Food Lipids: Chemistry, Nutrition, and Biotechnology

only enzymatic pathway of fatty acid peroxidation known to be operative in plants,
whereas animals have two primary enzymatic dioxygenation pathways: one initiated
by LOX and the other by cyclooxygenase.
II. �-OXIDATION PATHWAY
The �-oxidation pathway involves the oxidative degradation of a saturated acyl CoA
by a recurring sequence of four reactions: oxidation linked to flavin adenine dinucletide
(FAD), hydration, oxidation linked to NAD � , and thiolysis by CoA to form
acetyl CoA. The chain is broken between � and � carbon atoms, and the fatty acyl
chain is shortened by two carbon atoms as a result of these reactions. In the strict
sense, straight chain, saturated, common fatty acids (carbon chain length >3) can be
catabolized completely only by continuous repetition of the �-oxidation reaction
sequence.
Evidence for in vivo �-oxidation pathway in plants was first indicated by Grace
[1] and by Synerholm and Zimmerman [2] in experiments analogous to the classic
work of Knoop [3]. In 1956 Stumpf and Barber [4] demonstrated in vitro that lipidmobilizing
tissues of germinating oilseeds degraded fatty acids by the �-oxidation
pathway known for mammalian and microbial cells. The plant cell fractionation
studies showed the pathway was ascribed to mitochondria. However later studies
showed that in these seeds �-oxidation activity is associated primarily with the soluble
protein or extramitochondrial fraction unlike the case of the mammalian system
[5,6]. In 1969 both Beevers’ group [7] and Stumpf’s group [8] established that the
�-oxidation pathway was located within fragile organelles, referred to as glyoxysomes,
separated from the endosperm of the castor bean (Ricinus communis L.).
Subsequent studies showed clearly that all the enzymes of the �-oxidation complex,
together with the fatty acid activation enzyme, were associated with glyoxysomal
membrane [9,10]. Following these discoveries, it was demonstrated for lipid-mobilizing
tissues of germinating seeds of various species that �-oxidation was located
in glyoxysomes and was involved in conversion of oil to carbohydrate via the glyoxylate
bypass [11].
The first reports presenting evidence that the ability to carry out �-oxidation
is not restricted to glyoxysomes among plant peroxisomes were published between
1981 and 1983 [12,13]. Since then, investigators have demonstrated �-oxidation
enzymes and/or �-oxidation activity in peroxisomes isolated from photosynthetic
tissue, roots, and other plant tissues/organs that were devoid of storage lipids. Moreover,
recent studies established that peroxisomes are able to metabolize physiologically
relevant fatty acids of different molecular structure and to degrade completely
the carbon chain of these fatty acids to acetyl CoA. Thus, current experimental
evidence strongly supports the concept that the peroxisomes of higher plant cells are
a compartment competent for fatty acid catabolism by the �-oxidation pathway [14].
A. Peroxisomal �-Oxidation
The �-oxidation reaction sequence in higher plant peroxisomes appears to be identical
to those established for the �-oxidation reaction sequence in mammalian mitochondria
and in bacteria.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.