Food Lipids: Chemistry, Nutrition, and Biotechnology

Plant fatty acids provide a seminal source of polyunsaturated fatty acids
(PUFAs) in human nutrition. This dependence on plant fatty acids is due to the fact
that plants are capable of inserting double bonds into the �12 and �15 positions of
fatty acids. The desaturation of fatty acids produced de novo by plants involves an
initial �9 desaturation, followed by �12 and �15 desaturations. The insertion of
double bonds on the methylene side of the previously unsaturated �9 bond is in
opposition to animal metabolism, wherein additional double bonds are only inserted
on the carboxyl side of the �9 double bond. This difference allows dietary fatty
acids to be converted to forms not possible as a result of plant or animal metabolism
alone. Plants express two pathways for the desaturation of fatty acids. The first,
known as the prokaryotic pathway, is an array of desaturase activities that is present
in the plastid. The second eukaryotic pathway requires the translocation of the fatty
acids to the endoplasmic reticulum where desaturases act on the acyl groups. The
products of the eukaryotic pathway are either utilized directly by the cell or translocated
back to the plastid for further processing. Harwood [5] provides an extensive
review on plant fatty acid desaturation. An interesting phenomenon of plant fatty
acid biogenesis is that desaturation can utilize complex lipids as substrate. Both
plastid-and endoplasmic reticulum–based desaturation introduce a double bond into
fatty acids esterified in a phospholipid. In contrast, animals desaturate acyl CoA
complexes.
Fatty acid desaturation in animals utilizes fatty acyl CoA thioesters as substrate.
Despite multiple desaturase activities, cytochrome b5 reductase acts as an electron
donor common to all terminal desaturases. The desaturases are present on the cytoplasmic
face of the endoplasmic reticulum. Animals possess �9, �6, and �5 desaturase
activity. Like desaturases in plants, each of the desaturases has preferred
substrates, which can display organism- and tissue-specific differences. Because double
bonds are found inserted at the �4 position of 22:6n-3 and 22:5n-6, it was
originally assumed that animals also possessed a �4 desaturase. Such a desaturase
has not been identified, and work by Sprecher and colleagues [8,9] has demonstrated
that 22:6n-3 is produced via a �4 desaturase-independent pathway. Fatty acids containing
a �4 unsaturation are in fact the product of an additional elongation, a �6
desaturation, and a two-carbon chain shortening that takes place in the peroxisome
[9–11]. An interesting phenomenon specific to this process is the coordination of the
movement of fatty acid from the endoplasmic reticulum to the peroxisome and back
again for acylation into lysophospholipids. How the cell recognizes the production
of 22:6n-3 within the peroxisome and spares it from further chain shortening is not
understood.
The first double bond inserted into the saturated acyl CoA products of fatty
acid synthase is at the �9 position and is catalyzed by stearoyl CoA desaturase. In
contrast to desaturation in plants, further desaturation occurs only on the carboxyl
side of the initial unsaturation. Interestingly, most PUFAs are of plant origin, as
monounsaturated n-9 fatty acids produced de novo are not further desaturated by
animals except in times of essential fatty acid deficiency. However, fatty acids previously
polyunsaturated by plants are readily desaturated to form familiar PUFAs,
such as arachidonic acid and docosahexaenoic acid (DHA). These highly unsaturated
fatty acids are products of both plant and animal unsaturation, and they have double
bonds inserted on both sides of the original �9 double bond.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.