Carotenoids: Physical, Chemical, and Biological Functions and Properties

Book Preface

CAROTENOIDS: A COLORFUL AND TIMELY RESEARCH FIELD

For those readers who are less familiar with this fascinating fi eld of research, it is worth introducing a few key concepts about carotenoids. There are over 600 fully characterized, naturally occurring molecular species belonging to this class of essential pigments. Carotenoid biosynthesis occurs only in bacteria, fungi, and plants where they have established functions that include their role as antenna in the light harvesting proteins of photosynthesis, their ability to regulate light–energy conversion in photosynthesis, their ability to protect the plant from reactive oxygen species, and coloration. If these were the only known functions/properties of carotenoids in the natural world, it would be adequate; but these molecules are also part of the diet in higher species, and in animals and humans, carotenoids assume a completely different set of important functions/actions. In humans, some carotenoids (the provitamin A carotenoids) are best known for converting enzymatically into vitamin A; diseases resulting from vitamin A defi ciency remain among the most signifi cant nutritional challenges worldwide. Carotenoids serve a number of other roles in the animal kingdom including in the coloration of plumage in birds, which has now been recognized to play a signifi cant role in the selection of mates. In humans, the role that carotenoids play in protecting those tissues that are most heavily exposed to light (e.g., photoprotection of the skin, protection of the central retina) is perhaps most evident, while other potential roles for carotenoids in the prevention of chronic diseases are still being investigated. Because carotenoids are widely consumed and their consumption is a modifi able health behavior (via diets or supplements), health benefi ts for chronic disease prevention, if real, could be very signifi cant for public health.

This book on carotenoids spans the breadth of ongoing work by researchers around the world, ranging from basic studies to advanced applied biomedical research.

As in many fi elds of research, new tools and techniques for measuring carotenoids in various systems are critical to support research progress. Several chapters discuss new methodologies to measure carotenoids (see Chapter 4), carotenoid metabolites/radicals (see Chapter 9), or carotenoids in vivo in complex biological systems, especially in the human eye (e.g., see Chapters 5 and 6). Other chapters describe the oxygenase enzymes that are essential components of carotenoid metabolism to active metabolites (see Chapter 19). The study of active metabolites includes the in-depth evaluation of carotenoid cleavage products (see Chapter 11) and carotenoid radicals (see Chapter 14) that may account for some of the biological actions observed for these unique substances.

Carotenoids are highly lipophilic; an active area of research concerns how carotenoids interact with and affect membrane systems (see Chapters 2 and 10). Also, the lipid solubility of these compounds has important implications for carotenoid intestinal absorption (see Chapter 17); models such as the Caco-2 cell model are being used to conduct detailed studies of carotenoid absorption/ competition for absorption (Chapter 18). The lipid solubility of these carotenoids also leads to the aggregation of carotenoids (see Chapter 3). Carotenoids aggregate both in natural and artifi cial systems, with implications for carotenoid excited states (see Chapter 8). This has implications for a new indication for carotenoids, namely, serving as potential materials for harnessing solar energy.