Food Lipids: Chemistry, Nutrition, and Biotechnology

Although it appears clear that the retroconversion of fatty acids via peroxisomal
oxidation plays a significant role in fatty acid biosynthesis, several important questions
remain concerning the regulation of the process. It is known that retroconversion
takes place in the peroxisomes and not in mitochondria where the majority of
cellular � oxidation occurs [9]. In addition, the same acyl CoA oxidase may be
involved in all chain-shortening events [57]. A particularly intriguing aspect of the
specific production of acyl chains by retroconversion is how the fatty acids are spared
from further oxidation. Fatty acids with a �4 unsaturation are actively mobilized and
returned to the ER where they are acylated into lysophospholipids [11]. Therefore,
22:6n-3 produced de novo in peroxisomes from an n-3 precursor is found acylated
primarily in the phospholipids with remodeling pathways active at the ER. The distribution
of 22:6n-3 in cellular phospholipids is dependent on whether 22:6n-3 is
consumed intact or as a metabolic precursor [58]. A lack of peroxisomal retroconversion
activity has been associated with several pathologies, including Zellweger’s
syndrome [9,59]. The accumulation of very long chain fatty acids (C24–26) is a
hallmark of Zellweger’s syndrome, suggesting that peroxisomal oxidation is both a
normal and critical component of fatty acid metabolism.
III. PUFA METABOLISM IN MEMBRANES
Knowledge of tissue or even cellular fatty acid composition is not sufficient to predict
the effects of PUFAs in cell physiology. The realization that fatty acid location plays
an important part in the use and function of specific fatty acids has been a major
advancement in lipid metabolism. Unsaturated fatty acids, as well as their saturated
counterparts, are esterified in phospholipids of cell membranes. There are a variety
of phospholipid types, and each type has its own unique compositional fatty acid
identity. In addition to phospholipid identities, individual cell membranes have
unique phospholipid and fatty acid compositions. Although there are myriad data on
the fatty acid composition of plant and animal tissues, these fatty acid profiles are
subject to important changes brought about by diet, disease state, or a variety of
other factors. As a result of the variety of phospholipid pools in which fatty acids
can be esterified, very slight changes in total cell fatty acid content can have significant
effects on cell function. It is quite possible that lipids have been investigated
and rejected as causal agents of a number of pathologies on the basis of largely
unchanged fatty acid compositions. It is now clear that fatty acids exert their effects
from specific locations and that their physiological effects are mediated in part by
the movement of PUFAs into important phospholipid pools. The movement of fatty
acids into phospholipid pools is catalyzed by fatty acid carrier proteins, and the acyl
compositions of membranes and specific phospholipid pools are mediated by the
specificities of these enzymes.
A. Remodeling
Dietary fatty acids can exert significant effects on the fatty acid compositions of
phospholipid membranes. These effects must be attributed either to the synthesis of
new phospholipids de novo or to activities that change the composition of preexisting
phospholipids. Quite commonly, it is the latter of the two mechanisms. Phospholipid
fatty acid remodeling is both an important pathway for the incorporation of dietary
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