Photochemistry of lodopsin

Ultrafast Transient Absorption Spectra and Kinetics of Rod and Cone Visual Pigments

Rods and cones are the photoreceptor cells containing the visual pigment proteins that initiate visual phototransduction following the absorption of a photon. Photon absorption induces the photochemical transformation of a visual pigment, which results in the sequential formation of distinct photo-intermediate species on the femtosecond to millisecond timescales, whereupon a visual electrical signal is generated and transmitted to the brain. Time-resolved spectroscopic studies of the rod and cone photo-intermediaries enable the detailed understanding of initial events in vision, namely the key differences that underlie the functionally distinct scotopic (rod) and photopic (cone) visual systems. In this paper, we review our recent ultrafast (picoseconds to milliseconds) transient absorption studies of rod and cone visual pigments with a detailed comparison of the transient molecular spectra and kinetics of their respective photo-intermediaries. Key results include the characterization of the porphyropsin (carp fish rhodopsin) and human green-cone opsin photobleaching sequences, which show significant spectral and kinetic differences when compared against that of bovine rhodopsin. These results altogether reveal a rather strong interplay between the visual pigment structure and its corresponding photobleaching sequence, and relevant outstanding questions that will be further investigated through a forthcoming study of the human blue-cone visual pigment are discussed.

Rhodopsins: An Excitingly Versatile Protein Species for Research, Development and Creative Engineering

The first member and eponym of the rhodopsin family was identified in the 1930s as the visual pigment of the rod photoreceptor cell in the animal retina. It was found to be a membrane protein, owing its photosensitivity to the presence of a covalently bound chromophoric group. This group, derived from vitamin A, was appropriately dubbed retinal. In the 1970s a microbial counterpart of this species was discovered in an archaeon, being a membrane protein also harbouring retinal as a chromophore, and named bacteriorhodopsin. Since their discovery a photogenic panorama unfolded, where up to date new members and subspecies with a variety of light-driven functionality have been added to this family. The animal branch, meanwhile categorized as type-2 rhodopsins, turned out to form a large subclass in the superfamily of G protein-coupled receptors and are essential to multiple elements of light-dependent animal sensory physiology. The microbial branch, the type-1 rhodopsins, largely function as light-driven ion pumps or channels, but also contain sensory-active and enzyme-sustaining subspecies. In this review we will follow the development of this exciting membrane protein panorama in a representative number of highlights and will present a prospect of their extraordinary future potential.

Pathways and disease-causing alterations in visual chromophore production for vertebrate vision

All that we view of the world begins with an ultrafast cis to trans photoisomerization of the retinylidene chromophore associated with the visual pigments of rod and cone photoreceptors. The continual responsiveness of these photoreceptors is then sustained by regeneration processes that convert the trans-retinoid back to an 11-cis configuration. Recent biochemical and electrophysiological analyses of the retinal G-protein-coupled receptor (RGR) suggest that it could sustain the responsiveness of photoreceptor cells, particularly cones, even under bright light conditions. Thus, two mechanisms have evolved to accomplish the reisomerization: one involving the well-studied retinoid isomerase (RPE65) and a second photoisomerase reaction mediated by the RGR. Impairments to the pathways that transform all-trans-retinal back to 11-cis-retinal are associated with mild to severe forms of retinal dystrophy. Moreover, with age there also is a decline in the rate of chromophore regeneration. Both pharmacological and genetic approaches are being used to bypass visual cycle defects and consequently mitigate blinding diseases. Rapid progress in the use of genome editing also is paving the way for the treatment of disparate retinal diseases. In this review, we provide an update on visual cycle biochemistry and then discuss visual-cycle-related diseases and emerging therapeutics for these disorders. There is hope that these advances will be helpful in treating more complex diseases of the eye, including age-related macular degeneration (AMD).

