Retinyl Moiety Research Articles

Inhibition of RPE65 Retinol Isomerase Activity by Inhibitors of Lipid Metabolism

RPE65 is the isomerase catalyzing conversion of all-trans-retinyl ester (atRE) into 11-cis-retinol in the retinal visual cycle. Crystal structures of RPE65 and site-directed mutagenesis reveal aspects of its catalytic mechanism, especially retinyl moiety isomerization, but other aspects remain to be determined. To investigate potential interactions between RPE65 and lipid metabolism enzymes, HEK293-F cells were transfected with expression vectors for visual cycle proteins and co-transfected with either fatty acyl:CoA ligases (ACSLs) 1, 3, or 6 or the SLC27A family fatty acyl-CoA synthase FATP2/SLCA27A2 to test their effect on isomerase activity. These experiments showed that RPE65 activity was reduced by co-expression of ACSLs or FATP2. Surprisingly, however, in attempting to relieve the ACSL-mediated inhibition, we discovered that triacsin C, an inhibitor of ACSLs, also potently inhibited RPE65 isomerase activity in cellulo. We found triacsin C to be a competitive inhibitor of RPE65 (IC50 = 500 nm). We confirmed that triacsin C competes directly with atRE by incubating membranes prepared from chicken RPE65-transfected cells with liposomes containing 0-1 μM atRE. Other inhibitors of ACSLs had modest inhibitory effects compared with triascin C. In conclusion, we have identified an inhibitor of ACSLs as a potent inhibitor of RPE65 that competes with the atRE substrate of RPE65 for binding. Triacsin C, with an alkenyl chain resembling but not identical to either acyl or retinyl chains, may compete with binding of the acyl moiety of atRE via the alkenyl moiety. Its inhibitory effect, however, may reside in its nitrosohydrazone/triazene moiety.

Formation Of All-Trans Retinol In Mouse Rod Photoreceptors

Light detection destroys the visual pigment of vertebrate rod photoreceptors, rhodopsin, as its retinyl moiety is photoisomerized from 11-cis to all-trans. Rhodopsin is regenerated through a series of reactions that begin in the rod outer segment with the release of the all-trans retinal and its reduction to all-trans retinol. All-trans retinol is then transported to the neighboring retinal pigment epithelial cells where it is used to remake 11-cis retinal. The reduction of all-trans retinal to all-trans retinol is catalyzed by retinol dehydrogenase and requires metabolic input in the form of NADPH. We have used the fluorescence of all-trans retinol to monitor its concentration in isolated mouse rod photoreceptors. After the bleaching of rhodopsin, all-trans retinol formation proceeds with a rate of ∼0.06 min−1, which is faster than the rate of rhodopsin regeneration in whole animals; this would allow recycled chromophore to contribute to the 11-cis retinal used for regeneration. Inner segment metabolic pathways appear to make a significant contribution to the pool of NADPH needed for the reduction of all-trans retinal, as formation of all-trans retinol is suppressed in rod outer segments separated from the cell body. Finally, generation of all-trans retinol is suppressed in the absence of glucose, indicating a critical dependence of all-trans retinol formation on the level of metabolic activity.

Interphotoreceptor Retinoid-Binding Protein Is the Physiologically Relevant Carrier That Removes Retinol from Rod Photoreceptor Outer Segments

Light detection by vertebrate rod photoreceptor outer segments results in the destruction of the visual pigment, rhodopsin, as its retinyl moiety is photoisomerized from 11-cis to all-trans. The regeneration of rhodopsin is necessary for vision and begins with the release of the all-trans retinal and its reduction to all-trans retinol. Retinol is then transported out of the rod outer segment for further processing. We used fluorescence imaging to monitor retinol fluorescence and quantify the kinetics of its formation and clearance after rhodopsin bleaching in the outer segments of living isolated frog (Rana pipiens) rod photoreceptors. We independently measured the release of all-trans retinal from bleached rhodopsin in frog rod outer segment membranes and the rate of all-trans retinol removal by the lipophilic carriers interphotoreceptor retinoid binding protein (IRBP) and serum albumin. We find that the kinetics of all-trans retinol formation in frog rod outer segments after rhodopsin bleaching are to a good first approximation determined by the kinetics of all-trans retinal release from the bleached pigment. For the physiological concentrations of carriers, the rate of retinol removal from the outer segment is determined by IRBP concentration, whereas the effect of serum albumin is negligible. The results indicate the presence of a specific interaction between IRBP and the rod outer segment, probably mediated by a receptor. The effect of different concentrations of IRBP on the rate of retinol removal shows no cooperativity and has an EC50 of 40 micromol/L.

