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P. Ala-Laurila, R. Albert, P. Saarinen, A. Koskelainen, K. Donner; The Effect of the Chromophore Switch From A1 to A2 on Spectral and Thermal Properties of the Rod Visual Pigment in the Bullfrog, Rana catesbeiana . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2004.
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Purpose: To measure the change in the energy needed for photoactivation (Ea) of the rod visual pigment when the chromophore is switched from retinal (A1) to 3-dehydroretinal (A2) in rods of the adult bullfrog, Rana catesbeiana. This well-characterized model species1,2 offers a rhodopsin-porphyropsin pair for which the rates of thermal ("dark") isomerizations3 as well as the cDNA sequence of the opsin4 are available for functional comparisons. Methods: Absorbance spectra of rhodopsin and porphyropsin rods were recorded by single-cell microspectrophotometry (MSP) and spectral sensitivities were determined by electroretinogram (ERG) recording across the ventral (rhodopsin) and dorsal (porphyropsin) fields of the isolated retina at two temperatures, 8.5 oC and 28.5 oC. The photoactivation energies (Ea) of the two pigments were estimated based on the temperature effect on relative spectral sensitivities at long wavelengths.5, 6 Results: Our Ea-estimates (mean ± SEM) for rhodopsin and porphyropsin rods are 46.5 ± 0.8 kcal/mol and 44.2 ± 0.9 kcal/mol respectively. Thus the chromophore switch from A1 to A2 was associated with a statistically significant (P < 0.09) decrease of Ea. The absorbance maxima of the two pigments were at 501 nm and 525 nm, respectively. Conclusions: The lowering of the photoactivation energy due to the switch from A1 to A2 correlates both with the previously observed increase in thermal isomerization rate3 and the red-shift of the spectrum, as originally hypothesized by Horace Barlow.7 1Reuter et al., J. Gen. Physiol. 58, 351-371 (1971). 2Fong et al., Vision Res. 25, 1387-1397 (1985). 3Donner et al., J. Physiol. 428, 673-692 (1990). 4Kayada et al., Comp. Biochem. Physiol. 110B, 599-604 (1995). 5Koskelainen et al., Nature 403, 220–223 (2000). 6Ala-Laurila et al., Visual Neurosci. (in press). 7Barlow, Nature 179, 255-256 (1957).
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