growth regulator in plant tissues.
B. Mitochondrial �-Oxidation
�-Oxidation activity in mammalian cells is located in two subcellular compartments:
peroxisomes and mitochondria [33]. The mitochondrial pathway of fatty acid oxi-
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