Light and Chemical Energy

The basic energy-transfer reactions are assumed to be similar in plants and animals, even though environments differ. Excess light can cause excitation of porphyrin molecules (porphyrin triplets). These triplet-state porphyrin molecules can transfer their energy to oxygen-forming singlet oxygen, 1O2. Singlet oxygen can damage DNA and cause lipid peroxidation, thereby killing the cell. Carotenoids, having nine or more conjugated double bonds, can prevent damage by singlet oxygen through: (1) transfer of triplet energy from the excited porphyrin to the car-otenoid, forming a carotenoid triplet, which would be too low in energy for further transfer and would simply dissipate as heat; or (2) singlet oxygen energy could transfer to the carotenoid, also forming a triplet carotenoid, dissipating heat, and returning to the ground state. This ability to quench sensitized triplets has been useful in treating protoporphyria (PP) and congenital erythropoietic porphyria (CEP) in humans. Porphyrias are disorders resulting from a defect in heme biosynthesis. Precursor porphyrins accumulate and can be sensitized to the singlet state and drop to the lower triplet state. The triplet state is longer-lived and thus more likely to react with other molecules such as oxygen to form singlet oxygen, which can cause cellular damage. Because ^-carotene can transfer and dissipate either sensitized triplet or singlet oxygen energy it has been used to treat these disorders.

Light absorption and possibly scavenging of destructive oxygen species by the xanthophylls lutein and zeaxanthin are also important in the macula of the primate eye. Lutein and two isomers of zeaxanthin are selectively accumulated in the macula, creating a yellow area of the retina responsible for high visual acuity (smaller amounts are also found in the lens). Both carotenoids absorb light of about 450 nm 'blue light,' thus filtering light to the light receptors behind the carotenoid layer in the macula. Filtering blue light can reduce oxidative stress to retinal light receptors and chromatic aberration resulting from the refraction of blue light. A similar filter effect may occur in the lens, but the concentration of the xanthophylls is much lower, and further protection occurs with age when the lens yellows. Whether scavenging of destructive oxygen species by these carotenoids is useful here is unproven, but the retina is an area of higher blood flow and light exposure than other tissues.

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