Properties and Functions of Carotenes

Although many in vitro studies on the antioxidative property of food constituents have been reported, little is known about the biological functions of dietary antioxidants in vivo, except for several well-known antioxidants such as tocopherols, (i-carotene, and ascorbic acid. Because the bioavailability of food constituents is limited by their digestibility and metabolic fate, an oral administration trial of a dietary antioxidant is favored to evaluate its biological function. 

Table 3 summarizes some of the properties of PS I reaction center and specific functions of its individual subunits. The purified preparation contains about 100 Chi a molecules per P-700 [9,10]. However, this number can be decreased to about 40 while the order of the Chi a molecules increases [70]. Washing out more of the Chi molecules caused a decrease in the dichroic ratio, indicating that those 40 Chls are the highly oriented primary light-harvesting antenna of the reaction center. It was also shown that the S-carotene is in very close proximity to P-700 and it is highly oriented with respect to the latter [70]. P-700 as well as the primary electron acceptor (Aj) may be composed of specialized Chi a molecules [80]. There are at least three more electron acceptors which are part of the reaction center and their function is to slow down the rate of the reaction and thereby stabilize the redox potential difference [72]. It was not until Malkin and Bearden [81] discovered the bound ferredoxins that this part of PS I started to be understood. Today it appears that at least four different clusters are involved in the electron-accepting site of the... 

Britton, G. Structure and properties of carotenes in relation to function. FASEB Journal, 15,1551-1558. 

Carotenoids are a family of pigments that are divided into two main groups carotenes and xanthophyUs. They are introduced to the human body through dietary means alone, and although not considered to be essential micronutrients, they have important antioxidant and photoprotective properties. These functions have prompted interest in their potential role in prevention of disease. 

Britton G (1995) Structure and properties of carotenoids in relation to function. FASEB J 9 1551-1558 Erdman JW Jr, Bierer XL, Gugger ET (1993) Absorption and transport of carotenoids. In LM Canfield, NI Krinsky, JA Olson, (eds) Carotenoids in Human Health. New York Acad. Sci., New York, 691 76-86 Ben-Amotz A, Lers A, Avron M (1988) Stereoisomers of P-carotene and phytoene in the alga, Dunaliella bar-dawil. Plant Physiol 86 1286-1291... 

In the stomach, carotenoids are exposed to acid environments. This can lead to carotenoid isomerization, which can change carotenoid antioxidant properties, solubility, and absorption. In humans, (3-carotene absorption is reduced when the pH of the gastric fluids is below 4.5 (Tang and others 1995). Vitamin E consumption seems to reduce carotenoid absorption in animals, presumably because vitamin E and carotenoids compete for absorption (Furr and Clark 1997). Dietary sterols, such as those in sterol-supplemented functional foods, are also known to decrease carotenoid absorption. 

---