Primary Aminophospholipids in the External Layer of Liposomes Protect Their Component Polyunsaturated Fatty Acids from 2,2‘-Azobis(2-amidinopropane)- dihydrochloride-Mediated Lipid Peroxidation

Abstract

We showed in our previous study that docosahexaenoic acid-rich phosphatidylethanolamine in the external layer of small-size liposomes, as a model for biomembranes, protected its docosahexaenoic acid from 2,2'-azobis(2-amidinopropane)dihydrochloride- (AAPH-) mediated lipid peroxidation in vitro. Besides phosphatidylethanolamine, both phosphatidylserine and an alkenyl-acyl analogue of phosphatidylethanolamine, phosphatidylethanolamine plasmalogen, are reported to possess characteristic antioxidant activities. However, there are few reports about the relationship between the protective activity of phosphatidylethanolamine plasmalogen and/or phosphatidylserine against lipid peroxidation and their distribution in a phospholipid bilayer. Furthermore, it is unclear whether phosphatidylethanolamine plasmalogen and/or phosphatidylserine protect their component polyunsaturated fatty acids (PUFAs) from lipid peroxidation. In the present study, we examined the relationship between the transbilayer distribution of aminophospholipids, such as phosphatidylethanolamine rich in arachidonic acid, phosphatidylethanolamine plasmalogen, and phosphatidylserine, and the oxidative stability of their component PUFAs. The transbilayer distribution of these aminophospholipids in liposomes was modulated by coexisting phosphatidylcholine bearing two types of acyl chain: dipalmitoyl or dioleoyl. The amounts of these primary aminophospholipids in the external layer became significantly higher in liposomes containing dioleoylphosphatidylcholine than in those containing dipalmitoylphosphatidylcholine. Phosphatidylethanolamine rich in arachidonic acid, phosphatidylethanolamine plasmalogen or phosphatidylserine in the external layer of liposomes, as well as external docosahexaenoic acid-rich phosphatidylethanolamine, were able to protect their component PUFAs from AAPH-mediated lipid peroxidation.

Perspectives on the membrane fatty acid unsaturation/pacemaker hypotheses of metabolism and aging

“…For instance, phospholipids containing two DHA groups were peroxidized at the similar rates or sometimes a lower rate than phospholipids containing a DHA and a palmitic acyl group when put into liposomes composed of phospholipids with saturated fatty acyl groups, contrary to what would be predicted based on the peroxidizability indexes (Norris et al, 2012). Additionally, phosphatidylethanolamine with DHA appears to have protective effects against peroxidative damage when it is positioned in the outer leaflet of liposomes (Kubo et al, 2005). If the peroxidation index is potentially neither a good estimate of the actual peroxidizability of free fatty acids, nor of the peroxidizability of fatty acyl groups on phospholipids, it is unclear why it is often correlated with maximum lifespan, as well as actual measures of oxidative damage (Aguilar-Toral et al, 2014;Hulbert et al, 2014;Jeon et al, 2001;Naudí et al, 2013;Pamplona et al, 2004Pamplona et al, , 2000Pamplona et al, , 1999aPamplona et al, , 1998Pamplona et al, , 1996.…”

“…Moreover, the relative peroxidizabilities of free fatty acids may not always be relevant to how they act as acyl groups when attached to larger molecules (Cosgrove et al, 1987;Kubo et al, 2005;Norris et al, 2012;Wijesundera et al, 2008;Zerouga et al, 1995). For instance, phospholipids containing two DHA groups were peroxidized at the similar rates or sometimes a lower rate than phospholipids containing a DHA and a palmitic acyl group when put into liposomes composed of phospholipids with saturated fatty acyl groups, contrary to what would be predicted based on the peroxidizability indexes (Norris et al, 2012).…”

“…There is also evidence to suggest that phosphatidylethanolamine is more susceptible to oxidation than is phosphatidylcholine, even given the same fatty acyl composition, though that may be dependent on the oxidative agent (Kawakatsu et al, 1984;Wang and Shibata, 1995;Wang et al, 1994). Lastly, which phospholipid head group a fatty acyl group is attached to can affect important physical properties of membranes such as the partitioning of phospholipid species in the inner and outer leaflet of a membrane and the partitioning of phospholipid species within lipid rafts (Kubo et al, 2005;Shaikh et al, 2015). Phosphatidylcholine species with DHA as an acyl group displace cholesterol from lipid rafts, the opposite of the effect of phosphatidylethanolamine species with DHA in them, which tend to be more common phospholipid species (Shaikh et al, 2015).…”

Phospholipid Peroxidation: Lack of Effect of Fatty Acid Pairing

“…Similarly, the increased surface area of two 22:6 PUFA in PtdCho 22:6/22:6, even with its larger headgroup, might also favor its partitioning into the inner leaflet. It is well known that the charge on polar head groups, volume of the phosphoplipid, curvature of liposomes and molar ratios of phospholipids are all factors that can influence phospholipid distribution in liposomes [13,14].…”

Ethanol specifically alters the synthesis, acylation and transbilayer movement of aminophospholipids in rat-liver microsomes

“…It has recently been suggested that the synthesis of PE by this reaction may be directed towards repairing any damage to unsaturated fatty acid chains of phospholipids caused by reactive oxygen species under oxidative stress (Jasinka et al, 1996). In addition, as has already been observed in synthetic membranes, the arrangement of phospholipid molecules in the bilayer is a significant factor in determining resistance to freeradical attack during lipid peroxidation, and importantly in this respect, the primary amine group of PE and PS in the external layer possesses antioxidant activity, thus protecting the polyunsaturated fatty acids from lipid peroxidation (Kubo et al, 2005).…”