Circular dichroism of cephalopod rhodopsin and its intermediates in the bleaching and photoregeneration process

Rhodopsin inactivation is a modulated process in Limulus photoreceptors

Many G-protein-coupled receptors are only transiently active because an inactivation process stops the receptor from activating G protein molecules. Although this inactivation has been investigated in vitro, the real kinetics of the process can only be obtained from intact cells. Here we describe a method for measuring the inactivation of rhodopsin in intact photoreceptors and the application of this method to the ultraviolet rhodopsin of Limulus median eye. The results show that the inactivation process is very rapid (less than 150 ms) and occurs well before the peak of the receptor potential. We have also investigated whether the inactivation process can itself be modulated. Our results show that light-adaptation accelerates inactivation by about 10-fold, providing evidence that G-protein-mediated transduction can be modulated at this first stage.

Lipid fluidity of the outer segment membranes from cephalopod retina

Membranes from the retina outer segments of squid (Loligo sp.) were isolated by sucrose continuous gradient. In these membranes, phosphatidylcholine and phosphatidylethanolamine were shown to be the most abundant lipids, while rhodopsin was the predominant protein. Steady-state fluorescence depolarization of the probe 1,6 diphenyl-1,3,5-hexatriene was employed to characterize the microviscosity of the photosensitive membranes and of the vesicles prepared from their lipid extract. The lipid fluidity parameter (r0/r-1)-1 in these membranes ranged between approximately 3.8 and approximately 0.95 (0-38 degrees C), while the value of the lipid extract vesicles ranged between approximately 3 and approximately 0.6. No phase transition was observed in the membrane and vesicle preparation in the range of temperatures under study [0-38 degrees C). Photosensitive membranes from cuttlefish (sepia officinalis) gave similar results. The (r0/r-1)-1 value in this species varied between approximately 4 and approximately 1.2 (0-38 degrees C) with a linear dependence on temperature. In these outer segments the bleaching of rhodopsin did not cause any change of the membrane lipid fluidity value. The present results are compared with those obtained in bovine rod outer segments.

Specific photoisomerization of retinal in squid rhodopsin and metarhodopsin

The composition of retinal isomers in the photosteady-state mixtures formed from squid rhodopsin and metarhodopsin was determined by high-pressure liquid chromatography. A large amount of 9-cis-retinal was obtained at liquid N2 temperature when rhodopsin was irradiated with orange light, but only small quantities of 9-cis-retinal were obtained at 15 degrees C. Scarcely any 9-cis-retinal was produced from metarhodopsin by irradiation at liquid N2 temperature. A large quantity of 7-cis-retinal was found in the photoproduct of rhodopsin irradiated at solid carbon dioxide temperature, but not at 15 degrees C and liquid N2 temperature. 7-cis-Retinal was not produced from metarhodopsin at any temperatures. These results indicate that the photoisomerization of retinal is regulated by the structure of the retinal-binding site of this protein. The formation of 9-cis- and 7-cis-retinals is forbidden in the metarhodopsin protein.

Optical activity of octopus metarhodopsins

The optical activity of octopus rhodopsin, acid metarhodopsin and alkaline metarhodopsin was studied by a sensitive and rapid CD apparatus. For sometime it has been thought that cephalopod metarhodopsins do not have any optical activity associated with their main absorption band. However, the present work shows that acid metarhodopsin in digitonin has a positive CD band at 498 nm and a negative CD band at 436 nm and alkaline metarhodopsin has a negative CD band at 381 nm. Detergent affected the wavelength of the CD peak of the visual pigments though the pattern of the spectrum was similar. From these results it is concluded that the conformation of all-trans retinal in octopus metarhodopsin is influenced by the asymmetric conformation of the protein near the retinal and therefore inducing optical activity.

