2009_Book_FoodChemistry

3.7 Changes in Acyl Lipids of Food 207
the 13-hydroperoxide of linoleic acid by the
β-scission mechanism (cf. reaction 3.72). The
corresponding pathway for 16-hydroperoxide of
linolenic acid should then yield ethane.
3.7.2.2 Lipoxygenase: Occurrence
and Properties
(3.74)
Alkanes, Alkenes. The main constituents of
the volatile hydrocarbon fraction are ethane
and pentane. Since these hydrocarbons are
readily quantitated by gas chromatography
using head-space analysis, they can serve as
suitable indicators for in vivo detection of lipid
peroxidation. Pentane is probably formed from
A lipoxygenase (linoleic acid oxygen oxidoreductase,
EC 1.13.11.12) enzyme occurs in many
plants and also in erythrocytes and leucocytes.
It catalyzes the oxidation of some unsaturated
fatty acids to their corresponding monohydroperoxides.
These hydroperoxides have the same
structure as those obtained by autoxidation.
Unlike autoxidation, reactions catalyzed by
lipoxygenase are characterized by all the features
of enzyme catalysis: substrate specificity, peroxidation
selectivity, occurrence of a pH optimum,
susceptibility to heat treatment and a high
reaction rate in the range of 0–20 ◦ C. Also, the
activation energy for linoleic acid peroxidation
is rather low: 17 kJ/mol (as compared to the
activation energy of a noncatalyzed reaction,
see 3.7.2.1.5). Lipoxygenase oxidizes only fatty
acids which contain a 1-cis,4-cis-pentadiene
system. Therefore, the preferred substrates are
linoleic and linolenic acids for the plant enzyme,
and arachidonic acid for the animal enzyme;
oleic acid is not oxidized.
Lipoxygenase is a metal-bound protein with
an Fe-atom in its active center. The enzyme
Table 3.33. Regio- and stereospecificity of lipoxygenases (LOX)
Origin (isoenzyme) Hydroperoxide from 18:2 (9,12) a PH
13S 13R 9S 9R 8R
Soybean, seed (LOX I) 94 2 2 2 10.5
(LOX II) 77 3 18 2 7
Pea, seed (LOX I) 23 16 32 29 6.8
(LOX II) 87 2 6 5 6.8
Corn, germ 3.5 3.5 89 4 6.5
Tomato, fruit 13 2 84 <1 5.5
Potato, tuber 1.6 2.4 96 0 6.8
Barley, seed 92 3 7.0
Wheat, germ 10 5 83 2 6.8
Gaeumannomyces graminis 100 7.4
Marchantia polymorpha 89 2 9.0
a Composition of the hydroperoxide fraction in %.