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At least 991 records · Page 55Linked to original sources
PubMed1998

Evidence for the archaebacterial-type conformation about the bond between the beta-ionone ring and the polyene chain of the chromophore retinal in chlamyrhodopsin.

Previous studies using retinal analogs (Wada, A., Sakai, M., Kinumi, T., Tsujimoto, K., Yamauchi, M. and Ito, M. (1994) J. Org. Chem. 59, 6992-6997; Wada, A., Sakai, M., Imamoto, Y., Shichida., Y., Yoshizawa, T. and Ito, M. (1993) Chem. Pharm. Bull. 41, 793-795) revealed that both retinochrome and visual pigments share a common chromophoric conformation in which the ring region of retinal is twisted ca. 50 degrees with respect to the plane of the polyene chain, suggesting a highly conserved 6-s-cis conformation throughout rhodopsin-like pigments in animals. By contrast, 6-s-trans conformation was commonly observed or suggested in archaebacterial rhodopsins examined thus far. Here we have reconstituted in vivo both the photoreceptor for photobehavioral responses of the unicellular alga Chlamydomonas reinhardtii and the second archaeal sensory photoreceptor phoborhodopsin from a series of retinal analogs. Results exclusively point to the conclusion that in both photoreceptors retinal has the coplanar 6-s-trans conformation, though recent molecular cloning studies revealed no homology between Chlamydomonas photoreceptor (chlamyrhodopsin) and archaeal rhodopsins.

Animals↗
PubMed1979

Separation of geometric isomers of retinyl ester, retinal and retinol, pertaining to the visual cycle, by high-performance liquid chromatography.

A method for the analytical and/or preparative separation of the 13-cis, 11-cis, 9-cis and all all-trans isomers of retinyl palmitate ester, retinal and retinol by high-performance liquid chromatography is described. A straight-phase adsorption system consisting of Si 60 silica gel as the stationary phase and mixtures of n-hexane and dioxane as mobile phases were employed, using photometric detection at either 320 or 360 nm, depending on the nature of the compounds being studied. With simple adaptation of the phase system, all geometric isomers within each group could be separated. The precision of the method was 0.5% at the 10-microgram (about 40-nmole) level and 4% at the 10-ng (about 40-pmole) level (n = 3). The detection limit was about 0.5 ng (about 2 pmole). The suitability of the phase system is demonstrated by chromatograms of test mixtures and of eye extracts. The fractionation of 0.5 mg of an isomeric sample could be achieved on a column of length 250 mm and I.D. 10 mm.

Animals↗
PubMed1977

Simultaneous determination of cis-trans isomeric retinals by high-performance liquid chromatography.

A high-performance liquid chromatographic method for the simultaneous determination of geometric isomers of retinal in a mixture has been developed. The method can be applied to problems such as determination of optimal conditions for the preparation of a particular isomer, or the investigation of novel photoproducts from retinal. Aprotic solvent effects on cis-trans isomerization are discussed.

Chromatography, High Pressure Liquid↗
PubMed1988

Rapid, simultaneous determination of isomers of retinal, retinal oxime and retinol by high-performance liquid chromatography.

Thirteen geometric isomers of retinol, retinal and retinal oxime were resolved in a single chromatographic run on two 250 X 4 mm 5-micron LiChrosorb Si 60 columns in series, using 11.2% ethyl acetate, 1.4% 1-octanol and 2% dioxane in hexane as the mobile phase. All the 11-cis and all-trans isomers of retinol and retinal oxime were completely resolved from each other and from the 9-cis and 13-cis isomers. This chromatographic procedure should be particularly useful for quantifying geometric isomers of retinoids in methylene chloride-hydroxylamine extracts of biological samples such as ocular tissues.

Animals↗
PubMed1984

High-performance liquid chromatography of retinals, retinols (vitamin A1) and their dehydro homologues (vitamin A2): improvements in resolution and spectroscopic characterization of the stereoisomers.

A study of mobile phases for the improved high-performance liquid chromatographic resolution of retinal, 3-dehydroretinal, retinol and 3-dehydroretinol stereoisomers is described. By using 1-octanol as a phase modifier in n-hexane, the simultaneous separation of a complex mixture of vitamin A1 and A2 isomers was satisfactorily achieved, while the separation of the corresponding aldehydes required a ternary mixture of 2-propanol, dioxane and n-hexane. All peaks were spectroscopically characterized by recording their UV spectra, which are reported together with comparative tables of the absorbance maxima of the stereoisomers.

