[Retinoids]

Retinoid includes vitamins A and synthetic relatives except provitamins A such as carotenes. In this review, recent developments on the chemistry and physiological functions of retinoid (retinol, retinal and retinoic acid etc.) were surveyed, special emphasis being laid on the following subjects: (1) vision (retinal and photo-sensitivities; molecular mechanism of scotopic and photopic vision). (2) systemic actions of vitamins A (multiple physiological functions and their active intermediates). (3) reproduction and morphogenesis (extracellular-transport proteins, intracellular-cytoplasmic transfer proteins, and intracellular-nuclear transacting factors; RAR and RXR; morphogen). (4) chemical prevention of cancer (synthetic retinoid derived from retinoic acid). (5) biological significance of cis-retinoid.

Optical resolution of (+/-)-all-trans-3-hydroxyretinal by use of high-performance liquid chromatography

Racemic all-trans-3-hydroxyretinal (3-OH-RAL) (1) was converted by a reaction with (-)-camphanic acid chloride (CpCl) into a diastereomixture of camphanates (2a) and (2b) which was separated by preparative high-performance liquid chromatography (HPLC) to give two esters (2a) and (2b) in pure state, respectively. Saponification of (2a) and (2b) independently afforded optically active (3S)- and (3R)-3-OH-RALs (3a) and (3b), respectively, whose absolute structures were determined by circular dichroism (CD) spectra. Racemic 3-OH-RAL was separated to two peaks by HPLC using chiral column (ChiraSpher, Merck). Cochromatography with authentic specimens (3a) and (3b) showed that the peak with a short retention time corresponded to (3R)-isomer and the other to (3S).

Mechanism of isomerization of rhodopsin studied by use of 11-cis-locked rhodopsin analogues excited with a picosecond laser pulse

Primary photochemical behaviors of cattle rhodopsin analogues (Rh5 and Rh7) having cyclopenta- and cycloheptatrienylidene 11-cis-locked retinals (Ret5 and Ret7, respectively) were studied by excitation with a picosecond laser pulse (wavelength 532 nm; duration 21 ps). Picosecond absorption and fluorescence measurements of Rh5 showed formation of only a long-lived excited singlet state (tau l/e = 85 ps). The excited state of the retinal analogue having a five-membered ring was stabilized in protein (Rh5) more than in solvent (protonated Schiff base of Ret5; PSB5). Excitation of Rh7 produced two ground-state photoproducts, Rh7 (580) and Rh7 (630). According to the analysis of photon density dependency, Rh7 (580) was a single-photon product of Rh7, while Rh7 (630) was the photoproduct of Rh7 (580). Fluorescence emitted from a seven-membered ring system like Rh7 or a protonated Schiff base of Ret7 (PSB7) was weaker than that in a corresponding five-membered ring system, especially in protein (Rh7). The difference in photoreaction between Rh5 and Rh7 may originate from the difference in fixation of the 11-cis form. On the basis of the spectral and kinetic similarities between Rh7 (580) and photorhodopsin, a precursor of bathorhodopsin, it was proposed that both have twisted all-trans chromophores in the way of the isomerization. The protein moiety of rhodopsin which fixes the chromophore at both ends seems to accelerate the rotation of the C11-C12 double bond and to prevent it from going through relaxation processes other than the isomerization. This may be a plausible reason why rhodopsin has a large quantum yield (0.67).

A fly, Drosophila melanogaster, forms 11-cis 3-hydroxyretinal in the dark

All-trans and 11-cis 3-hydroxyretinals were synthesized and the presence of these substances in the head of Drosophila melanogaster was shown by using high performance liquid chromatography. Even when the head extract was prepared in the dark from the flies reared successively in the dark, both of the 3-hydroxyretinal isomers were detected. In the culture medium, they were not present. D. melanogaster must have an 11-cis 3-hydroxyretinal forming-system that does not need light.