All- Trans Retinol in Rod Photoreceptor Outer Segments Moves Unrestrictedly by Passive Diffusion

The visual pigment protein of vertebrate rod photoreceptors, rhodopsin, contains an 11- cis retinyl moiety that is isomerized to all- trans upon light absorption. Subsequently, all- trans retinal is released from the protein and reduced to all- trans retinol, the first step in the recycling of rhodopsin’s chromophore group through the series of reactions that constitute the visual cycle. The concentration of all- trans retinol in photoreceptor outer segments can be monitored from its fluorescence. We have used two-photon excitation (720 nm) of retinol fluorescence and fluorescence recovery after photobleaching to characterize the mobility of all- trans retinol in frog photoreceptor outer segments. Retinol produced after rhodopsin bleaching moved laterally in the disk membrane bilayer with an apparent diffusion coefficient of 2.5 ± 0.3 μm 2 s −1. The diffusion coefficient of exogenously added retinol was 3.2 ± 0.5 μm 2 s −1. These diffusion coefficients are in close agreement with those reported for lipids, suggesting that retinol is not tightly bound to protein sites that would be diffusing much more slowly in the plane of the membrane. In agreement with this interpretation, a fluorescent-labeled C-16 fatty acid diffused laterally with a similar diffusion coefficient, 2.2 ± 0.2 μm 2 s −1. Retinol also moved along the length of the rod outer segment, with an apparent diffusion coefficient of 0.07 ± 0.01 μm 2 s −1, again suggesting that it is not tightly bound to proteins that would confine it to the disks. The axial diffusion coefficient of exogenously added retinol was 0.05 ± 0.01 μm 2 s −1. In agreement with passive diffusion, the rate of axial movement was inversely proportional to the square of the length of the rod outer segment. Diffusion of retinol on the plasma membrane of the outer segment can readily account for the measured value of the axial diffusion coefficient, as the plasma membrane comprises ∼1% of the total outer-segment membrane. The values of both the lateral and axial diffusion coefficients are consistent with most of the all- trans retinol in the outer segments moving unrestricted and not being bound to carrier proteins. Therefore, and in contrast to other steps of the visual cycle, there does not appear to be any specialized processing for all- trans retinol within the rod outer segment.

Binding of lipophilic nutrients to beta-lactoglobulin prepared by bioselective adsorption.

The binding of the lipophilic nutrients, retinal, vitamin D2, and retinyl palmitate by beta-lactoglobulin was measured by analysis of changes in the fluorescence of the tryptophanyl residue of beta-lactoglobulin or the retinyl moiety. The fluorescence intensity of the tryptophanyl residue was quenched by retinoid or vitamin D binding but was enhanced by palmitate binding. The analysis of competitive binding experiments with palmitate indicated that retinal and palmitate did not compete for the same site; however, vitamin D2, which binds with a stoichiometry of 2, appeared to displace palmitate at higher concentrations. Also, the retinoids and vitamin D2 were bound more tightly than was palmitate. The results are consistent with the model in which the retinoids and vitamin D2 bind in the calyx formed by the beta-barrel; palmitate and a second molecule of vitamin D2 bind in a surface pocket near the dimer contact region. Retinyl palmitate, which has both moieties, appeared to bind at both sites.

High-resolution solid-state 13C-NMR study of carbons C-5 and C-12 of the chromophore of bovine rhodopsin. Evidence for a 6-S-cis conformation with negative-charge perturbation near C-12.

Solid-state 13C magic-angle spinning NMR spectroscopy has been employed to study the conformation of the 11-cis-retinylidene Schiff base chromophore in bovine rhodopsin. Spectra were obtained from lyophilized samples of bovine rhodopsin selectively 13C-labeled at position C-5 or C-12 of the retinyl moiety, and reconstituted in the fully saturated branched-chain phospholipid diphytanoyl glycerophosphocholine. Comparison of the NMR parameters for carbon C-5 presented in this paper with those published for retinyl Schiff base model compounds and bacteriorhodopsin by Harbison and coworkers [Harbison et al. (1985) Biochemistry 24, 6955-6962], indicate that in bovine rhodopsin the C-6-C-7 single bond has the unperturbed cis conformation. This is in contrast to the 6-S-trans conformation found in bacteriorhodopsin. The NMR parameters for bovine [12-13C]rhodopsin present evidence for the presence of a negative charge interacting with the retinyl moiety near C-12, in agreement with the model for the opsin shift presented by Honig and Nakanishi and coworkers [Kakitani et al. (1985) Photochem. Photobiol. 41, 471-479].

