1. What is biological oxidation?

Oxidation is the chemical process by which an atom, molecule or ion robs another of one or more of its electrons. Chemicals exhibiting this tendency for stealing electrons are referred to as oxidizing agents. Perhaps the most familiar oxidizing agent is oxygen itself. We can see many examples of oxygen doing its electron-stealing in our everyday lives: the browning of an apple, the rusting of an iron nail, the slow fading of blue jeans.

When a material is doxidized, its chemical structure is altered, often irreversibly. In biological systems, such as the human body, a number of powerful oxidising oxidizing agents can cause damage to cells. Electron-stealing molecules known as free radicals (e. g., hydroxyls and peroxyl radicals), as well as highly reactive forms of oxygen, such as singlet oxygen, are produced in the body during various normal metabolic reactions and processes.

Physiological stress, air pollution, tobacco smoke, exposure to chemicals, and exposure to ultraviolet (UV) light or other forms of ionizing radiation can all enhance the production of these unwanted oxidising oxidizing agents². Phagocytes involved in the immune response against micro-organisms can also generate an excess of free radicals to aid in their defensive degradation of the invader.

Within cells, free radicals can damage DNA, proteins, and lipid membranes. Such damage has been linked to aging^{3, 4} and a number of pathological conditions including atherogenesis^{5, 6}, ischemia-reperfusion injury^{7, 8}, infant retinopathy⁹, age-related macular degeneration¹⁰, and carcinogenesis^{11, 12,13}.

2. What are biological antioxidants?

Biological antioxidants are defined as "compounds that protect biological systems against the potentially harmful effects of processes or reactions that can cause excessive oxidations" ¹⁴.

There are two broad classes of biological antioxidants:

• Preventative antioxidants, and
• Radical-scavenging antioxidants.

Preventative antioxidants, such as catalase and superoxide dismutase, suppress the formation of free radicals. Radical-scavenging antioxidants, such as the flavinoid compounds and vitamin C, serve to "mop up" excess free radicals¹⁵. Vitamin E and the carotenoids are very important biological antioxidants that act in both preventative and radical-scavenging roles.

3. Carotenoids – powerful natural antioxidants

• Carotenoids are a class of natural lipid-soluble pigments found principally in plants, algae and photosynthetic bacteria, where they play a critical role in photosynthesis. They also occur in some non-photosynthetic bacteria, yeast and mold, where they may carry out a protective function against damage by light and oxygen. Although animals appear to be incapable of synthesising carotenoids de novo, many animals incorporate carotenoids from their diet. Within animals, carotenoids provide bright coloration, serve as antioxidants, and can be a precursor of vitamin A^{16, 17}.

• Carotenoids are responsible for many of the red, orange and yellow hues of plant leaves, fruits and flowers, as well as the colors of some birds, insects, fish and crustaceans. Some familiar examples of carotenoid coloration are the oranges of carrots and citrus fruits, the reds of peppers and tomatoes, and the pinks of flamingoes and salmon¹⁸. Some 600 different carotenoids are known to occur naturally¹⁶.

• Carotenoids can act as potent biological antioxidants, especially as quenchers of singlet oxygen and other reactive species, by absorbing the excited energy of singlet oxygen onto the carotenoid chain, leading to the degradation of the carotenoid molecule, but preventing other molecules or tissues from being damaged¹⁹.

• Carotenoids can act also as chain-breaking anti-oxidants: free radicals generated within the body can lead to the degradation of unsaturated fatty acids, and create a chain reaction leading to the degradation of lipid-rich membranes within a short time.

4. Astaxanthin as an antioxidant

Astaxanthin’s ability to quench singlet oxygen and scavenge free radicals has been demonstrated by a number of in vitro studies^{1, 20-24}. Astaxanthin shows very good capability at protecting membranous phospholipids²⁵ and other lipids^{1, 24} against peroxidation.

One of these studies demonstrated that astaxanthin was best among carotenoids at preventing peroxidation of lipids, with up to 10-times higher anti-oxidant efficacy of astaxanthin over beta-carotene¹, while another one demonstrated a superior capacity of astaxanthin over zeaxanthin, cantaxanthin or beta-carotene at reducing peroxidation of unsaturated fatty acids. Consider the following:

• Superior singlet oxygen quenching ability of astaxanthin has also been demonstrated over other carotenoids such as beta-carotene (up to 1.7^26,27 to 38²⁸ times higher, depending on testing conditions) or lutein and zeaxanthin ²⁸.
• Another important factor is that in humans and other mammals, although this is not the case in most aquatic animals, and unlike beta-carotene and other carotenoids, astaxanthin has no pro-vitamin A activity.
• It can therefore not be diverted from its main function as an antioxidant to become part of the pro-vitamin A pool.
• The risk of hyper-vitaminosis excessive accumulation of vitamin A is not a concern.
• Finally, astaxanthin, unlike beta-carotene has the ability to cross the blood-brain barrier and therefore, directly exert its antioxidant properties in this organ²⁹.

Astaxanthin has also been compared to the well-known non-carotenoid antioxidant: alpha-tocopherol (Vitamin E) and proved, in in vitro studies, to have a superior capability to quench singlet oxygen (80^26,27 to 550²⁸ times higher) and to prevent lipid peroxidation^{1, 20}. In vitro experiments with red blood cells and mitochondria from rats have shown that astaxanthin can be 100 to 500 times more effective at inhibiting lipid peroxidation than vitamin E^{1, 20}. In vivo studies with rats given dietary astaxanthin confirmed its antioxidant abilities^1,20.

These antioxidant properties are believed to be at the source of most potential benefits of astaxanthin in human health. Those include among others³⁰:

• Support of the immune system
• Health of the eye and central nervous system
• Anti-cancer properties
• Protection against UV light damage
• Blood cholesterol regulation and prevention of arteriosclerosis and related ailments
• Response to bacterial infections
• Anti-inflammatory response

Compounds	Physical quenching rate constant (in-vitro)1 kq x 10-9 (M-1 s-1)
	Substrate 1 (CDCl3/CD3OD)(2:1)	Substrate 2 (CDCl3)
Astaxanthin Zeaxanthin Lutein Beta-carotene Alpha-tocopherol	1.8                (3,673%) 0.12               (245%) n.d. 2 0.049             (100%) n.d.	2.2               (100%) 1.9               (82%) 0.8               (41%) 2.2               (100%) 0.004            (0.2%)