Astaxanthin, a naturally occurring carotenoid pigment, is a powerful biological antioxidant. Astaxanthin exhibits strong free radical scavenging activity and protects against lipid peroxidation and oxidative damage of LDL-cholesterol, cell membranes, cells, and tissues. Astaxanthin has been the focus of a large and growing number of peer-reviewed scientific publications.
Astaxanthin is a red pigment occurring naturally in a wide variety of living organisms. Although the word astaxanthin may not be commonly encountered in everyday speech, the pigment itself is found in many human foods, and you are quite likely to be consuming it in your diet already. Most crustaceans, including shrimp, crawfish, crabs and lobster, are tinted red by accumulated astaxanthin. The coloration of fish is often due to astaxanthin; the pink flesh of a healthy wild salmon is a conspicuous example. In commercial fish and crustacean farms, astaxanthin is commonly added to feeds in order to make up for the lack of a natural dietary source of the pigment (Torrissen et al. 1989). Not only does astaxanthin provide for pigmentation in these farmed animals, it also has been found to be essential for their proper growth and survival (Torrissen and Christiansen 1995).
Astaxanthin is one of a group of natural pigments known as carotenoids. In nature, carotenoids are produced principally by plants and their microscopic relatives, the microalgae. Animals cannot synthesize carotenoids de novo, thus ultimately they must obtain these pigments from the plants and algae that support their food chains (Britton et al. 1995). Commercial production of astaxanthin from the microalga Haematococcus pluvialis is a growing business worldwide, primarily due to the rapid growth of this microorganism and its high astaxanthin content. Other commercial ventures for natural astaxanthin production utilize fermentation of the pink yeast Xanthophyllomyces dendrorhous or extraction of the pigment from by-products of crustacea such as the Antarctic krill (Euphausia superba). In addition to production from natural sources, astaxanthin may be chemically synthesized, and synthetic astaxanthin is the major form currently being used in fish feeds (McCoy 1999).
The astaxanthin molecule is similar to that of the familiar carotenoid beta-carotene (Fig. 1), but the small differences in structure confer large differences in the chemical and biological properties of the two molecules. In particular, astaxanthin exhibits superior antioxidant properties to beta-carotene in a number of in vitro studies (Terao 1989; Miki 1991; Palozza and Krinsky 1992; Lawlor and O'Brien 1995). While the positive effects of astaxanthin on farmed fish and crustaceans have been recognized for years, the potential benefits of this powerful antioxidant to human health are only now being revealed.
How is synthetic astaxanthin different from natural astaxanthin?
Synthetic astaxanthin is produced as the free (unesterified) xanthophyll and as a 1:2:1 mixture of the three stereoisomers: 3S,3'S, 3R,3'S, and 3R,3'R. The industrial producers of synthetic astaxanthin are Hoffmann-La Roche AG and BASF AG.
Are there different forms of natural astaxanthin?
In its natural state, astaxanthin is usually associated with other molecules (Bernhard, 1990). It is often complexed with proteins, producing an array of colors in different organisms. For example, it is the chromophore in the blue, green, and yellow pigments of lobsters. In other cases, astaxanthin may simply be dissolved in the lipid fraction of complex molecules such as egg lipoproteins, or it may actually be bound chemically to molecules such as fatty acids to form esters. Reddening of some snow algae (Bidigare et al. 1993) and Haematococcus is the result of such esters accumulating in cytoplasmic lipid droplets. Less often, because it is not as stable, astaxanthin occurs in cells as a free, unbound molecule.
Whether free or complexed, the atoms comprising an astaxanthin molecule can be oriented in different ways, producing different isomers. The most common geometric configuration in both synthetic and natural astaxanthin is the most thermodynamically stable all-E (all-trans) isomer. Astaxanthin from natural sources tends to occur predominantly as either the 3S,3'S or 3R,3'R form, while the meso (3R,3'S) isomer is the most abundant in synthetic astaxanthin (Bernhard 1990)
Is all astaxanthin the same?
Astaxanthin has chemical features that result in the existence of several forms of astaxanthin:
Stereoisomers. Astaxanthin has two chiral (pronounced "ky-ral"), or asymmetric, centers. These are the carbons numbered 3 and 3' (pronounced "three prime") on the two rings in the structure. One can think of chiral asymmetry as analogous to "handedness". A left hand and a right hand are mirror images of each other--they are similar but not identical, and are not superimposable. Similarly, a chiral center can exist in either of two configurations; the same atoms are bonded to the chiral center, but the three-dimensional arrangements are different and not superimposable. Chemists identify chiral centers as being either R or S (from rectus or sinister, Latin for "right" or "left"). The two chiral centers in astaxanthin, carbons 3 and 3', can each exist either in the R or the S form, and thus there are a total of three stereoisomers: 3S,3'S, 3R, 3'S, or 3R,3'R. The 3S,3'S and 3R,3'R stereoisomers are mirror images of each other and are termed "enantiomers". Each enantiomer has the opposite optical activity of the other, i.e., a solution of a pure enantiomer will rotate plane-polarized light in a direction opposite to that observed for the other enantiomer. The 3R,3'S form is sometimes termed "meso" and is optically inactive because there is a plane of symmetry through the center of the molecule