Lutein is a xanthophyll and one of 600 known naturally-occurring carotenoids. Found in green leafy vegetables such as spinach and kale, lutein is employed by organisms as an antioxidant and for blue light absorption. Lutein is also found in egg yolks, animal fats, and the retina (zeaxanthin predominates at the macula lutea while lutein predominates elsewhere in the retina).
The principal natural stereoisomer of lutein is -beta,epsilon-carotene-3,3-diol. Lutein is a lipophilic molecule and is generally insoluble in water. The presence of the long chromophore of conjugated double bonds (polyene chain) provides the distinctive light-absorbing properties. The polyene chain is susceptible to oxidative degradation by light or heat and is chemically unstable in acids.
Lutein is present in plants as fatty-acid esters, with one or two fatty acids bound to the two hydroxyl-groups. For this reason, saponification (de-esterfication) of lutein esters to yield free lutein may yield lutein in any ratio from 1:1 to 1:2 molar ratio with the saponifying fatty acid.
Lutein is isomeric with zeaxanthin, differing only in the placement of one double bond.