Food Lipids: Chemistry, Nutrition, and Biotechnology

IV. MONOXYGENASE PATHWAYS
A. �-Oxidation
Fatty acids with oxygen functional group (alcoholic, carbonyl or carboxyl) at the
methyl terminal end (�-end) are formed by �-oxidation and frequently occur as
components of cutin and suberin. The initial oxidation step that brings about the �hydroxylation
of a fatty acid is catalyzed by a monoxygenase. The �-hydroxy fatty
acid is then catalyzed by NADP-specific dehydrogenases to form oxo-fatty acid and
dicarboxyl acid (Fig. 8).
The hydroxylation reaction in the �-oxidation pathway was first demonstrated
in plant tissues by Soliday and Kolattukudy [67]. In the presence of NADPH and
O2, the endoplasmic reticulum fraction from embryonic shoots of Vicia faba converted
16:0 to �-hydroxy 16:0. Microsomal preparations from the root tips and the
excised epidermis of expanding leaves of V. faba also catalyzed a similar hydroxylation
reaction. Neither acyl CoA nor fatty acid ester was used in the oxidation. In
addition to 16:0, other fatty acids especially 18:1 were a good substrate for the
reaction. The �-hydroxylation catalyzed by the microsomal preparation was inhibited
by classic mixed-function oxidase inhibitors such as metal ion chelators, NaN3, and
thiol-directed reagents. The involvement of a cytochrome (cyt) P450-type protein in
the �-hydroxylation was suggested by the inhibition of this reaction by CO. The
degree of sensitivity to CO and lack of photoreversibility by light, however, implied
Figure 8 The �-oxidation pathway in plant tissues.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.