Food Lipids: Chemistry, Nutrition, and Biotechnology

cotyledons and partially purified. The activity has been attributed to three distinct
proteins. Two of them were the isoforms of the multifunctional protein. The third
monofunctional protein carried the predominant proportion of the overall epimerase
activity [29]. However, Engeland and Kindl [48] also purified two homodimeric
isoforms of the novel D-3-hydroxyacyl-CoA dehydratase from cucumber cotyledons.
They exhibited kinetic and molecular properties very similar to the earlier described
monofunctional ‘‘epimerase’’ protein. Both D-3-hydroxyacyl-CoA dehydratases reversibly
catalyzed the conversion of D-3-hydroxydecanoyl-CoA to 2E-decenoyl-CoA,
as demonstrated by product identification, and were inactive toward L-3-hydroxydecanoyl-CoA
or 2Z-decenoyl-CoA.
The following evidence indicates that the D-3-hydroxyacyl-CoA dehydratase
pathway may be favored over the 2,4-dienoyl-CoA reductase pathway during 18:2
catabolism in higher plant peroxisomes.
1. Complete degradation of 18:2 as well as the rate of 18:2 catabolism were
unaffected by NADPH (or NADH), which is required for participation of
the 2,4-dienoyl-CoA reductase in 18:2 degradation [41].
2. The activity of the D-3-hydroxyacyl-CoA dehydratase in glyoxysomes from
cucumber cotyledons is 100 times higher than the 2,4-dienoyl-CoA reductase
activity.
3. The 2,4-dienoyl-CoA reductase activity amounts to only one-tenth of the
activity of thiolase [48], which evidently is the rate-limiting enzyme of the
�-oxidation reaction sequence in vitro.
4. The low, 2,4-dienoyl-CoA reductase activity, which should lead to intermediate
accumulation at or above the C 10 level, did not elicit this result
[47].
Complete degradation of 18:2 by higher plant peroxisomes has been demonstrated.
Other than [ 14 C]acetyl CoA, no other products finally accumulated when
peroxisomes metabolized [U- 14 C]18:2. The amount of [ 14 C]acetyl CoA formed after
nearly complete consumption of the 18:2 corresponds to that calculated for complete
degradation of the 18:2 consumed. Under steady state conditions of [U- 14 C]18:2
degradation, accumulation was not observed at the C12, C10, orC8 intermediate level,
where the barriers to continuous passages through the �-oxidation reaction sequence
must be surmounted [26].
Based on the modified �-oxidation pathway of linoleoyl CoA, the catabolism
of 18:1, linolenic acid (18:3), as well as other unsaturated, straight chain fatty acids
can easily be deduced.
2. Ricinoleic Acid
An in-chain hydroxyl (or oxo) group located at an even-numbered carbon atom of
a straight chain fatty acid also forms a barrier, 2-hydroxyacyl-CoA intermediate, to
continuous degradation by the �-oxidation pathway. A different modified �-oxidation
pathway, which allows circumvention of this barrier, is demonstrated during the
complete catabolization of ricinoleate in plant peroxisomes (Fig. 4).
Following activation of ricinoleic acid by acyl-CoA ligase, ricinoleoyl CoA is
degraded by �-oxidation and modified �-oxidation (see above). At the C8 intermediate
level, the hydroxyl group of D-2-hydroxyl-octanyl-CoA prevents repetitive passages
through the �-oxidation reaction sequence. To surmount this barrier, the 2-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.