Alcohols↗
PubMed1985

Rapid analysis of the major classes of retinoids by step gradient reversed-phase high-performance liquid chromatography using retinal (O-ethyl) oxime derivatives.

A rapid step-gradient reversed-phase high-performance liquid chromatography (HPLC) method is presented for analysis of the major classes of retinoids in tissues. Retinal was converted into a new derivative, retinal (O-ethyl) oxime, since the standard derivative, retinaloxime, co-elutes with retinol on reversed-phase HPLC. The most abundant naturally occurring retinyl esters, retinyl palmitate and retinyl stearate, were eluted within 12 min to complete the separation. Retinoids were extracted in the presence of an antioxidant, butylated hydroxytoluene, and a lipid carrier, cholesterol. Recoveries of 98-100% were obtained from tissue samples by internal addition for the retinoids tested (retinol, retinal and retinyl palmitate); and the absolute recovery of endogenous retinal from rat eyecups was confirmed by spectrophotometric measurements of rhodopsin. Extraction was carried out in an air atmosphere and under subdued incandescent light rather than requiring inert atmosphere and safe-light conditions used in most methods. Cis-trans isomers were not separated under the reversed-phase HPLC conditions employed. Quantitation was carried out using retinyl acetate as internal standard and the day to day precision was better than 3.5%. A sensitivity of about 1 ng is obtained for all retinoids using absorbance monitoring at 325 nm and a C18 5 micrometers column with 12% reversed-phase loading. The tocopherols can also be separated and detected simultaneously with similar sensitivity by this method using a fluorescence detector in series [G. J. Handelman, L. J. Machlin, K. Fitch, J. J. Weiter and E. A. Dratz, J. Nutr., 115 (1985) 807].

Animals↗
PubMed1999

A reversed-phase high-performance liquid chromatographic method to analyze retinal isomers.

A high-performance liquid chromatographic (HPLC) procedure was developed to separate all-trans-, 13-cis-, 11-cis- and 9-cis-retinal isomers. Two reversed-phase Vydac C18 columns in series were used with an isocratic solvent system of 0.1 M ammonium acetate-acetonitrile (40:60, v/v) as mobile phase and all-trans-9-(4-methoxy-2,3,6-trimethylphenyl)-3,7-dimethyl-2,4,6,8-no natetraene-1-ol (TMMP) as internal standard. Prior to HPLC, the retinal isomers were efficiently extracted in their original isomeric conformation using dichloromethane-n-hexane in the presence of formaldehyde. This technique is suitable for the assay of 11-cis- and all-trans-retinal isomers in retina.

Animals↗
PubMed2002

Proton translocation by bacteriorhodopsin in the absence of substantial conformational changes.

Unlike wild-type bacteriorhodopsin (BR), the BR triple mutant D96G/F171C/F219L has been shown to undergo only minor structural rearrangements during its photocycle. Nonetheless, the mutant is capable of transporting protons at a rate of 125(+/-40) H+/BR per minute under light-saturating conditions. Light adaptation of the triple mutant's retinal proceeds in a pH-dependent manner up to a maximum of 63% all-trans. These two findings imply that the transport activity of the triple mutant comprises 66% of the wild-type activity. Time-resolved spectroscopy reveals that the identity and sequence of intermediates in the photocycle of the triple mutant in the all-trans configuration correspond to that of wild-type BR. The only differences relate to a slower rise and decay of the M and O intermediates, and a significant spectral contribution from a 13-cis component. No indication for accumulation of the N intermediate is found under a variety of conditions that normally favor the formation of this species in wild-type BR. The Fourier transform infrared (FTIR) spectrum of the M intermediate in the triple mutant resembles that of wild type. Minor changes in the amide I region during the photocycle suggest that only small movements of the protein backbone occur. Electron microscopy reveals large differences in conformation between the unilluminated state of the mutant protein and wild-type but no light-induced changes in time-resolved measurements. Evidently, proton transport by the triple mutant does not require the major conformational rearrangements that occur on the same time-scale with wild-type. Thus, we conclude that large conformational changes observed in the photocycle of the wild-type and many BR mutants are not a prerequisite for the change in accessibility of the Schiff base nitrogen atom that must occur during vectorial catalysis to allow proton transport.

