Food Lipids: Chemistry, Nutrition, and Biotechnology

the required cofactors [73]. This activity involving cyt P450 was absent in fresh
tissue and was induced by aging the disks for 3 days. The biological significance of
this observation and the relevance of choosing C 12 acid as the substrate are not clear.
A different enzyme appears to be responsible for the formation of hydroxy
groups in ricinoleic acid from 18:1 in the developing endosperm of the castor bean.
In elegant double-labeling studies, Morris [74] established that the hydrogen is directly
replaced by a hydroxy group rather than via an unsaturated, keto, or epoxy
intermediate. In this work, developing endosperm slices were incubated with a mixture
of [1- 14 C]oleic acid and erythro-12,13-ditritio-oleic acid. The 3 H/ 14 C ratio of
ricinoleate synthesized was 0.75. This result can be obtained only by a hydroxyl
substitution mechanism. For a keto intermediate, it should be 50% and 50%; for an
unsaturated intermediate, 50% and 25%; and for an epoxy intermediate, 50% and
25%. Hydroxylation used oleoyl CoA and required NAD(P)H and O 2 [75]. Investigation
of the substrate specificity of the hydroxylase showed that chain length and
double-bond position with respect to both ends were important and caused reduced
activity relative to 18:1 [78]. However, the position at which the hydroxyl was introduced
was determined by the position of the double bond, always being three
carbons distal toward the � end. The substrate for hydroxylation in vivo is most
likely 18:1 esterified in the sn-2 position of phosphatidylcholine, from which ricinoleate
was released as the free acid before activation and incorporation into triacylglycerol
[77]. Antibodies raised against purified plant cytochrome b 5 inhibited
the hydroxylation reaction, pointing to cytochrome b 5 as the electron donor to the
hydroxylase. Carbon monoxide did not inhibit hydroxylation, and the characteristics
of the hydroxylase were generally similar to those of the microsomal fatty acid
desaturases. Van de Loo et al. [78] capitalized on the assumed homology to desaturases
to clone the 12-hydroxylase gene from developing castor endosperm.
V. LIPID/FATTY ACID PEROXIDATION
Peroxidation is a chemical reaction that results in the generation of peroxides and
their degradation products. One of the most typical properties of the unsaturated fatty
acids, particularly the polyenoic acids, is their susceptibility to peroxidation. In animal
tissues, PUFAs are chiefly peroxidized by autoxidation. In plant tissues, however,
lipid peroxidation has been thought to be predominantly a result of enzymatic
processes, such as those of the oxylipin/LOX pathway. Lipid peroxidation produces
a number of fatty acid derivatives and directly impacts various aspects of plant food
product qualities.
A. Oxylipin/Lipoxygenase Pathway
‘‘Oxylipins’’ is the generic name for a family of oxygenated compounds formed from
fatty acids by enzymatic reaction(s) involving at least one step of dioxygen-dependent
oxidation [79]. In mammalian tissues, the C20 PUFA, arachidonic acid (20:4),
serves as the most important precursor of oxylipins, while plant oxylipins are mainly
formed from the C18 PUFA 18:2 and 18:3. Oxylipins include fatty acid hydroperoxides,
hydroxy fatty acids, epoxy fatty acids, keto fatty acids, volatile aldehydes,
and cyclic compounds such as jasmonic acid. Although a number of interesting
hydroperoxide metabolites have been defined in detail with respect to structure, an
explanation of their physiological function appears to be less than complete.
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