Studies on structure and function of rhodopsin by use of cyclopentatrienylidene 11-cis-locked-rhodopsin

The photochemical reaction of cyclopentatrienylidene 11-cis-locked-rhodopsin derived from cyclopentatrienylidene 11-cis-locked-retinal and cattle opsin was spectrophotometrically studied. The difference absorption spectrum between the cyclopentatrienylidene 11-cis-locked-rhodopsin and its retinal oxime had its maximum at 495 nm (P-495). Irradiation of P-495 at -196 degrees C with either blue light or orange light caused no spectral change, supporting the cis-trans isomerization hypothesis for formation of bathorhodopsin. Upon irradiation of P-495 at 0 degree C with orange light, however, its absorption spectrum shifted to a shorter wavelength owing to formation of a hypsochromic product. The difference absorption spectrum between this product (P-466) and its retinal oxime showed its maximum at 466 nm. Analysis of retinal isomers by high-performance liquid chromatography showed that this spectral shift was not accompanied by photoisomerization of the chromophore. P-466 could almost completely be photoconverted to the original pigment (P-495) by irradiation at 0 degree C with blue light with little formation of the other isomeric form of its chromophore. The alpha-band of the circular dichroism spectrum of P-495 was very small in comparison with that of rhodopsin, while that of P-466 was comparable to it. These facts suggest that P-495 has a planar conformation in the side chain of the chromophore and that P-466 has a twisted one, probably at the C8-C9 single bond. Cyclic-GMP phosphodiesterase in frog rod outer segment was activated by neither P-495 nor P-466. This result suggests that the isomerization of the retinylidene chromophore of rhodopsin is indispensable in the phototransduction process.

Cytotoxic action of retinoidal butenolides on mouse neuroblastoma and rat glioma cells

Proliferation and death were measured in cultures of mouse neuroblastoma N18TG -2 and rat glioma C6BU -1 cells when treated with up to 100 micron retinoidal butenolides (RB 1-6). The number of viable cells in each case was measured with various concentrations of the compounds, of which RB-3 (5-hydroxy-4-[2-(2,6,6,-trimethyl-l- cyclohex en-l-yl) ethenyl ]-2(5H)- furanone ) was the most potent in destroying the cells after 2 days' incubation. ED50 of RB-3 was about 5 X 10(-7) M for both types of cell. RB-3 was 80 times more potent than retinoic acid. Ten analogues of RB-3 had a similar inhibitory effect on DNA synthesis in N18TG -2 cells. The degenerative changes caused by RB-3 in C6BU -l cells were irreversible even when the cells were exposed to it for 2 h. Tumor weights of N18TG -2 cells that had been inoculated subcutaneously onto the backs of A/J mice were 30-40% lower than those of untreated controls after 14 days of single daily i.p. injections of RB-3 doses of 100 mg/kg of body weight. The results indicate that RB-3 is cytotoxic in murine tumor cells originating from the nervous system and has an inhibitory effect on neuroblastoma-tumor growth in mice.

Thermal stereoisomerization of all-(E)-beta-carotene: (Z)-beta-carotenes and electrocyclized beta-carotenes

Thermal isomerization of synthetic all-(E)-beta-carotene yielded a multi-component mixture composed of more than sixteen (Z)-beta-carotenes and two thermo-cyclized isomers. They were separated and purified by high-performance liquid chromatography. Ten of these were characterized by spectroscopic methods as four mono-(Z)-, five di-(Z)-, and one tri-(Z)-beta-carotenes; the 7-(Z)-, 9-(Z)-, 13-(Z)-, 15-(Z)-, 9.13-(Z)-, 9.15-(Z)-, 9.13'-(Z)-, 13.15-(Z)-, 7.13'-(Z)-, and 9.9'.13-(Z)-isomers were included. Stereochemistry of six additional (Z)-isomers obtained is, as yet, unknown, though one of them might have the 9.9'-(Z)-geometry in the molecule. This is the first report on the formation and identification of the 'sterically hindered' 7-(Z)-isomers as well as the (5 leads to 10)-thermo-cyclized products in the field of alicyclic carotenoids.