Archaea↗
PubMed2002

Crystallographic structure of the K intermediate of bacteriorhodopsin: conservation of free energy after photoisomerization of the retinal.

The K state, an early intermediate of the bacteriorhodopsin photocycle, contains the excess free energy used for light-driven proton transport. The energy gain must reside in or near the photoisomerized retinal, but in what form has long been an open question. We produced the K intermediate in bacteriorhodopsin crystals in a photostationary state at 100K, with 40% yield, and determined its X-ray diffraction structure to 1.43 A resolution. In independent refinements of data from four crystals, the changes are confined mainly to the photoisomerized retinal. The retinal is 13-cis,15-anti, as known from vibrational spectroscopy. The C13=C14 bond is rotated nearly fully to cis from the initial trans configuration, but the C14-C15 and C15=NZ bonds are partially counter-rotated. This strained geometry keeps the direction of the Schiff base N-H bond vector roughly in the extracellular direction, but the angle of its hydrogen bond with water 402, that connects it to the anionic Asp85 and Asp212, is not optimal. Weakening of this hydrogen bond may account for many of the reported features of the infrared spectrum of K, and for its photoelectric signal, as well as the deprotonation of the Schiff base later in the cycle. Importantly, although 13-cis, the retinal does not assume the expected bent shape of this configuration. Comparison of the calculated energy of the increased angle of C12-C13=C14, that allows this distortion, with the earlier reported calorimetric measurement of the enthalpy gain of the K state indicates that a significant part of the excess energy is conserved in the bond strain at C13.

Bacteriorhodopsins↗
PubMed2002

Crystallographic structure of the retinal and the protein after deprotonation of the Schiff base: the switch in the bacteriorhodopsin photocycle.

We illuminated bacteriorhodopsin crystals at 210K to produce, in a photostationary state with 60% occupancy, the earliest M intermediate (M1) of the photocycle. The crystal structure of this state was then determined from X-ray diffraction to 1.43 A resolution. When the refined model is placed after the recently determined structure for the K intermediate but before the reported structures for two later M states, a sequence of structural changes becomes evident in which movements of protein atoms and bound water are coordinated with relaxation of the initially strained photoisomerized 13-cis,15-anti retinal. In the K state only retinal atoms are displaced, but in M1 water 402 moves also, nearly 1A away from the unprotonated retinal Schiff base nitrogen. This breaks the hydrogen bond that bridges them, and initiates rearrangements of the hydrogen-bonded network of the extracellular region that develop more fully in the intermediates that follow. In the M1 to M2 transition, relaxation of the C14-C15 and C15=NZ torsion angles to near 180 degrees reorients the retinylidene nitrogen atom from the extracellular to the cytoplasmic direction, water 402 becomes undetectable, and the side-chain of Arg82 is displaced strongly toward Glu194 and Glu204. Finally, in the M2 to M2' transition, correlated with release of a proton to the extracellular surface, the retinal assumes a virtually fully relaxed bent shape, and the 13-methyl group thrusts against the indole ring of Trp182 which tilts in the cytoplasmic direction. Comparison of the structures of M1 and M2 reveals the principal switch in the photocycle: the change of the angle of the C15=NZ-CE plane breaks the connection of the unprotonated Schiff base to the extracellular side and establishes its connection to the cytoplasmic side.

Bacteriorhodopsins↗
PubMed2003

Mechanism of proton transport in bacteriorhodopsin from crystallographic structures of the K, L, M1, M2, and M2' intermediates of the photocycle.

We produced the L intermediate of the photocycle in a bacteriorhodopsin crystal in photo-stationary state at 170 K with red laser illumination at 60% occupancy, and determined its structure to 1.62 A resolution. With this model, high-resolution structural information is available for the initial bacteriorhodopsin, as well as the first five states in the transport cycle. These states involve photo-isomerization of the retinal and its initial configurational changes, deprotonation of the retinal Schiff base and the coupled release of a proton to the extracellular membrane surface, and the switch event that allows reprotonation of the Schiff base from the cytoplasmic side. The six structural models describe the transformations of the retinal and its interaction with water 402, Asp85, and Asp212 in atomic detail, as well as the displacements of functional residues farther from the Schiff base. The changes provide rationales for how relaxation of the distorted retinal causes movements of water and protein atoms that result in vectorial proton transfers to and from the Schiff base.

Bacteriorhodopsins↗
PubMed2003

Crystallographic structures of the M and N intermediates of bacteriorhodopsin: assembly of a hydrogen-bonded chain of water molecules between Asp-96 and the retinal Schiff base.