Circular dichroism of squid rhodopsin and its intermediates

Circular dichroism (CD) and absorption spectra of squid (Todarodes pacificus) rhodopsin, isorhodopsin and the intermediates was measured at low temperatures. Squid rhodopsin has positive CD bands at wavelengths corresponding the alpha- and beta-absorption bands at liquid nitrogen temperature (CD maxima: 485 nm at alpha-band and 348 nm at beta-band) as well as at room temperature (CD maxima: 474 nm at alpha-band and 347 nm at beta-band). The rotational strength of the alpha-band has a molecular ellipticity about twice that of cattle rhodopsin. The CD spectrum of bathorhodopsin displays a negative peak at 532 nm, the rotational strength of which has an absolute value slightly larger than that of rhodopsin. The reversal in sign at alpha-band of the CD spectrum may indicate that the isomerization of retinal chromophore from twisted 11-cis form to twisted 11-trans form has occurred in the process of conversion from rhodopsin to bathorhodopsin. Lumirhodopsin has a small negative CD band at 490 nm, the maximum of which lies at 25 nm shorter wavelengths than the absorption maximum (515 nm), and a large positive CD band near 290 nm, which is not observed in rhodopsin and the other intermediates. This band may de derived from a conformational change of the opsin. In the process of changing from lumirhodopsin to LM-rhodopsin, The CD bands at visible and near ultraviolet regions disappear. Both alkaline and acid metarhodopsins have no CD bands at visible and near ultraviolet regions.

Kinetic study of photoregeneration process of digitonin-solubilized squid rhodopsin

In the photoregeneration process of squid rhodopsin, an intermediate has been found at neutral pH values (phosphate buffer) with a flash light (lambda greater than 540 nm). An intermediate R430, with the 11-cis retinal as chromophore, is produced from metarhodopsin in light and is converted to rhodopsin through the processes R430 leads to P380 and P380 leads to rhodopsin. The pH dependence of the velocity of the conversions suggests that processes R430 leads to P380 and P380 leads to rhodopsin involve a protolytic reaction and that the ionized group is a histidine residue of opsin. Kinetic parameters show that the largest conformational change in opsin occurs in the conversion of R430 leads to P380.

Reversible changes in circular dichroism spectra of cattle rhodopsin and isorhodopsin

When the disk membrane of rod outer segment is treated with detergents, the alpha-band CD of rhodopsin decreases and the gamma-band CD increases. This tendency of CD change is most prominent in the purified rhodopsin in cholic acid obtained by the ammonium sulfate fractionation of disk membranes, and the gamma-band CD is three times larger than the alpha-band CD. The beta-band CD of rhodopsin is only slightly influenced by detergents. The gamma-band of isorhodopsin shows two CD bands, one negative and one positive. Both in rhodopsin and isorhodopsin the gamma-band CD is lost by light irradiation. It is supposed that both chromophore retinal and aromatic amino acid residues of opsin are responsible for the gamma-band CD. When ammonium sulfate is added to the sonicated disk membranes suspended in cholic acid solution, the alpha-band CD of rhodopsin decreases to about a third and the gamma-band CD increases remarkably. The CD spectrum goes back to the original one on eliminating ammonium sulfate from the solution with dialysis. However, the purified rhodopsin recovers native CD spectrum on addition of lipids extracted from disk membranes. The retinal-opsin interaction that induces optical activity depends upon the property of a local environment formed by lipid and detergent.

Studies on cephalopod rhodopsin: photoisomerization of the chromophore

Photoisomerization of the chromophore of squid rhodopsin is dependent upon the irradiation temperature. Above 0 degrees C, only 11-cis in equilibrium all-trans reaction proceeds and the all-trans leads to 9-cis reaction is limited to extremely low efficiency. At liquid nitrogen temperature, 11 cis in equilibrium all-trans in equilibrium 9-cis reaction takes place. At intermediary low temperatures (-80 degrees C to -15 degrees C) another isomer of retinal may be produced by the irradiation, which forms a pigment having an absorbance maximum at 465 nm (P-465). The formation of P-465 decreases remarkably in the narrow temperature range from -30 degrees C to 0 degrees C where mesorhodopsin converts to metarhodopsin. Medsorhodopsin is quite different from metarhodopsin in the photoisomerization of the chromophore because P-465 is produced from the former but not from the latter. No P-465 is produced both at liquid nitrogen temperature and above 0 degrees C. P-465 is more labile than any of the other photoproducts so far known, that is isorhodopsin, alkaline and acid metarhodopsins. P-465 is converted to metarhodopsin by irradiation.