Flavonoids of the Orange Subfamily Aurantioideae

Abstract

The flavonoids and related compounds of the orange subfamily Aurantioideae have attracted the attention of generations of chemical researchers, beginning with the first description of hesperidin by Lebreton (1828) to the many current pharmacological studies of these compounds in living systems. For many reasons (medicinal, herbal, agricultural), citrus fruit have been collected and used by societies throughout the centuries (Webber, 1967). However, our modern focus on the impact of citrus flavonoids on human health was perhaps started by the work of Szent-Györgyi, who, in calling citrus flavonoids Vitamin P, first indicated the importance of flavonoids in capillary function (Armentano et al., 1936; Rusznyak and Szent-Györgyi, 1936; Bentsath et al., 1937). While the term Vitamin P fell into disuse, the importance of flavonoids and ascorbic acid in proper capillary function was firmly established. Without question, the importance of the capillaries in many different aspects of human health cannot be overstated, and aspects of this are discussed in the chapters by Middleton and Kandaswami (1998), Gerritsen (1998), and Attaway and Buslig (1998). Extending from this, many pharmacological studies now show the important antioxidant and anticancer activities that citrus flavonoids contribute to human health through the diet. Much of this research relies directly on the isolation and structural characterizations of these diverse citrus phenolics, much of which was done by chemists at the U.S. Department of Agriculture. Although many of the major citrus flavonoids have now been well characterized, much still remains unclear about the biological activities of these compounds in mammalian systems, and about the biosynthesis, transport, and physiological roles of these compounds in the plants in which these compounds occur. It has been noted that in developing citrus plant tissue tremendous amounts of metabolic energy are expended in the biosynthesis of these compounds. In fact, flavonoids can constitute well above 50 percent of the dry weight of immature citrus fruit and leaf tissue undergoing rapid cell division. Yet, very little is known why this occurs, or how the biosyntheses of the different groups of flavonoids in citrus are connected. As part of this chapter, the remarkable diversity and distribution of the flavonoids in the orange subfamily Aurantioideae are reviewed, and evidence pertinent to the biosynthetic pathways of citrus flavonoids is reported.