PHOTOCHEMICAL STUDIES OF ARTIFICIAL BACTERIORHODOPSINS

Abstract— Three artificial bacteriorhodopsins are prepared [from synthetic aromatic and bicyclic analogues of retinal and exposed to spectroscopic and pulsed lader photolysis studies. The spectra of the pigments, all perturbed in the ring region of the molecule, are markedly blue shifted in respect to natural bacteriorhodopsin. The shift is attributed to a decreased effect of a protein charge in the vicinity of the ring, in agreement with the point‐charge model of Nakanishi et al., 1980. The photocycles of the synthetic pigments exhibit a primary red‐shifted (K) intermediate and a blue shifted (M) transient, analogous to those observed for the natural pigment. Such observations impose considerable limitations, both on the possible chromophore conformational changes and on the effects of neighbouring protein charges associated with the photocycle.It is concluded that only the Schiff base counter‐ion, but not the ring charge, may be associated with the generation of the primary red shifted K species. Moreover, the rigidity imposed on the polyene by the additional ring in the bicyclic analogue shows that the photocycle can not be initiated by conformational changes in the retinyl moiety up to the C9 carbon in the polyene chain. It is also observed that the K→L process in the photocycle is considerably slower in the case of the synthetic pigments. The observation is rationalized by attributing the process to a conformational change in the polyene moiety catalyzed by the ring protein charge.

Lysine 216 is a binding site of the retinyl moiety in bacteriorhodopsin

Lysine 216 is a binding site of the retinyl moiety in bacteriorhodopsin

The incorporation of tritiated retinyl moiety into the active-site lysine residue of bacteriorhodopsin.

Purple membranes were isolated from Halobacterium halobium bleached and regenerated with all-trans-[15-3H]retinal. The incorporation of label was 1.2 mol of retinal/mol of bacterio-opsin. The [3H]retinyl-bacterio-opsin obtained from regeneration was hydrolysed to give tritiated retinyl-lysine, which, on hydrogenation to N-epsilon-perhydro[3H]retinyl-lysine and reaction with 1-fluoro-2,4-dinitrobenzene, gave bis-(2,4-dinitrophenyl)-N-epsilon-perhydro[3H]retinyl-lysine. This result confirmed that the retinyl moiety of the chromophore is attached to an epsilon-amino group of lysine.

Specificity of the retinal binding site of bacteriorhodopsin: chemical and stereochemical requirements for the binding of retinol and retinal.

The complexes formed from bacteriopsin and various retinyl compounds were analyzed by fluorescence and absorption spectroscopy. The binding of retinol occurs in two steps. In the first reaction the molecule is fixed in the retinal binding site of the protein. In this state, energy transfer from aromatic amino acid residues to the retinyl moiety is observed. all-trans-Retinal and the 13-, 11-, and 9-cis-retinols are bound in the chromophoric site. In the second reaction the cyclohexene ring and the side chain of the retinyl moiety are forced into a planar conformation. This reaction is mediated by a base (B1) with a pK of 3.8 and requires the oxygen atom but not the free hydroxyl group of retinol, indicating interaction with a group AH (pK greater than or equal to 10.5). The ring-chain planarization reaction is blocked for the 9-cis isomer of retinol. Binding studies with bacterioopsin and retinal isomers reveal that, as in the case of the corresponding retinols, B1 mediates ring-chain planarization in the case of the all-trans, 13-cis, and 11-cis isomers but not with the 9-cis isomer. Reconstitution of the purple complex from the intermediate 430-460-nm chromophore requires the presence of a second base (B2) with a pK of 4.6. This reaction is exclusive for all-trans- and 13-cis-retinal

PROPERTIES OF THE RETINAL BINDING SITE IN BACTERIORHODOPSIN. USE OF RETINOL AND RETINYL MOIETIES AS FLUORESCENT PROBES

Abstract— The fluorescence spectra of various reduced bacteriorhodopsin chromophore species indicate energy transfer from aromatic amino acid side chains of the protein to the retinyl moiety. Binding studies with retinol reveal that energy transfer occurs only when the retinyl moiety is bound in the chromophoric site of the protein. Retroretinol is a fluorescent probe for the binding site.

Studies on the retinal-protein interaction in bacteriorhodopsin.

1 Different types of bacteriorhodopsin chromophores and their reactions are described. This includes intermediates of the reconstitution reaction of the purple complex, intermediates of the photochemical cycle and a photochemically active 500-nm chromophore. In contrast to the native chromophore (purple complex) some of these species are reducible by borohydride yielding stable retinyl proteins. 2 Hydrolysis and/or extraction of the retinyl proteins reveals that in the corresponding non reduced parent chromophores the retinal is bound either noncovalently or covalently to lysyl residues of the polypeptide chain. 3 Retinol, retinyl lysine and their retro isomers isolated from the various reduced chromophores are identified by thin-layer chromatography and mass spectrometry. 4 The absorption spectra of the retinyl proteins and binding studies with retinol and retinal indicate that the cyclohexene ring and the side chain of the retinyl moiety are forced into a coplanar conformation by interaction with the protein. The three peaked absorption band of the reduced chromophores is due to this planarized conformation and not to a retro configuration of the retinyl moiety. 5 Isomerization to retro compounds can be achieved by HCl as is known to be the case for retinyl compounds in solution. In addition photochemical isomerization is observed in the case of planarized retinyl moieties. Thus the native protein structure is responsible not only for a specific conformation of the retinyl moiety but also for its specific reactivity.