Cone visual pigments

Cone visual pigments are visual opsins that are present in vertebrate cone photoreceptor cells and act as photoreceptor molecules responsible for photopic vision. Like the rod visual pigment rhodopsin, which is responsible for scotopic vision, cone visual pigments contain the chromophore 11-cis-retinal, which undergoes cis-trans isomerization resulting in the induction of conformational changes of the protein moiety to form a G protein-activating state. There are multiple types of cone visual pigments with different absorption maxima, which are the molecular basis of color discrimination in animals. Cone visual pigments form a phylogenetic sister group with non-visual opsin groups such as pinopsin, VA opsin, parapinopsin and parietopsin groups. Cone visual pigments diverged into four groups with different absorption maxima, and the rhodopsin group diverged from one of the four groups of cone visual pigments. The photochemical behavior of cone visual pigments is similar to that of pinopsin but considerably different from those of other non-visual opsins. G protein activation efficiency of cone visual pigments is also comparable to that of pinopsin but higher than that of the other non-visual opsins. Recent measurements with sufficient time-resolution demonstrated that G protein activation efficiency of cone visual pigments is lower than that of rhodopsin, which is one of the molecular bases for the lower amplification of cones compared to rods. In this review, the uniqueness of cone visual pigments is shown by comparison of their molecular properties with those of non-visual opsins and rhodopsin. This article is part of a Special Issue entitled: Retinal Proteins - You can teach an old dog new tricks.

Effects of long-term administration of 9-cis-retinyl acetate on visual function in mice

PURPOSE

Long-term effects of treatment with 9-cis-retinyl acetate (9-cis-R-Ac), an artificial retinoid prodrug, were tested on changes in rod and cone visual functions in mice.

METHODS

The acetyl ester of the functional geometric chromophore 9-cis-retinal was delivered by oral gavage to C57BL/6 female mice. In initial experiments, 10-month-old mice were used for the single treatment with 9-cis-R-Ac or the control vehicle. In long-term experiments, 4-month-old mice were treated with 9-cis-R-Ac monthly for 6 and 10 months. Photoreceptor status was evaluated by various electroretinographic (ERG) techniques, retinoid analyses, and retinal morphology. Opsin, the predicted target of oxidized 9-cis-R-Ac, was purified and its chromophore was characterized.

RESULTS

Age-related changes observed in vehicle-treated mice at 10 months of age, compared with those in 4-month-old mice, included a progressive decline in ERG responses, such as a decreased rate of dark adaptation and a lowered rhodopsin/opsin ratio. Administration of 9-cis-R-Ac increased the rhodopsin regeneration ratio, and improved ERG responses and dark adaptation. Compared with vehicle-treated control animals, 10- and 14-month-old mice treated monthly with 9-cis-R-Ac for 6 or 10 months exhibited improved dark adaptation. In 14-month-old mice treated monthly, changes in the expression of retina-specific genes in the eye were detected by mRNA expression profiling, but no significant effects in gene expression were detected in the liver and kidney.

CONCLUSIONS

Deteriorating photoreceptor function documented in mice at 10 and 14 versus 4 months of age was improved significantly by long-term, monthly administration of 9-cis-R-Ac. These findings suggest a potential therapeutic approach to prevent age-related retinal dysfunction.

Switchable ErSc2N rotor within a C80 fullerene cage: an electron paramagnetic resonance and photoluminescence excitation study

Motivated by the possibility of observing photoluminescence and electron paramagnetic resonance from the same species located within a fullerene molecule, we initiated an EPR study of Er3+ in ErSc2N@C80. Two orientations of the ErSc2N rotor within the C80 fullerene are observed in EPR, consistent with earlier studies using photoluminescence excitation (PLE) spectroscopy. For some crystal field orientations, electron spin relaxation is driven by an Orbach process via the first excited electronic state of the 4I(15/2) multiplet. We observe a change in the relative populations of the two ErSc2N configurations upon the application of 532 nm illumination, and are thus able to switch the majority cage symmetry. This photoisomerization, observable by both EPR and PLE, is metastable, lasting many hours at 20 K.

