Astaxanthin

Astaxanthin is an abundant oxycarotenoid (Structure 12.1). It is found in green algae and in the plumage of many birds including the flamingo. It is bound to the carotenoproteins present in the carapace of crustacea, and it is present in the flesh of salmon (Britton, 1983). Salmon is certainly a well known, but occasional, human source of astaxanthin. Astaxanthin is not found in common food plants and consequently is not a regular dietary carotenoid. Nor is it found in detectable levels in normal human serum. In many animals astaxanthin is produced through a biosynthetic

Astaxanthin 3,3'-dihydroxy-p,p-carotene-4,4'-dione pathway from P-carotene, lutein, or zeaxanthin. There is no evidence that this pathway exists in humans (Scheidt, 1990).

Martin et al. and others have shown that astaxanthin is a highly effective antioxidant carotenoid in vitro (Martin et al., 1999; Terao et al., 1989; Woodall et al., 1997; Rengel et al., 2000; Miki, 1991). The antioxidant behavior of many carotenoids is characterized by a U-shaped graph (Figure 12.2). The rate of substrate oxidation decreases with increasing carotenoid concentration up to a concentration that is roughly 1 mM, but reverses and increases at higher concentration. This is typical of carotenoids such as P-carotene, which are designated mixed antioxidant/prooxidant, and the extent of the prooxidant upturn is dependent upon oxygen partial pressure. Astaxanthin is a pure antioxidant and lacks the right-hand arm of the U-curve at high carotenoid concentrations.

It has been suggested that interception of peroxy radicals and singlet oxygen by astaxanthin could be more effective than P-carotene, particularly in tissues where oxygen partial pressures are high. Astaxanthin has been regarded as having exceptional antioxidant properties. The relative antioxidant ability of several carotenoids is shown in Figure 12.3 (Martin et al., 1999). The high antioxidant ability of astaxanthin is ascribed to the presence of two keto groups at the 4 and 4' positions. These are conjugated to the polyene chain of the carotenoid through the 5,6 and 5',6' double bonds in the respective ionone rings. Several investigations using animal models, and a few investigations in humans, have been conducted to determine the ability of astaxanthin to function as an antioxidant.

Log of the Carotenoid Concentration

Figure 12.2 These qualitative curves illustrate the comparative antioxidant abilities of P-carotene and astaxanthin. The U-shape of the P-carotene curve (dashed) at high partial pressures of oxygen seen here is consistent with antioxidant function at low carotenoid concentrations. This function undergoes a transition at higher concentrations of oxygen to participate in chain-propagating oxidation steps characterized by an increased rate of substrate oxidation. At low partial pressures of oxygen, P-carotene (dotted) shows little prooxidant behavior. Astaxanthin (solid) is almost independent of the oxygen partial pressure. (Adapted from Martin, H.D. et al., Pure Appl. Chem., 71, 2253-2262, 1999. With permission.)

Log of the Carotenoid Concentration

Figure 12.2 These qualitative curves illustrate the comparative antioxidant abilities of P-carotene and astaxanthin. The U-shape of the P-carotene curve (dashed) at high partial pressures of oxygen seen here is consistent with antioxidant function at low carotenoid concentrations. This function undergoes a transition at higher concentrations of oxygen to participate in chain-propagating oxidation steps characterized by an increased rate of substrate oxidation. At low partial pressures of oxygen, P-carotene (dotted) shows little prooxidant behavior. Astaxanthin (solid) is almost independent of the oxygen partial pressure. (Adapted from Martin, H.D. et al., Pure Appl. Chem., 71, 2253-2262, 1999. With permission.)

Figure 12.3 Relative antioxidant ability of P-carotene, lycopene, zeaxanthin, and astaxanthin illustrating the greater ability of carotenoids with oxygen functionalities to effectively disrupt the chain-propagation mechanism of peroxy radical oxidation. (Adapted from Martin, H.D. et al., Pure Appl. Chem., 71, 2253-2262, 1999. With permission.)

Figure 12.3 Relative antioxidant ability of P-carotene, lycopene, zeaxanthin, and astaxanthin illustrating the greater ability of carotenoids with oxygen functionalities to effectively disrupt the chain-propagation mechanism of peroxy radical oxidation. (Adapted from Martin, H.D. et al., Pure Appl. Chem., 71, 2253-2262, 1999. With permission.)

0 0

Post a comment