Food Lipids: Chemistry, Nutrition, and Biotechnology

deposition of storage lipids, indicating developmental control (138,139), similar to
control mechanisms for other units of FAS discussed in Sec. III.C.3. Another regulatory
feature of ACP action in E. coli is the presence of membrane-binding sites
(141), although this issue has not received much attention in plant FAS systems.
The significance of ACP isoforms was perhaps best illustrated (139) by their
influence on trafficking of 18:1-ACP residues toward the competing pathways of
plastidic glycerolipid synthesis and export to the endoplasmic reticulum (cf. Fig. 3).
In spinach leaf, acyl-ACP isoform I is a better substrate than acyl-ACP isoform II
for TE , and the reverse is true for plastidic GPAT activities. Thus, ACP isoform I
tends to favor channeling newly synthesized fatty acids to the endoplasmic reticulum
(for further elongation, desaturation, hydroxylation, or glycerolipid assembly in the
‘‘eukaryotic’’ configuration), whereas ACP isoform II favors intraplastidic utilization
of newly synthesized fatty acids for glycerolipid synthesis, giving rise to the ‘‘prokaryotic’’
configuration of plastid-derived polar glycerolipids. Despite these differences,
the two ACP isoforms are equally supportive of plant fatty acid biosynthesis
in general and yield similar fatty acid profiles form plant FAS systems.
IV. EXTRAPLASTIDIC MODIFICATION REACTIONS
With reference to triacylglycerol-accumulating plant tissues, 16:0 and 18:1 �9 are principal
products of plant FAS systems. To account for the diversity among the 16–24
acyl carbon derivatives that can be found in seed/fruit oils, modification reactions
must take place, and these occur in the endoplasmic reticulum. (Fatty acids of fewer
than 18 acyl carbons exported from the plastid generally become incorporated into
triacylglycerols without further modification in the endoplasmic reticulum.) The basic
modification processes are oxygenation (hydroxylation, epoxidation), elongation, and
desaturation, and these processes give rise to many unusual fatty acids (some of
which appear in Table 2). The unusual fatty acids (>200) that exist in plants are
generally confined to storage lipids (142), in keeping with the strict requirements
that functional lipids have for costitutive fatty acids (13,15,23). Most of these unusual
fatty acids are found at trace levels and are potential targets for use in the oleochemical
industries. Later in the chapter we consider a few such modification processes
(only those that yield fatty acid derivatives relevant to use in, or as, foods).
Figure 4 provides an overview of common fatty acid modification reactions taking
place in the endoplasmic reticulum, and the enzymes associated with these reactions
are listed in Table 4.
A. Oxygenation
Hydroxylation of fatty acids yields derivatives that can have fragrance/flavoring potential
under conditions favoring the induction of lactonization. In addition, many
hydroxylated fatty acid derivatives are components of natural waxes, which are effective
moisture barriers when used in foods. The most common hydroxylated fatty
acid is ricinoleic acid (18:1�9,12-OH), which is abundant in the developing endosperm
of castor bean. The oleate hydroxylase (12-MO) is localized in the endoplasmic
reticulum; it utilizes 18:1�9-phosphatidylcholine, oxygen, and NAD(P)H channeled
through cytochrome b5, and it is specific for both chain length and double-bond
position (143,144). Efforts to purify the 12-MO have not been successful because
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.