NADH photo-oxidation is enhanced by a partially purified λ-crystallin fraction from rabbit lens

A function of the eye lens is to absorb near-ultraviolet light (near-UV) transmitted through the cornea and aqueous humor in the wavelength range of about 300-400 nm [1-3], thereby preventing UV-induced photo-damage to the retina [4,5] and reducing chromatic aberration [3]. It has been reported that the majority of environmental UV radiation reaching the lens is UV-A (320-400 nm) and that only approximately 3% is UV-B (290-320 nm) [6]. Several investigators [7-9] have shown that long-term exposure of mice and guinea pigs to near-UV can damage the lens, probably due to photo-oxidation. It is assumed that normally the lens is protected from photo-oxidation by anti-oxidative systems utilizing the ascorbate [10] and glutathione [11] redox cycles. However, the antiphoto-oxidation systems in the lens are not yet fully understood. Zigler and Rao [12] reported that the lenses of vertebrate species such as bullfrogs, guinea pigs, ducks, and rabbits contain high concentrations (0.4-1.5 mM) of the reduced pyridine nucleotides reduced nicotinamide adenine dinucleotide phosphate (NADPH) or reduced nicotinamide adenine dinucleotide (NADH), which may function as near-UV filters. High nucleotide levels in the lens are likely due to specific nucleotides binding to taxon-specific enzyme-crystallins related to NAD(P)H/NAD(P)-dependent oxidoreductases (i.e., ρ-crystallin/aldo-keto reductase in bullfrogs, ζ-crystallin/alcohol dehydrogenase in guinea pigs, ε-crystallin/lactate dehydrogenase B in ducks, and λ-crystallin/hydroxyacyl-CoA dehydrogenase in rabbits) [12-14]. Rao and Zigler [14] have further shown that bullfrogs, guinea pigs, and rabbits lenses exhibiting high levels of NAD(P)H are less susceptible to photo-oxidative damage compared with lenses from the rat, Xenopus, and a mutant strain of guinea pig with lower levels of NAD(P)H. Cunningham et al. [15,16] have reported that when irradiated with near-UV, NAD(P)H gives rise to low but significant levels of superoxide radicals via a type 2 photosensitization. Matsukura et al. [17] reported that soluble ascorbate free radical (AFR) reductase activity tends to be high in animal lenses and functions as a near-UV filter, enhancing the antiphoto-oxidative capacity of ascorbate. Animal lenses with a near-UV filter may be able to efficiently and rapidly scavenge for superoxides that are photosensitized by NAD(P)H using ©2007 Molecular Vision