Diseases caused by defects in the visual cycle: retinoids as potential therapeutic agents

Absorption of a photon by an opsin pigment causes isomerization of the chromophore from 11-cis-retinaldehyde to all-trans-retinaldehyde. Regeneration of visual chromophore following light exposure is dependent on an enzyme pathway called the retinoid or visual cycle. Our understanding of this pathway has been greatly facilitated by the identification of disease-causing mutations in the genes coding for visual cycle enzymes. Defects in nearly every step of this pathway are responsible for human-inherited retinal dystrophies. These retinal dystrophies can be divided into two etiologic groups. One involves the impaired synthesis of visual chromophore. The second involves accumulation of cytotoxic products derived from all-trans-retinaldehyde. Gene therapy has been successfully used in animal models of these diseases to rescue the function of enzymes involved in chromophore regeneration, restoring vision. Dystrophies resulting from impaired chromophore synthesis can also be treated by supplementation with a chromophore analog. Dystrophies resulting from the accumulation of toxic pigments can be treated pharmacologically by inhibiting the visual cycle, or limiting the supply of vitamin A to the eyes. Recent progress in both areas provides hope that multiple inherited retinal diseases will soon be treated by pharmaceutical intervention.

Seeing the light: New insights into the molecular pathogenesis of retinal diseases

In the past, most treatments for retinal diseases have been empirical. Steroids and/or laser photocoagulation and/or surgery have been tried for almost every condition with little or no understanding of the underlying disease. Over the past several years vision researchers have uncovered molecular components of processes, such as visual transduction and the visual cycle, that are critical for visual function, and identified other molecules that lead to dysfunction and disease processes such as neovascularization and macular edema. It is becoming clear that dysregulation of certain molecules can have major effects on retinal structure and function. Studies in animal models have suggested that inhibiting or augmenting levels of a single molecule can have major effects in complex disease processes. Although several molecules probably contribute to neovascularization and excessive vascular permeability in the eye, blockade of vascular endothelial growth factor (VEGF) has remarkable beneficial effects in animal models that have now been proven to apply to human diseases in clinical trials. Intraocular injection of VEGF antagonists has revolutionized the treatment of choroidal neovascularization (CNV) and macular edema and serves as a model of targeted ocular pharmacotherapy. Significant progress elucidating the molecular pathogenesis of several disease processes in the eye may soon lead to new treatments following the lead of VEGF antagonists. Initial treatments that provide benefit from frequent intraocular injections are likely to be followed by sustained delivery of drugs and/or prolonged protein delivery by gene transfer. The eye has entered the era of molecular therapy.

Examples of hula-twist in photochemical cis- trans isomerization

ALiu and Hammond recently reasoned that the hula-twist (HT), a volume-conserving cis-trans isomerization mechanism, is involved in reactions of confined systems. We now show that HT can be applied to various reported photochemical isomerization of chromophores (small organic systems as well as photoactive bio-pigments). The results, when taken as a whole, argue powerfully that HT is a common supramolecular photoisomerization reaction mechanism.

Large-scale production and purification of the human green cone pigment: characterization of late photo-intermediates

We present the first characterization of the late photo-intermediates (Meta I, Meta II and Meta III) of a vertebrate cone pigment in a lipid environment. Marked differences from the same pathway in the rod pigment were observed. The histidine-tagged human green cone pigment was functionally expressed in large-scale suspension cultures in Sf9 insect cells using recombinant baculovirus. The recombinant pigment was extensively purified in a single step by immobilized metal affinity chromatography and displays the expected spectral characteristics. The purified pigment was able to activate the rod G-protein transducin at about half the rate of the rod pigment. Following reconstitution into bovine retina lipid proteoliposomes, identification and analysis of the photo-intermediates Meta I, Meta II and Meta III was accomplished. Similar to the rod pigment, our results indicate the existence of a Meta I-Meta II equilibrium, but we find no evidence for pH dependence. Replacement of native Cl- by NO3- in the anion-binding site of the cone pigment affected the spectral position of the pigment itself and of the Meta I intermediate, but not that of Meta II and Meta III. The decay rate of the 'active' intermediate Meta II did not differ for the Cl- and NO3- state. However, in qualitative agreement with results reported before for chicken cone pigments, the rate of Meta II decay was significantly higher in the human cone pigment than in the rod pigment.

