Absorption spectra of rhodopsin and its intermediates and orientational change of the chromophore

Activation of rhodopsin phosphorylation is triggered by the lumirhodopsin-metarhodopsin I transition

The absorption of light by the chromophore of rhodopsin initiates a series of interconversions between spectrally distinct intermediates. The possibility has been raised that one of these transitions is accompanied by a change in the state of rhodopsin, and that it is this change which instigates visual excitation via a cascade of enzyme catalysed reactions. It has been suggested that the initial step of this cascade, which leads to the activation of cyclic GMP phosphodiesterase (PDE), involves the interaction of a GTP-binding regulatory (G) protein with rhodopsin. The ability of rhodopsin to activate PDE may be inhibited by the phosphorylation of sites exposed on the opsin surface as a result of light-induced conformational changes. To obtain more information about the relationship between the postchemical and biochemical reactions of rhodopsin we have investigated which transition leads to the activation of rhodopsin as a substrate for rhodopsin kinase, and report here that it is the transition from lumirhodopsin to metarhodopsin I.

Two forms of intermediates of frog rhodopsin in rod outer segments

Using frog rod outer segments, we measured changes of the absorption spectrum during the conversion of rhodopsin to a photosteady-state mixture composed of rhodopsin, isorhodopsin and bathorhodopsin by irradiation with blue light (440 nm) at -190 degrees C and during the reversion of bathorhodopsin to a mixture of rhodopsin and isorhodopsin by irradiation with red light (718 nm) at -190 degrees C. The reaction kinetics was expressed by one exponential in the former case and by two exponentials in the latter. These results suggest that rhodopsin is composed of a single molecular species, while bathorhodopsin is composed of two kinds of molecular species designated as batho1-rhodopsin and batho2-rhodopsin. On warming the two forms of bathorhodopsin, each bathorhodopsin converted to its own lumirhodopsin, metarhodopsin I and finally a free all-trans-retinal plus opsin. The absorption spectra of the two forms of bathorhodopsin, lumirhodopsin and metarhodopsin I were measured at -190 degrees C. We infer that a rhodopsin molecule in the excited state relaxes to either batho1-rhodopsin or batho2-rhodopsin, and then converts to its own intermediates through one of the two parallel pathways.

Photochemical reactions of 13-demethyl visual pigment analogues at low temperatures

The photobleaching reaction of 13-demethylisorhodopsin (hereafter designated as 9-cis- 13-dm-rhodopsin), which was synthesized from 9-cis- 13-demethylretinal and cattle opsin, was investigated by low-temperature spectrophotometry in order to elucidate the role of the 13-methyl group of retinal in photobleaching. When 9-cis- 13-dm-rhodopsin was irradiated at-190 degrees C, batho-13-dm-rhodopsin was produced. Its absorption maximum lay at 532 nm, 11 nm shorter than that of cattle bathorhodopsin (gamma max 543 nm), and batho-13-dm-rhodopsin had an extinction coefficient about 0.6 times that of bathorhodopsin. Batho-13-dm-rhodopsin was thermally unstable. Above-180 degrees C, it converted to a new intermediate, BL-13-dm-rhodopsin, which in turn changed to lumi-13-dm-rhodopsin- above -140 degrees C. BL-13-dm-rhodopsin was "photosensitive" at temperatures around -188 degrees C, though batho-13-dm-rhodopsin and lumi-13-dm-rhodopsin was "photosensitive" at the same temperature. In the photobleaching process, lumi-13-dm-rhodopsin and meta-I-13-dm-rhodopsin were observed. Their thermostabilities were very similar to those of lumirhodopsin and metarhodopsin I, but each dm intermediate differed from its methylated counterpart in its value of gamma max and extinction coefficient.

Photochemical reaction of 9-cis-retro-gamma-rhodopsin at low temperatures

9-cis-Retro-gamma-rhodopsin (lambda max = 420 nm) was prepared from 9-cis-retro-gamma-retinal and cattle opsin. After cooling to liquid nitrogen temperature (77 K), the pigment was irradiated with light at 380 nm. The spectrum shifted to the longer wavelengths, owing to formation of a batho product. This fact indicates that the conjugated double bond system from C-5 to C-8 of the chromophoric retinal in rhodopsin was not necessary for formation of bathorhodopsin. Reirradiation of the batho product with light at wavelengths longer than 520 nm yielded a mixture composed of presumably 9- or 11-cis forms of retro-gamma-rhodopsin. These three isomers are interconvertible by light at liquid nitrogen temperature. Thus the retro-gamma-rhodopsin system is similar in photochemical reaction at 77 K to cattle rhodopsin system. Each system has its own batho product. Based on these results, it was infered that the formation of batho-rhodopsin is due to photoisomerization of the chromophoric retinal of rhodopsin and is not due to translocation of a proton on the ring or on the side chain from C-6 to C-8 of the chromophoric retinal to the Schiff-base nitrogen.

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.

Formation of hyposorhodopsin in frog retina

Hypsorhodopsin was formed in frog retina by irradiation at liquid helium temperature and converted into bathorhodopsin above about 29K.