Spectral analysis and high-performance liquid chromatography of the cis-trans geometrical isomers of 5,6-Epoxyretinal

Spectral characterization of 5,6-epoxyretinal isomers (II) prepared by a regioselective epoxidation of the parent retinal isomers have been described. High-performance liquid chromatographic separation of epoxyretinals and a photoisomerization behavior of all-trans-5,6-epoxyretinals (IIa) have been investigated.

Activation of phosphodiesterase in frog rod outer segment by rhodopsin analogues

Activation of guanosine 3',5'-cyclic monophosphate (cGMP) phosphodiesterase (EC 3.1.4.35) in frog rod outer segment membrane by rhodopsin analogues has been investigated. A rhodopsin analogue modified at the Schiff-base linkage (N-retinyl-opsin) or the beta-ionone ring (3-dehydro-rhodopsin) in the retinylidene chromophore of rhodopsin has some ability in activation of the enzyme. In consideration of our previous observation that opsin including a retinal-oxime can activate the enzyme, it seems likely that the Schiff-base linkage is not always necessary for the phosphodiesterase activation. On the other hand, a change in the length of the side chain of retinal (complex of opsin and beta-ionone, beta-ionylideneacetaldehyde or retinylideneacetaldehyde) or dissection of the conjugate double-bond system of the side chain (retro-gamma-rhodopsin) remarkably reduces the activation ability. However, 5,8-epoxy-rhodopsin having a similar dissected conjugate double-bond system induces some enzyme activation because of its rigid conformation around C7-C8-C9 single bonds. Consequently, it is suggested that the necessary portion of rhodopsin chromophore for the activation of the enzyme is the rigid conjugate double-bond system between the beta-ionone ring and the Schiff-base linkage in its all-trans form.

Phosphorylated polyenols of biochemical interest: cis=trans stereoisomeric retinylphosphates

All-trans biscyclohexylammonium retinylphosphate was synthesized using bis (triethylammonium) phosphate as the phosphate donor. It was shown to be pure from physico-chemical evidence including ultraviolet, infrared, 1H- and 13C-nuclear magnetic resonance and mass spectrometry. In contrast to previous reports, the synthesized product is quite stable toward alkaline hydrolysis. Direct or iodine-catalyzed photoisomerization of the all-trans compound can produce the corresponding 13-cis isomer. Simultaneous quantification of cis-trans stereoisomeric retinylphosphates was demonstrated by ion-pairing reversed-phase liquid chromatography with an isocratic mobile phase consisting of aqueous methanol and with ultraviolet detection at 300 nm. The mobile phase also contains tetrabutylammoniumphosphate as the counter-ion. The feasibility and extent of stereoisomerization of all-trans retinylphosphate have been compared with those of carotenoidal and retinoidal polyenes.

Structural elucidation of the main cis beta-carotenes

Different isomerization experiments on all trans-beta-carotene (I) yielded multi-component mixtures of cis isomers. Separation and isolation of these isomers were achieved by classic adsorption chromatography on a lime column. Investigation of their 200 MHz 1H- and 50.3 MHz 13C-NMR spectra led to the structural elucidation of the main cis-beta-carotenes, namely the 9-monocis structures (II) for neo beta-carotene U, and the 13-monocis structure (III) for neo beta-carotene B. Formation of other hindered monocis (7-cis and 11-cis) and 15-monocis isomers should be carefully examined by a suitable method such as high-performance liquid chromatography.

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.

Mechanism for interference of hydrochlorothiazide with the Kober reaction for determination of urinary estriol in pregnancy

Investigation of the mechanism of the lowering effect of hydrochlorothiazide, which was administered orally, in the determination of estriol from the pregnancy urine has been carried out. The isolation and determination of the structure of a copolymer formed from estriol and formaldehyde have suggested the mechanism by which, during the acid hydrolysis process of the glucuronide conjugate of estriol, some copolymers of estriol and formaldehyde generated from hydrochlorothiazide, were formed, thus lowering the observed values of the estriol contents in the urine.