Structure and photobleaching process of chicken iodopsin

Iodopsin, a dominant cone pigment in a chicken retina, has an absorption spectrum in longer wavelength region than rhodopsin. To account for this red-shift of iodopsin, we had proposed a structural model from retinal analogue experiments, in which iodopsin would have a relatively long distance between the protonated Schiff base nitrogen and the counterion. This was confirmed by a resonance Raman spectroscopy. The photochemical properties of iodopsin were studied and compared with those of rhodopsin, which revealed the following differences. The regeneration rate of iodopsin with 11-cis-retinal was 240 times faster than rhodopsin. Meta-iodopsin II, the signalling state of iodopsin, decayed about 100 times faster than meta-rhodopsin II. The Km value of meta-iodopsin II and rhodopsin kinase was lower than meta-rhodopsin II. These results are in consistent with rapid adaptation and low photosensitivity of cones relative to those of rods.

Difference in molecular properties between chicken green and rhodopsin as related to the functional difference between cone and rod photoreceptor cells

Using low-temperature spectroscopy, we have investigated the photobleaching process of chicken green, a green-sensitive cone visual pigment present in chicken retina, and compared it to that of rhodopsin, a rod visual pigment. Like rhodopsin, chicken green converts to all-trans-retinal and opsin through batho, lumi, and meta I, II, and III intermediates. However, all of the intermediates of chicken green except lumi, are less stable than the corresponding intermediates of rhodopsin. While early intermediates, batho and lumi are similar in absorption maxima between chicken green and rhodopsin, the meta intermediates of chicken green are about 20 nm blue shifted from those of rhodopsin. Low-temperature time-resolved spectroscopy was applied to estimate the thermodynamic properties of meta intermediates, and it indicated that the less stable properties of meta II and III intermediates of chicken green originate from the smaller activation enthalpies. The decay of the meta II intermediate of chicken green is greatly suppressed when a chicken green sample is irradiated at alkaline conditions while the net charge becomes similar to that of rhodopsin at neutral conditions. These results strongly suggest that the functional properties of chicken green that are different from those of rhodopsin are regulated by the dissociative amino acid residue(s).

Thermal recovery of iodopsin from its meta I-intermediate

The thermal reaction of meta I-intermediate of iodopsin (metaiodopsin I), a chicken red-sensitive cone pigment, was studied by low-temperature spectrophotometry at -20 degrees C. Irradiation of iodopsin at -20 degrees C produced metaiodopsin I, whose absorption maximum was at about 470 nm. An incubation of metaiodopsin I at -20 degrees C resulted in a conversion to metaiodopsin II having absorption maximum at about 380 nm, as well as a concurrent formation of a red-shifted product stable at room temperature. Since the absorption spectrum and photo-reactivity of the red-shifted product were identical with those of iodopsin, the red-shifted product should be iodopsin. Thus a part of metaiodopsin I can revert to iodopsin by the thermal reaction unlike metarhodopsin I.

Iodopsin, a red-sensitive cone visual pigment in the chicken retina

The vertebrate retina contains two kinds of visual cells: rods, responsible for twilight (scotopic) vision (black and white discrimination); and cones, responsible for daylight (photopic) vision (color discrimination). Here we attempt to explain some of their functional differences and similarities in terms of their visual pigments. In the chicken retina there are four types of single cones and a double cone; each of the single cones has its own characteristic oil droplet (red, orange, blue, or colorless) and the double cone is composed of a set of principal and accessory members, the former of which has a green-colored oil droplet. Iodopsin, the chicken red-sensitive cone visual pigment, is located at outer segments of both the red single cones and the double cones, while the other single cones and the rod contain their own visual pigments with different absorption spectra. The diversity in absorption spectra among these visual pigments is caused by the difference in interaction between chromophore (11-cis retinal) and protein moiety (opsin). However, the chromophore-binding pocket in iodopsin is similar to that in rhodopsin. The difference in absorption maxima between both pigments could be explained by the difference in distances between the protonated Schiff-bases at the chromophore-binding site and their counter ions in iodopsin and rhodopsin. Furthermore, iodopsin has a unique chloride-binding site whose chloride ion serves for the red-shift of the absorption maximum of iodopsin. Visual pigment bleaches upon absorption of light through several intermediates and finally dissociates into all-trans retinal and opsin. That the sensitivity of cones is lower than rods cannot be explained by the relative photosensitivity of iodopsin to rhodopsin, but may be understood to some extent by the short lifetime of an enzymatically active intermediate (corresponding to metarhodopsin II) produced in the photobleaching process of iodopsin. The rapid formation and decay of the meta II-intermediate of iodopsin compared with metarhodopsin II are not contradictory to the rapid generation and recovery of cone receptor potential compared with rod receptor potential. The rapid recovery of the cone receptor potential may be due to a more effective shutoff mechanism of the visual excitation, including the phosphorylation of iodopsin. The rapid dark adaptation of cones compared with rods has been explained by the rapid regeneration of iodopsin from 11-cis retinal and opsin. One of the reasons for the rapid regeneration and susceptibility to chemicals of iodopsin compared with rhodopsin may be a unique structure near the chromophore-binding site of iodopsin.

