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A T Tsin

Publications and source records attributed to A T Tsin.

At least 37 records · Page 2Linked to original sources
PubMed1990

Vitamin A metabolism in the baboon eye.

The vitamin A metabolism of eight baboon (Papio cynocephalus anubis) eyes were examined. High performance liquid chromatography was utilized for quantitative analysis of retinyl esters and retinol in four eyes. Retinyl ester levels in pigment epithelium (PE) were greater (3043 pmol/eye) than in neuroretina (1084 pmol/eye). However, the reverse was true for retinol levels (neuroretina = 4610 pmol/eye; PE = 1451 pmol/eye). Neuroretina and PE of four remaining eyes were analyzed for retinyl ester hydrolase (REH) activity. Specific activity of 11-cis REH in neuroretina was 2.14 Units/mg while that of PE was 1.11 Units/mg (1 Unit of activity = 1 pmol of product formed/minute). In contrast, all-trans REH activity was greater in PE (1.78 Units/mg) than in neuroretina (0.41 Units/mg). Because the visual cells, vitamin A distribution and metabolism in the baboon eye are similar to those found in human ocular tissues, the baboon eye serves as an important primate model of the human visual system.

Animals↗
PubMed1989

Retinoids in the bovine pineal gland.

Using high performance liquid chromatography, the level of retinoids was determined in the bovine pineal gland, retina, retinal pigment epithelium, cortical and subcortical brain tissues, skeletal muscle and the liver. Except the retina, the bovine pineal gland possesses levels of retinol and retinyl esters significantly higher than other brain tissues and muscle. However, unlike the retina, the bovine pineal gland does not possess a significant level of retinal, the chromophore for visual pigments in the retina. This finding suggests that the bovine pineal gland does not possess rhodopsin nor the property of phototransduction which has been fully established in the retina and pineal glands of lower vertebrates.

Animals↗
PubMed1989

Retinyl esters in the vertebrate neuroretina.

High-performance liquid chromatography (HPLC) was employed to measure retinyl esters in the vertebrate retina. Both retina and retinal pigment epithelium (RPE) from frog, chicken, and bovine eyes were studied. In comparison to the RPE, the retina possessed a significant level of 11-cis and all trans retinyl palmitate. Using a sensitive radioassay, we also detected the presence of retinyl ester hydrolase (REH) activity in homogenates prepared from both retina and RPE. The rate of retinyl ester hydrolysis in these retinas was sufficiently high to supply retinal chromophores for the metabolic renewal and for the regeneration of visual pigments. In comparison to retinyl esters in the RPE, retinyl esters in the retina are located much closer to the sites of visual pigment synthesis and regeneration. Hence it is possible that these retinyl esters play a more important role in the visual cycle than those in the RPE.

Animals↗
PubMed1989

Selective utilization of serum vitamin A for visual pigment synthesis.

Two groups of goldfish (Carassius auratus) were subjected to light and temperature conditions known to promote a contrast in their scotopic visual pigment compositions. After 3 weeks, the porphyropsin/rhodopsin ratio in the neuroretina of these goldfish ranged from 99% porphyropsin in one group to 59% in the other. Samples of blood, liver and retinal pigment epithelium (RPE) were also removed from these animals and analysed by high-performance liquid chromatography (HPLC) for vitamin A composition. There was consistently more vitamin A2 than vitamin A1 (over 50% vitamin A2) in both vitamin A alcohol and vitamin A esters extracted from the liver and the RPE. In contrast, only 30% of all vitamin A extracted from the blood was vitamin A2. These observations suggest that it is mainly vitamin A1 that is transported in the blood, whereas vitamin A2 is selectively retained in the liver and in the RPE and used to form porphyropsin in the eye.

Animals↗
PubMed1988

The fluorescence quantum yield of vitamin A2.

The fluorescence quantum yield of all-trans 3,4-didehydroretinol (vitamin A2) was measured in hexane at room temperature, using quinine sulfate as a standard. Unlike all-trans retinol (vitamin A1) which possessed a relative quantum yield of 0.0298, 3,4-didehydroretinol was 37 times lower in fluorescence (i.e. 0.0008). In addition, a significant bathochromic shift (both excitation and emission maxima) and a general broadening of the fluorescence spectra were noted for 3,4-didehydroretinol. This information is important not only for the understanding of the basic structure of vitamin A but also the photochemistry of vision.

Chromatography, High Pressure Liquid↗
PubMed1987

Visual pigments and retinoids in the Mongolian jird.