Electrocyclization reaction of higher conjugated polyenals: photochemical behaviors of retinal (vitamin A1 aldehyde) homologues

During the studies on a photoreaction of retinal, involving various kinds of (Z)-(E) isomerization, a heretofore unknown photoproduct of retinal was isolated in a pure state and was characterized unambiguously. Thus, direct irradiation of all-(E)-retinal (I) and of all-(E)-beta-ionylidenecrotonaldehyde (II) in acetonitrile solution gave the corresponding 6e-electrocyclized photoproducts, (III) and (IV), both via the possible 7-(Z)-isomer intermediates of the parent conjugated polyenals. Unlike the lower members in the retinal series, it was also confirmed that sigmatropic rearrangement or photo-Diels-Alder reaction hardly proceeds in these higher members of the series mentioned above.

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.

Simultaneous determination of eight vitamin D2 isomers by proton magnetic resonance spectroscopy

Simultaneous determination of vitamin D2 and its isomers, with the aid of pmr employing a paramagnetic shift reagent Eu (dpm)3, is presented. Thus, a mixture of vitamin D2 isomers is acetylated and its pmr spectra are run with and without the addition of the shift reagent. Relative paramagnetic shift values (gamma rel) of the acetoxyl protons of the acetylated isomers are estimated for solutions in CCl4 against the reference acetoxyl signal of vitamin D2 acetate. Estimated values are as follows: vitamin D2 acetate = 100, 5,6-trans-vitamin D2 acetate = 71 +/- 1, ergosteryl acetate = 86 +/- 1, lumisteryl2 acetate = 123 +/- 3, isovitamin D2 acetate = 82, isotachysteryl2 acetate = 97, tachysteryl2 acetate = 100 (105 +/- 1 in C6D6), and previtamin D2 acetate = 80 +/- 2. On the basis of a dramatic shift of an acetoxyl signal and a constancy of each gamma rel, this method permits immediate identification of the isomers and provides valuable information on their stereochemistry. This technique also facilitates a simultaneous determination of these isomers in a mixture by estimating each signal area.

Simultaneous determination of eight vitamin D2 isomers by high-speed liquid chromatography

The simultaneous determination of vitamin D2 and its isomers by high-speed liquid chromatography (HSLC) is described. Preferred operating conditions for the resolution and quantitation of vitamin D2 5,6-trans-vitamin D2, ergosterol, lumisterol2, isovitamin D2, isotachysterol2, tachysterol2, and previtamin D2 are as follows: column, "Zorbax" SIL (25 cm x 2.1 mm i.d.); pressure, 100-120 kg/cm2; temperature, ambient; detector, UV 254 nm; mobile phase, 0.15% methanol + 2% ether in pentane (for the simultaneous determination), 10% ether in hexane (for quantitation of the practical sets of isomers), or 55% CHCl3 (distilled) in pentane (for separation of isovitamin D2 and 5,6-trans-vitamin D2); sample size, 1 mul (greater than 1-5 ng); internal standard, p-cresol or alpha-naphthol. High-speed, good resolution, precise and accurate trace analysis, greater analysis flexibility, no necessity for preparing derivatives, and gentle operating conditions are main potential advantages of our method which has proved to be very efficient and surpasses all analytical procedures hitherto proposed.

Carbon-13 nuclear magnetic resonance spectra of vitamins D and related compounds

1. The natural abundance carbon-13 nmr of vitamins D (D2 and D3) and several isomers (5, 6-trans-vitamin D2, isotachysterol2 and isovitamin D2) have been completely assigned by employing off-resonance noise-decoupling, acetylation shifts, and lanthanide-induced shifts experiments. The last two techniques were especially useful for the present study. 2. Carbon-13 nmr spectral characteristics of the three main conjugated triene moieties (SE-Z-SZ, SE-E-SZ, or SE-E-SE), involved in the molecules of vitamin D and its isomers, were revealed. Thus, the striking dependence of the shieldings on molecular geometries and high sensitivity of the resonances to the environments of conjugated systems were surveyed. 3. Conformational preferences in solutions of the hydroxyl groups in vitamins D2 and D3 as well as 5, 6-trans-vitamin D2 were conveniently determined.