Abstract
Purpose:
Macular xanthophylls (MXs) lutein and zeaxanthin are dietary carotenoids that are specifically concentrated in the macula lutea of human eyes where they are thought to help protect against age-related macular degeneration (AMD) by multiple mechanisms including filtration of phototoxic blue light and quenching of singlet oxygen and triplet states of photosensitizers. These physical protective mechanisms require that MXs be in their intact structure. Here we investigated how water- and/or membrane-soluble small nitroxide free radicals protect the intact structure of MXs incorporated into model membranes subjected to oxidative stress.
Methods:
Model membranes were formed from saturated, mono-unsaturated and poly-unsaturated phosphatidylcholines (PCs). The extent of peroxidation was evaluated using the TBA method and was correlated with the extent of chemical destruction (bleaching) of MXs evaluated from their absorption spectra. Small nitroxide free radicals (spin labels) with different polarity (membrane/water partition coefficients) were used as protectors of MXs. The redox stage of the nitroxides was monitored using electron paramagnetic resonance spectroscopy.
Results:
The rate of PC autoxidation increased with the degree of their unsaturation and correlated with the rate of bleaching of incorporated MXs. Formation of TBA products and bleaching of MXs was not observed in fully saturated membranes. Moderate bleaching was observed in mono-unsaturated membranes and fast bleaching, which was accompanied by the fast autoxidation of PCs, was observed in poly-unsaturated membranes. Membrane located nitroxides (spin labeled stearic acids and spin labeled phospholipids) completely protected MXs from bleaching in all investigated membranes. Water soluble, polar nitroxides practically have no effects on the rate of MXs bleaching. However, less polar nitroxides, which can penetrate from water phase into the membrane interior, showed moderate protective effects.
Conclusions:
The body of evidence supporting a protective role for MXs against AMD has been considered strong enough to justify their inclusion in the National Eye Institute's AREDS2 study. We theorize that new strategies like the one employed here aimed at maintaining the intact structure of MXs during oxidative stress, which is implicated in AMD pathogenesis, should enhance their protective potential.