Properties of the Chromophore Binding Site of Retinolbinding Protein from Human Plasma

Abstract Apo-retinol-binding protein (apo-RBP) from human plasma was found to bind all-trans-3-dehydroretinol and to protect the chromophore from degradation. The all-trans-3-dehydroretinol-RBP complex, in turn, binds tightly to prealbumin at physiological ionic strength as judged by gel filtration chromatography. All-trans-3-dehydroretinal also formed a complex with RBP but this complex did not bind to prealbumin. Both the all-trans-3-dehydroretinol and all-trans-3-dehydroretinal-RBP complexes showed induced optical activity of the chromophore absorption spectrum. In marked contrast to the retinol- and retinal-RBP complexes the sign of the long wavelength circular dichroism band of the all-trans-3-dehydro chromophore-RBP complexes was reversed. The chromophoric binding site of apo-RBP was probed for binding with hydrophobic compounds such as β-ionone, crocetin, and 1-phenyldecane. All of these compounds did not bind to apo-RBP, as was judged by the availability of the binding site to the native chromophore-retinol. This suggests that the retinyl moiety is necessary for binding. The capacity of the chromophoric binding site to bind to various retinyl derivatives was tested with several retinyl-Schiff bases. Retinylideneaniline did not bind to apo-RBP, but when the phenyl group was displaced by adding a C4 chain between the retinyl and the phenyl moieties as in retinylidene-4-phenylbutylamine, binding to RBP occurred. Aliphatic retinyl-Schiff bases shorter than and including retinylidenehexylamine bind to apo-RBP. With retinylidenedecylamine only 30 to 40% of the binding site was occupied. Retinylidene-1-tetradecylamine did not bind at all to apo-RBP. Similarly, retinyl palmitate did not bind to apo-RBP. The fact that liver retinol is stored as retinyl palmitate suggests that the palmitate group must be enzymatically hydrolyzed before the retinol can bind to apo-RBP, so as to be released from the liver stores. This enzymatic step makes it possible to control the rate of retinol release from the liver and its level in the plasma.

A convenient synthesis of labelled rhodopsin and studies on its active site.

Digitonin solutions of labelled rhodopsin, containing (3)H in the retinyl moiety, were prepared by two related methods. Labelled rhodopsin was also prepared for the first time in cetyltrimethylammonium bromide and purified by column chromatography. It was shown that only certain rhodopsin preparations on denaturation in the dark and the reduction with sodium borohydride gave up to 60% of the radioactivity in a fraction characterized as N-retinylphosphatidylethanolamine. Such preparations also gave a lipid-linked retinyl moiety at the metarhodopsin-I stage, but, as expected, a protein-linked retinyl moiety at the metarhodopsin-II stage. Other preparations however, gave exclusively protein-bound radioactivity at the native-rhodopsin, metarhodopsin-I and metarhodopsin-II stages. It is therefore conceivable that the formation of N-retinylphosphatidylethanolamine is due to a non-enzymic reaction resulting from the transfer of the retinyl moiety from its native site to an amino group of a favourably oriented phospholipid molecule. The only firmly established aspect of the rhodopsin active site remains the demonstration in our previous work that at the metarhodopsin-II stage the retinyl moiety is linked to an in-amino group of lysine. On the basis of chemical reactivity it is argued that the light-induced conversion of rhodopsin into metarhodopsin II involves a profound conformational change resulting in the dislocation of the retinylideneiminium chromophore from a non-polar environment in rhodopsin to a polar environment in metarhodopsin II.

Studies on vision. The nature of the retinal-opsin linkage.

1. Convenient methods for the preparation of tritiated 11-cis-retinol, 11-cis-retinal and rhodopsin are described. Irradiation of labelled rhodopsin in the presence of sodium borohydride resulted in the irreversible binding of the retinyl moiety to the active site. Degradative studies established that the retinyl moiety in this reduced derivative of rhodopsin was attached to the in-amino group of lysine. In connexion with this investigation the synthesis of a number of N-retinylidene- and N-retinyl-amino acids was achieved. Derivatives of lysine with the retinyl moiety attached either to the alpha-amino group or to the in-amino group were also synthesized and characterized. 2. It is suggested that the involvement of a charge-transfer interaction between the retinylidene chromophore and a suitable group -X or -X.H on the opsin best explains the spectroscopic properties of rhodopsin and other visual proteins.