Monkey photoreceptor calycal processes and interphotoreceptor matrix as observed by scanning electron microscopy

Photoreceptors and the interphotoreceptor matrix of monkey retina were observed by scanning electron microscopy. Cone photoreceptors are easily distinguished from rod photoreceptors by their wide conical inner segments. The calycal processes of 50 rods and 50 cones were counted and measured. The calycal processes of cones were distinct, short, and uniform in diameter (0.1 micron). They were arranged equidistantly and, in most cases, were not continuous with longitudinal inner segment ridges, as previously suggested. In contrast to cones, rod calycal processes were fewer in number, were about one-fourth the number of the cones, were of variable length (0.7 micron to 3.0 micron), and tapered to a fine point at their distal ends. The interphotoreceptor matrix appeared spongelike, made up of anastomosing plates and strands filling the interphotoreceptor space. Other than an increased amount of matrix around cones, no structural difference between rod- and cone-associated interphotoreceptor matrix was observed.

Photoreaction cycle of phoborhodopsin studied by low-temperature spectrophotometry

The photochemical and subsequent thermal reactions of phoborhodopsin (pR490), which mediates the negative phototaxis (phobic reaction) of Halobacterium halobium, were investigated by low-temperature spectrophotometry. At room temperature, the absorption spectrum of pR490 displayed vibrational structure with a maximum at 490 nm and a shoulder at 460 nm, which were remarkably sharpened by cooling, resulting in the appearance of two well-separated peaks. On irradiation of pR490 at -170 degrees C, a photo-steady-state mixture composed of pR490 and two photoproducts, P520 and P480, was formed. P480 had an absorption maximum at 480 nm and thermally converted to pR490 above -160 degrees C, while P520 had an absorption maximum at 515 nm and thermally converted to P350, the next intermediate, above -60 degrees C. Above -30 degrees C, P350 was converted to P530, and then reverted to pR490. P520, P350, and P530 may correspond to K, M, and O intermediates of bacteriorhodopsin, respectively, on the basis of their absorption spectra, but the intermediates corresponding to L and N intermediates were not observed. On the basis of these results, a new scheme of the photoreaction cycle of pR490 was presented.

Bathoiodopsin, a primary intermediate of iodopsin at physiological temperature

Measurement of the primary photochemical reaction of iodopsin, a chicken red-sensitive cone visual pigment, was carried out at room temperature by using picosecond (ps) laser photolysis. Excitation of iodopsin with a ps green pulse (pulse width, 21 ps) caused the instantaneous formation of a bathochromic product, which was stable on a ps time scale. This product may correspond to "bathoiodopsin," which was detected by low-temperature spectrophotometry. Although bathoiodopsin produced at the temperature of liquid nitrogen or helium reverted to the original pigment (iodopsin) on warming (above -170 degrees C), the bathoiodopsin produced at physiological temperature decayed to all-trans-retinal and R-photopsin (the protein moiety of iodopsin) presumably through several intermediates. The absorption maximum of bathoiodopsin at room temperature was at 625 nm, a wave-length slightly shorter than that measured at low temperature (lambda max, 640 nm). The extinction coefficient of bathoiodopsin at room temperature was lower than that at low temperature and close to that of the original iodopsin at room temperature.

