ABSTRACT

Carotenoids are a large class of naturally occurring pigments that are ubiquitous in nature. They are synthesized by plants and some microorganisms, as well as some non-photosynthetic bacteria and fungi. Carotenoids are an important and obligatory dietary constituent for many species. 1 In higher plants, carotenoids are present in plastids, in the chloroplasts of photosynthetic tissues and the chromoplasts of fruits and flowers. 1,2 The roles that carotenoids play in photosynthesis give important clues to their functions in humans. As accessory light-harvesting compounds, carotenoids transfer absorbed energy to chlorophylls to help fuel photosynthesis. This absorbed energy comes specifically from the 450–550 nm region of the light spectrum, the blue light region. Carotenoids also prevent damage from singlet oxygen, a free radical generated in plants as chlorophyll enters an energetic “triplet state” and reacts with molecular oxygen in the presence of light. The subsequent 56formation of excited triplet states of carotenoids results in the dissipation of excess heat as carotenoids relax to a ground state. 3 Photosynthetic organisms also benefit from the ability of carotenoids to stabilize pigment-protein complexes. Such complexes form the basic architecture of photosynthetic systems. These systems, although diverse among different organisms, follow a basic pattern of a light harvesting complex (LHC) coupled to a reaction center (RC). It appears that higher plants require lutein in order to establish stable LHCII complexes, alluding to the importance of lutein as a structural component in these assemblies. 3 The carotenoids relative to humans are distinctly different in number from those that occur in nature: from a family of over 750 identified compounds, only about 40 carotenoids are present in the typical human diet, only about 20 are found in human blood, only 5 have been reported in the human infant brain, and only 3 (lutein, zeaxanthin, and meso-zeaxanthin; Figure 4.1) accumulate specifically in the human retina. 4,5 Bioselection of carotenoids therefore seems evident in the human body. Major roles of carotenoids in human health derive from their varying degrees of vitamin A and antioxidant activity. As one of the main carotenoids in human tissue and serum, lutein has many applications to human health. This chapter will focus on the chemical and biological properties of lutein in relation to human health.