Food Lipids: Chemistry, Nutrition, and Biotechnology

sosomes, etc. A comparison of the lipid content of several of these subcellular
membranes (Table 3) shows clearly that as the percentage of one membrane type
changes in the isolate, the batch lipid composition also changes [36]. Even within
one membrane type, such as SR, fractions isolated from cisternal and longitudinal
SR [37] have revealed different ratios of phospholipid to protein [38]. Here also the
lipid composition of the isolated membranes changes if more of one fraction is
isolated. Ultimately, these changes in lipid content will lead to differences in the
oxidative susceptibility of the membrane isolate. The next section reviews the effects
of lipid composition on oxidative stability.
D. Lipid Substrate
1. Fatty Acid Unsaturation
Carbon–hydrogen bond dissociation energies of a fatty acid are lowest at bisallylic
methylene positions. These are the positions between adjacent double bonds [26,39].
Consequently, these positions are the thermodynamically favored sites for attack by
lipid peroxyl radicals in polyunsaturated fatty acids (PUFAs). In studies involving
the use of homogeneous solutions of purified lipids, a linear correlation has been
found between the number of bisallylic methylene positions and the oxidizability of
the lipids [40]. More recently, Wagner et al. [41] subjected cultured cells to oxidative
stress following systematic alteration of the lipid unsaturation through supplementation
of the growth medium with various PUFAs. In that study, the apparent oxidizability
of the cellular lipids correlated exponentially with the number of bisallylic
methylene positions in the cellular fatty acids. Different responses by the homogeneous
and cellular systems to changes in PUFA content may be explained by a
clustering of lipids within cell membranes that increases the apparent substrate concentration.
Alternatively, Ursini et al. [42] suggested that unsaturated fatty acids are
drawn into clusters of peroxidized lipids as part of a phase-compensating behavior.
Such a process would ‘‘feed’’ the peroxyl radical–propagating reactions within the
clusters of peroxidized lipid. In any event, Wagner et al. [43] found no apparent
effect on the rate or extent of radical formation with fatty acid chain length, whereas
Yin and Faustman [44] found with their liposomal model that both increased unsaturation
and increased chain length resulted in greater phospholipid and oxymyoglobin
oxidation. Location of methylene-interrupted double bonds also appears to affect
the rate of oxidation as n-3 fatty acids autoxidized faster than n-6 fatty acids [45].
Table 3 Phospholipid Composition (wt %) of Subcellular Membranes in Muscle
Sarcolemma
Sarcoplasmic
reticulum Mitochondria
Phosphatidylcholine 45.5 58.2 48.9
Phosphatidylethanolamine 22.4 29.4 39.3
Phosphatidylserine 17.6 9.4 9.0
Sphingomyelin 14.4 3.2 2.9
Source: Ref. 36.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.