The primary structure of iodopsin, a chicken red-sensitive cone pigment

A purified iodopsin was digested by CNBr or several proteolytic enzymes into fragments, the amino acid sequences of which were determined. A partial sequence of the C-terminal fragment was utilized for synthesizing an oligonucleotide probe which identified the iodopsin cDNA (1339 bases). The deduced amino acid sequence (362 residues) had 80%, 42%, or 43% homology to that of human red-sensitive cone pigment, cattle or chicken rhodospin, respectively. Although the hydropathy profile implies that iodopsin, like rhodopsin, has 7 transmembrane alpha-helical segments, iodopsin may have a hydrophilic pocket near the seventh segment on the basis of the unexpected cleavages in the middle of the segment VII by chymotrypsin under nondenaturing conditions.

Effects of chloride on chicken iodopsin and the chromophore transfer reactions from iodopsin to scotopsin and B-photopsin

Spectroscopic properties of chicken iodopsin were investigated in correlation with the concentration of chloride in digitonin extracts. When chloride in the extract was depleted by extensive dialysis, chloride-depleted iodopsin (absorption maximum, 512 nm) was formed. It was converted to chloride-bound iodopsin (absorption maximum, 562 nm) by the addition of chloride in the extract. There existed an equilibrium between two forms of iodopsin with a dissociation constant of 0.8 mM chloride. The chromophore-transfer reaction from iodopsin to scotopsin or B-photopsin, the protein moiety of chicken rhodopsin or chicken blue-sensitive cone pigment, respectively, in digitonin extract was also investigated in correlation with the concentrations of chloride, other monovalent and divalent anions, and detergent. The chromophore of chloride-depleted iodopsin was easily transferred to scotopsin in the extract, resulting in formation of rhodopsin. On the other hand, chloride-bound iodopsin was fairly stable even in the presence of scotopsin, indicating that the reaction is inhibited by binding of chloride to iodopsin. The chromophore-transfer reaction to B-photopsin was also observed from chloride-depleted iodopsin but not from chloride-bound iodopsin. The reaction was observable in the 10% digitonin extract as well as in the 2% digitonin extract. The reaction was also observed when 25 mM Na2SO4 was present in the mixture instead of NaCl, but was not when 67 mM NaNO3 was present. All these facts suggest that the chloride binding site of iodopsin does not accept a divalent anion such as SO4(2+), but does accept a monovalent anion such as Cl- or NO3-, which causes inhibition of the chromophore transfer.

Monoclonal antibodies specific to chicken iodopsin

Monoclonal antibodies (mABs) from hybridoma cells were raised and screened with a purified cone pigment, iodopsin, from the chicken retina. Four different methods were used to test these antibodies: (1) dot-immunobinding assay; (2) enzyme-linked immunoabsorbent assay (ELISA); (3) one dimensional immunoblotting and (4) two dimensional immunoblotting. Three classes of antibody producing cell lines were identified. One class produces a mAB specific to iodopsin. The mAB from the second class crossreacts with iodopsin and probably one of the other three cone pigments. The mAB from the third class probably crossreacts with all the four cone pigments in the chicken retina. The mABs from all these classes of hybridoma cell lines were selected so that they do not crossreact with rhodopsin. Two dimensional immunoblotting indicated that iodopsin has a much higher isoelectric point than rhodopsin, as suggested from the known amino acid sequences of human rod and cone pigments.

Primary intermediates of rhodopsin studied by low temperature spectrophotometry and laser photolysis. Bathorhodopsin, hypsorhodopsin and photorhodopsin

The primary photochemical processes of rhodopsin studied by low temperature spectrophotometry and picosecond laser spectroscopy in our group was summarized. Low temperature spectroscopic experiments demonstrated that the retinylidene chromophores of hypso- and bathorhodopsins are in a twisted all-trans forms. Excitation of rhodopsin with 532 nm laser pulse (width: 25 psec) yielded a new bathochromic photoproduct "photorhodopsin"; its spectrum was located at longer wavelengths than that of bathorhodopsin. Photorhodopsin decays to bathorhodopsin with time constants of about 200 psec in squid and 40 psec in cattle. Squid and octopus hypsorhodopsins were produced within 25 psec by high energy pulse, but not by low energy pulse. Thus hypsorhodopsin is produced by two photon reactions (sequential two photochemical reactions) and decayed to bathorhodopsin with time constant of 125 psec.

