Food Lipids: Chemistry, Nutrition, and Biotechnology

potato, and corn have LOXs that oxygenate primarily at C-9. LOX regiospecificity
can vary with LOX isozymes. At pH 6.8, pea L-1 produces mainly C-9 HPOD;
however, pea L-2 forms predominantly C-13 HPOD [116]. With LOX from flaxseed
(Linum usitatissiumum L.), 80% of the 13-isomer was formed when 18:2 served as
substrate and 88% of this isomer was formed when the linolenic acid was substituted
for 18:2 [117]. The pH at which the reaction is conducted, the O 2 concentration, and
the temperature also influence the 9:13 ratio of the products [118]. Soybean LOX-1
catalyzes the oxidation of 18:2 into both 13S-HPOD and 9S-HPOD at pH values
less than 9. However, the negligible percentage of 9S-HPOD is produced at pH
higher than 9. Replacement of histidine-608 by valine in amino acid sequence of
cucumber lipid body LOX altered the positional specificity of this 13-LOX in favor
of 9-lipoxygenation [117a].
The regiospecificity of oxygen addition to the linoleoyl radical also depends
on the ability of the enzyme to recognize the carboxyl or methyl terminal end of the
substrate. At pH 9, soybean L-1 recognizes and orients the methyl end of the 18:2.
The enzyme abstracts the pro(S)-hydrogen from C-11, and catalyzes the formation
of 13S-HPOD [119]. LOXs from wheat, potato tubers, and maize kernels [86] recognize
the carboxyl group, abstract the R-hydrogen from C-11, and catalyze the
formation of 9S-HPOD.
When the oxygen supply is depleted, the activated enzyme abstracts a hydrogen
atom from C-11 as usual to form a linoleoyl radical, and the enzyme returns to the
native E-Fe(II) form. Because no oxygen atoms are available to react with the radical,
it dissociates from the enzyme to form a free radical, which results in the formation
of a variety of products such as fatty acid dimers, ketone, and epoxides. The native
E-Fe(II) could be reactivated by oxidation with hydroperoxy products. Reduction
products of the hydroperoxide during the activation are hydroxyl ions and alkoxy
radicals, which rearrange or combine to form oxodienoic acids, dimers, and pentane.
If PUFAs containing more than three double bonds are used as substrates, LOX
can catalyze the incorporation of a second oxygen molecule into a fatty acid hydroperoxide.
Soybean and potato tuber LOXs convert �-18:3 to 9,16-dihydroperoxy-
10E,12Z,14E-octadecatrienoic acid [120,121]. Soy LOX also catalyzes the formation
of 8S,15S-dihydroperoxy 5Z,9E,11Z,13E-eicosatetraenoic acid [122] or 5S,15S-dihydroperoxy-6E,8Z,11Z,13E-eicosatetraenoic
acid [123], using 20:4 as a substrate.
It is widely accepted that LOX recognizes the (1Z,4Z)-pentadiene moiety. However,
unsaturated fatty acids which have C 8�12 chains do not act as substrates for tea
chloroplast LOX even though they possess the (1Z,4Z)-pentadiene moiety in their
structure. All the C 18-fatty acid acting as substrates had the (1Z,4Z)-pentadiene moiety
between C-9 and C-12 positions numbered from the carboxyl group (ninth and sixth
from methyl group; �6 and �9). The geometric isomers of 18:2, (9E,12E)-, (9E,
12Z)-, and (9Z,12E)-ocatadecadienoic acid did not act as substrates. Ten positional
isomers of 18:2 of (3Z,6Z)- to (13Z,16Z)-ocatadecadienoic acid that do not occur
naturally [except for the (9Z,12Z)-acid 18:2] are oxygenated by tea chloroplast LOX.
However, the rate of oxygen uptake is 30–60% lower than 18:2 [124]. When the
entire series of (�6Z, �9Z)-C 14�24 dienoic and (�3Z, �6Z, �9Z)-C 14�24 trienoic acids
are used as substrates, soybean LOX1 activity increases from C 16 to C 20 and decreases
thereafter. No appreaciable activity was detected with C 14 and C 15 substrate. When
the entire series of C 14�24 dienoic acid with a fixed (9Z,12Z)-C 13–diene carboxyl
moiety are used as substrates, LOX activity is minimal compared with the C 18, lin-
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