An M intermediate of wild-type bacteriorhodopsin and an N intermediate of the V49A mutant were accumulated in photostationary states at pH 5.6 and 295 K, and their crystal structures determined to 1.52A and 1.62A resolution, respectively. They appear to be M(1) and N' in the sequence, M(1)<-->M(2)<-->M'(2)<-->N<-->N'-->O-->BR, where M(1), M(2), and M'(2) contain an unprotonated retinal Schiff base before and after a reorientation switch and after proton release to the extracellular surface, while N and N' contain a reprotonated Schiff base, before and after reprotonation of Asp96 from the cytoplasmic surface. In M(1), we detect a cluster of three hydrogen-bonded water molecules at Asp96, not present in the BR state. In M(2), whose structure we reported earlier, one of these water molecules intercalates between Asp96 and Thr46. In N', the cluster is transformed into a single-file hydrogen-bonded chain of four water molecules that connects Asp96 to the Schiff base. We find a network of three water molecules near residue 219 in the crystal structure of the non-illuminated F219L mutant, where the residue replacement creates a cavity. This suggests that the hydration of the cytoplasmic region we observe in N' might have occurred spontaneously, beginning at an existing water molecule as nucleus, in the cavities from residue rearrangements in the photocycle.

Aspartic Acid↗
PubMed1983

Fluorescence energy transfer studies of transmembrane location of retinal in purple membrane.

A diffusion-enhanced energy transfer technique was employed for the determination of transmembrane location of the retinal chromophore in the purple membrane. Theoretical considerations showed that the rate of energy transfer from an energy donor embedded within a membrane to acceptors dissolved in solvent could be described by an analytical function of the distance a of closest approach between the donor and acceptor, if the "rapid-diffusion limit" was attained. The criterion for this limit was given by the relation: (RO)6 much less than 20D tau Da4, where RO is the characteristic distance of energy transfer, D is the diffusion coefficient of the acceptor and tau D is the fluorescence lifetime of the donor in the absence of acceptor. By photo-reduction of the purple membrane with sodium borohydride, the retinal chromophore was converted to a highly fluorescent derivative, which showed a broad emission band in the visible region. From analysis of the fluorescence decay curves of the photo-reduced purple membrane in the presence of various concentrations of cobalt-ethylenediamine tetraacetate (Co-EDTA: energy acceptor), the depth of the chromophore from the membrane surface was estimated to be 8 (+/-3) A. This result was supported by investigations of energy transfer processes in a system where the native purple membranes and the photo-reduced membranes were stacked in parallel: the energy acceptor in this system was the native retinal chromophore.

Bacteriorhodopsins↗
PubMed1999

Inhibition by retinoids of benzo(A)pyrene metabolism catalyzed by 3-methylcholanthrene-induced rat cytochrome P-450 1A1.

Benzo(a)pyrene, a well-known procarcinogen, is believed to be activated by microsomal cytochrome P-450 1A1 (CYP 1A1). We recently reported that rat CYP 1A1 induced by 3-methylcholanthrene (3-MC) catalyzed the conversion of retinal to retinoic acid. In this study, we investigated retinoid inhibition of the metabolism of benzo(a)pyrene and 7-ethoxyresorufin, another specific substrate of CYP 1A1, using liver microsomes prepared from control and 3-MC-pretreated rats as the enzyme source. In 3-MC-treated rats, retinal and retinol, but not retinoic acid, inhibited benzo(a)pyrene metabolism. The 50% inhibitory concentration (IC50) of retinal was about 11.5 micromol/L and the inhibition was competitive, with a Ki value of 5.8 micromol/L. Retinol is a more potent inhibitor than retinal. The IC50 was about 5 micromol/L and the inhibition was mixed, with a Ki value of 19.2 micromol/L and a Ki' value of 4.2 micromol/L. Almost the same results were obtained for the reaction of 7-ethoxyresorufin deethylation. In contrast, the metabolic activity of both benzo(a)pyrene and 7-ethoxyresorufin was much lower in untreated versus 3-MC-treated rats, and only weak inhibition by the retinoids was observed. The results suggest that retinoids inhibit the activation of benzo(a)pyrene and that the substrate specificity of cytochrome P-450 isozymes associated with retinoid metabolism should be taken into account when studying the anticarcinogenic action of retinoids.

Animals↗