Incorporation of Lutein and Zeaxanthin into membranes in comparison to other carotenoids and under consideration of their abundance in retinal epithelial cells

The incorporation yields of four structurally different carotenoids : b-carotene, canthaxanthin, lutein and zeaxanthin in a model [small unilamellar liposomes] and natural membranes [microsomes and retinal pigment epithelium] were comparatively investigated. We applied and quantified two methods of preparations to incorporate carotenoids into liposomes and microsomes. Moreover, we used retinal pigment epithelium cells to incorporate the same carotenoids using as well two different procedures. The incorporation of b-carotene at different concentrations resulted in a very small miscibility of b-carotene with the phospholipid bilayer while a high miscibility was seen for lutein and zeaxanthin. Canthaxanthin exhibited an intermediate miscibility. The stability of carotenoid formulations in liposomes and microsomes under storage was measured by monitoring the shape of UV-Vis spectra and the percentage of carotenoid content which remained incorporated into the membrane. The stability test revealed that lutein was the most stable carotenoid upon storage, followed by zeaxanthin. b-carotene and canthaxanthin decayed rapidly in liposomes and gradually in microsomes. The quenching ability of carotenoids incorporated into membrane systems simultaneously loaded with the fluorescent dyes Laurdan and DPH has been investigated. Lutein and zeaxanthin were the strongest quenchers of fluorescence intensity compared to other carotenoids. The Laurdan quenching even offers a method to determine the carotenoid contents intrinsically from membranes. The inclusion of lutein and zeaxanthin into the lipid bilayer is associated with increased rigidity of the membrane which is not observed in membranes prepared with b-carotene.