Activation of phosphodiesterase by chicken iodopsin

Activation of guanosine 3',5'-cyclic monophosphate phosphodiesterase in outer-segment membrane of chicken retina was investigated. Irradiation of dark-adapted chicken outer segment membrane for bleaching of iodopsin increased the enzyme activity twice as much as that in the dark in the presence of GTP. Further irradiation of the sample for bleaching of rhodopsin in the membrane induced some additional activation of the enzyme. However, chicken iodopsin activated the enzyme in frog rod outer segment membrane without irradiation, while chicken rhodopsin did not. Irradiation of chicken iodopsin increased the enzyme activity twice as much as that in the dark.

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.

Bicycle-pedal model for the first step in the vision process

Computer simulation of the molecular dynamics of retinal during its photoisomerisation inside a restrictive active site gives a detailed model for the sequence of events in the first step of the vision process. It is proposed that the prelumirhodopsin intermediate contains a strained all-trans retinal molecule produced directly and rapidly from the 11-cis, 12-s-trans conformation in rhodopsin by a bicycle-pedal isomerisation. The model reproduces the main experimental observations and explains how the protein makes the photoisomerisation path unique.

Accessibility of the iodopsin chromophore

Iodopsin can replace its chromophore (11-cis retinal) by added 9-cis retinal, resulting in the formation of isoiodopsin. NaBH4 bleaches iodopsin in the dark. In a relatively low concentration of digitonin, the scotopsin (the protein moiety of chicken rhodopsin) removes 11-cis retinal from iodopsin in the dark. These facts suggest that the linkage of the chromophore to opsin in the iodopsin molecule (presumably a Schiff-base linkage) is accessible to these reagents, which is different from the situation in rhodopsin.

Psychophysical estimates of visual pigment densities in red-green dichromats

1. The spectral sensitivity of red-green dichromats was determined using heterochromatic flicker photometric matches (25-30 c/s) on the fovea. These matches are upset after a bright bleach and consequently the spectral sensitivity is altered.2. Preliminary experiments indicate that under the conditions in which these experiments were performed, the blue cone mechanism of deuteranopes and protanopes cannot follow 20 c/s flicker. If dichromats lack one of the normal pigments then the upset of these matches monitors the change in spectral sensitivity of a single mechanism.3. After a bleach which removes all the cone pigments, the spectral sensitivity recovers with the time course of pigment kinetics as measured by densitometry.4. An intense background also changes the relative spectral sensitivity of the dichromats. On real equilibrium backgrounds, the changes in spectral sensitivity follow those predicted by the pigment changes measured by densitometry. The predicted changes are obtained by modifying the Rushton equilibrium equation to take into account the density of pigment.5. The relationship of these changes to the luminance of the background is independent of the colour of the background light.6. In contradistinction the effect is dependent on the colour of the lights which were flickered. These experiments indicate that a narrowing of the spectral sensitivity curves takes place on both sides of the dichromats' lambda(max).7. The change in relative spectral sensitivity as a function of background intensity was also determined by increment threshold measurements. These changes can be expressed in terms of deviations from Weber's law (DeltaI/I = const.) if DeltaI and I represent the number of chromophores destroyed by the test and background.8. The relative spectral sensitivity of the dichromat was changed by decentering the point of pupil entry. This upset was abolished by bleaching. The size of the upset was correlated with the magnitude of the S-C I effect.9. Given the hypothesis of pigment density (self-screening), the results of expts. (3)-(8) are consistent and allow the calculation of a maximum optical density for those pigments which underlie the dichromats' long-wave mechanism. For the deuteranope a D(lambdamax) of 0.5-0.6 is calculated and for the protanope a D(lambdamax) of 0.4-0.5 is obtained.