Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology
Carotenoid and retinoid — two pigments in a gecko eye lens
Introduction
The vertebrate eye lens is an avascular, transparent organ focusing the incoming light onto the retina. Its optical properties are defined by densely packed, highly stable structural proteins called crystallins. These can be divided into three main classes. The first class comprises the α- and β/γ-crystallins, which are ubiquitous in vertebrate lenses. These crystallins are more or less closely related to stress response proteins (Wistow and Piatigorsky, 1988, de Jong et al., 1994). The second class of crystallins involves ten taxon-specific crystallins which are found only in scattered groups of vertebrates and which are related to or even identical with enzymes, mostly pyridine nucleotide binding oxidoreductases (Wistow, 1995, Röll et al., 1995, Röll and de Jong, 1996).
The third class of crystallins so far comprises only the recently described ι-crystallin, which is not an enzyme (Röll et al., 1996, Röll and Schwemer, 1999). ι-Crystallin is identical with the cellular retinol-binding protein type I (CRBP I) which belongs to the family of small, intracellular lipid-binding proteins (Werten et al., 1998). It occurs exclusively in lenses of several diurnal geckos, absorbs shortwave radiation and may promote visual quality by reducing glare and chromatic aberration and by enhancing contrast (Röll et al., 1996, Röll and Schwemer, 1999).
The endogenous ligand of ι-crystallin turned out to be 3,4-didehydroretinol (vitamin A2). Dependent on its concentration, the ι-crystallin/3,4-didehydroretinol complex accounts for the characteristic pale or dark yellow colouration of lenses. In lenses of the Morrocan day gecko Quedenfeldtia trachyblepharis, ι-crystallin reaches only up to 2% of the total amount of crystallins. The colour of the lenses, however, is dark yellow, similar to lenses of Pristurus, which contain up to 9–12% ι-crystallin (Röll and Schwemer, 1999). Thus, the deep yellow colour of the lens of Quedenfeldtia should be due to another pigment.
The present paper reports the presence of a second pigment, a carotenoid, in the lens of Quedenfeldtia. This is the first example of a carotenoid in the lens of a terrestrial vertebrate reaching a sufficiently high concentration to be physiologically effective as a UV-filter. Additionally, it is the first example of a carotenoid associated with γs-crystallin.
Section snippets
Preparation of lens extracts
Geckos used in this study were adult specimens with a snout to vent length of 35–40 mm and a hatchling 1-week-old. Geckos were chilled, killed by decapitation, and enucleated. The eyes were carefully incised at their posterior segments so that the lenses could be removed. Freshly isolated or frozen lenses were homogenized in distilled water or buffer solution, depending on the different experiments. Insoluble fractions were removed by centrifugation at 4°C for 15 min at 15 000×g. Protein
Lens pigments
In order to identify lipid-soluble pigments of the lenses, extracts of them were analysed by HPLC. The chromatograms reveal two main peaks. One peak (Fig. 1A, peak 5) contains all-trans 3,4-didehydroretinol, the endogenous ligand of ι-crystallin. In contrast, the other peak (Fig. 1A, peak 4) does not contain a retinoid, but a substance with a conspicuous absorption spectrum (Fig. 2, solid line). Its major peak has a definite fine structure with an absorption maximum at 392 nm, two secondary
Discussion
The Moroccan day gecko Q. trachyblepharus inhabits predominantly rocky areas of the Atlas Mountains, where it is exposed to high ambient light intensities. Its eyelids are fused to a transparent spectacle and its pupil diameter is virtually constant, as in other diurnal geckos. Thus, the only alternative to reduce radiation of potentially harmful wavelengths before reaching the retina is to absorb it in the lens. In the lens of Quedenfeldtia, this is achieved by the presence of two different
Acknowledgements
I am grateful to Professor Dr J. Schwemer, Bochum, for generous help with the samples for HPLC analysis.
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