A SPIN LABELED RETINAL PIGMENT ANALOGUE OF THE PURPLE MEMBRANE

Low aqueous solubility of 11-cis-retinal limits the rate of pigment formation and dark adaptation in salamander rods

We report experiments designed to test the hypothesis that the aqueous solubility of 11-cis-retinoids plays a significant role in the rate of visual pigment regeneration. Therefore, we have compared the aqueous solubility and the partition coefficients in photoreceptor membranes of native 11-cis-retinal and an analogue retinoid, 11-cis 4-OH retinal, which has a significantly higher solubility in aqueous medium. We have then correlated these parameters with the rates of pigment regeneration and sensitivity recovery that are observed when bleached intact salamander rod photoreceptors are treated with physiological solutions containing these retinoids. We report the following results: (a) 11-cis 4-OH retinal is more soluble in aqueous buffer than 11-cis-retinal. (b) Both 11-cis-retinal and 11-cis 4-OH retinal have extremely high partition coefficients in photoreceptor membranes, though the partition coefficient of 11-cis-retinal is roughly 50-fold greater than that of 11-cis 4-OH retinal. (c) Intact bleached isolated rods treated with solutions containing equimolar amounts of 11-cis-retinal or 11-cis 4-OH retinal form functional visual pigments that promote full recovery of dark current, sensitivity, and response kinetics. However, rods treated with 11-cis 4-OH retinal regenerated on average fivefold faster than rods treated with 11-cis-retinal. (d) Pigment regeneration from recombinant and wild-type opsin in solution is slower when treated with 11-cis 4-OH retinal than with 11-cis-retinal. Based on these observations, we propose a model in which aqueous solubility of cis-retinoids within the photoreceptor cytosol can place a limit on the rate of visual pigment regeneration in vertebrate photoreceptors. We conclude that the cytosolic gap between the plasma membrane and the disk membranes presents a bottleneck for retinoid flux that results in slowed pigment regeneration and dark adaptation in rod photoreceptors.

Quantum mechanical/molecular mechanical structure, enantioselectivity, and spectroscopy of hydroxyretinals and insights into the evolution of color vision in small white butterflies

Since Vogt's discovery of A(3)-retinal or 3-hydroxyretinal in insects in 1983 and Matsui's discovery of A(4)-retinal or 4-hydroxyretinal in firefly squid in 1988, hydroxyretinal-protein interactions mediating vision have remained largely unexplored. In the present study, A(3)- and A(4)-retinals are theoretically incorporated into squid and bovine visual pigments by use of the hybrid quantum mechanics/molecular mechanics [SORCI+Q//B3LYP/6-31G(d):Amber96] method, and insights into structure, enantioselectivity, and spectroscopy are gathered and presented for the first time. Contrary to general perception, our findings rule out the formation of a hydrogen bond between the hydroxyl-bearing β-ionone ring portion of retinal and opsin. Compared to A(1)-pigments, A(3)- and A(4)-pigments exhibit slightly blue-shifted absorption maxima due to increase in bond-length alternation of the hydroxyretinal. We suggest that (i) the binding site of firefly squid (Watasenia scintillans) opsin is very similar to that of the Japanese common squid (Todarodes pacificus) opsin; (ii) the molecular mechanism of spectral tuning in small white butterflies involve sites S116 and T185 and breaking of a hydrogen bond between sites E180 and T185; and finally (iii) A(3)-retinal may have occurred during the conversion of A(1)- to A(2)-retinal and insects may have acquired them, in order to absorb light in the blue-green wavelength region and to speed up the G-protein signaling cascade.

The absorbance spectrum and photosensitivity of a new synthetic "visual pigment" based on 4-hydroxyretinal

The firefly squid, Watasenia scintillans, is the only animal known to possess a visual pigment in which the chromophore is 4-hydroxyretinal. This paper describes the absorbance spectrum and some properties of a synthetic "A4" visual pigment generated from bovine opsin and 4-hydroxyretinal. The absorbance spectrum of this pigment is compared with (a) bovine rhodopsin and (b) a rhodopsin template with the same lambda max as the synthetic visual pigment. The A4 pigment is shown to have an absorbance spectrum that is almost identical to that of a rhodopsin template. It is also shown that the photosensitivity and thermal stability of the A4 pigment, dispersed in detergent micelles, is essentially similar to that of rhodopsin.

An investigation of the electrochemical cycle of bacteriorhodopsin analogs with the modified ring

5,6-Epoxy-, 4-methoxy-, 4-hydroxy-, and 3,4-dehydrobacteriorhodopsins can generate delta psi coupled to a photochemical cycle with intermediate M. The kinetics of delta psi comprises three main electrogenic phases: the fast small negative, the microsecond, and the millisecond positive phases. The photocycle efficiency is lower in all the analogs. The photocycle is modified insignificantly only in 3,4-dehydrobacteriorhodopsin. In the other pigments the decay of the flash-induced bleaching in the chromophore main absorption band is slower than the decay of M or long-wave intermediates, especially in the 4-hydroxy analog. In the latter analog, such distinctions, according to delta pH measurements, are partly due to deceleration of the decay of the novel intermediate (P). In 5,6-epoxybacteriorhodopsin, at all wavelengths, the decay of the intermediates takes seconds upon M formation. According to our and literature data, no bacteriorhodopsin analogs are known to have a cycle which preserves the M-intermediate and does not transport a proton.

4-Hydroxyretinal, a new visual pigment chromophore found in the bioluminescent squid, Watasenia scintillans

The bioluminescent squid, Watasenia scintillans has three visual pigments. The major pigment, based on retinal (lambda max 484 nm), is distributed over the whole retina. Another pigment based on 3-dehydroretinal (lambda max approximately 500 nm) and the third pigment (lambda max approximately 470 nm) are localized in the specific area of the ventral retina just receiving the downwelling light. Visual pigment was extracted and purified from the dissected retina. The chromophores were then extracted and analyzed with HPLC, NMR, infrared and mass spectroscopy, being compared with the synthetic 4-hydroxyretinal. A new retinal derivative, 11-cis-4-hydroxyretinal, is identified as the chromophore of the third visual pigment of the squid.