The visual cells, visual pigments and major retinoids of the Mongolian jird (Meriones unguiculatus) were examined. Light and electron microscope analyses show that these jirds had mainly rod photoreceptors. Octylglucoside extracts prepared from their retinas contained only rhodopsin with a maximum absorption at 497 nm and a concentration of 0.51 nmol per retina. Employing a standard method of high performance liquid chromatography (HPLC), the pigment epithelium from each eye was found to possess 0.52 nmol of retinyl palmitate (the most abundant form of retinyl ester) along with a small amount of retinol (0.02 nmol). Most of the retinoids in the body of these animals are stored in the liver, in the form of retinyl palmitate (1228.80 nmol per gram liver). As the Mongolian jird is small, inexpensive and readily available, this animal is a mammalian species suitable for the research of the biochemistry of retinoids and vision.

Animals↗
PubMed1987

Pineal and retinal lysosomal enzyme rhythms.

Pineal glands and the neurosensory portion of the retinae of adult male rats adapted to a 24-h cycle with lights on from 06.00 to 20.00 h were collected at 9 timepoints during the cycle. Significant rhythms in both pineal and retinal hexosaminidase, beta-glucuronidase, acid phosphatase and beta-glucosidase were observed. For each enzyme, pineals had greater overall activity per unit amount of protein than did retinae. All 4 significantly rhythmic pineal enzymes peaked within 30 min of each other (18.30-19.00 h) whereas the retinal enzymes peaked some 6 h earlier, between 11.30 and 13.45 h. To our knowledge, this is the first report demonstrating 24-h rhythms in lysosomal enzymes in the pineal gland and retina. Since the acrophases (peak times) of these enzymes within each tissue are tightly synchronized yet different between tissues, lysosomes may play unique roles in the physiology of different structures in the photoneuroendocrine system with respect to time in the light-dark cycle.

Acid Phosphatase↗
PubMed1986

Decreased hepatic retinyl palmitate hydrolase activity in protein-deficient rats.

28-day-old weanling rats were fed a diet containing 3% casein as the only source of protein for eight weeks to induce protein deficiency. When compared to control animals (fed a diet containing 25% casein), these rats had significantly lowered body (5.2-fold reduction) and liver (2.5-fold reduction) weights. The circulatory level of retinol (nmol per ml plasma) as well as retinol (nmol per g tissue) in the liver of these protein-deficient animals were also reduced significantly, although their liver concentration of retinyl palmitate (nmol per g tissue) was comparable to that of the control group. Assay of liver tissue for retinyl palmitate hydrolase activity revealed a 4-fold reduction (compared to that of control animals) of specific enzyme activity (nmol retinol formed per g protein per h). These findings suggest that severe protein deficiency results in a decreased hydrolysis of retinyl esters in the liver, which may be in part responsible for the reduced level of metabolically 'active' retinoids available for normal physiological functions.

Animals↗
PubMed1986

Retinyl palmitate hydrolase activity in the bovine retina.

Retinyl palmitate hydrolase (RPH) activity of bovine tissues was estimated from retinol formation following incubation of tissue homogenates with all-trans retinyl palmitate. The quantity of retinol produced in the incubation mixture was analyzed by high-performance liquid chromatography. RPH activities of retinal pigment epithelium (RPE), liver, retina, muscle and brain were 194.2, 138.0, 72.5, 25.0 and 5.1 units/gm protein respectively. The RPH activity in the retina was far above that attributable solely to RPE contaminations. The presence of RPH in the retina suggests that retina can utilize retinyl esters for the formation of visual pigments and/or cellular metabolism.

Animals↗
PubMed1986

Enzymatic esterification of exogenous retinol and 3,4-didehydroretinol in the retinal pigment epithelium.

The kinetics of esterification of exogenous retinol by cell membranes prepared from the crude homogenate of the frog retinal pigment epithelium was studied. The formation of retinyl palmitate from added retinol was directly assayed by high performance liquid chromatography (HPLC). A linear relationship was observed between the amount of protein (up to 2 mg) in the incubation medium and the amount of retinyl palmitate formed. At room temperature, this reaction took less than 2 hours to complete. By varying the substrate concentration in the incubation medium, the reciprocal of initial velocity of the reaction (nmol retinyl palmitate formed per hour) was plotted against the reciprocal of substrate concentration (nmol of retinol). This double-reciprocal plot shows that the apparent Km of the reaction was 10 microM with an apparent Vmax of 9.1 nmol of retinyl palmitate per hour per mg protein. When this assay was repeated in the presence of 3,4-didehydroretinol (20 microM), the kinetics of the reaction showed the pattern of that of a competitive inhibitor, suggesting that 3,4-didehydroretinol competes with retinol for the same active site for esterification. The esterification of 3,4-didehydroretinol resulted in the formation of 3,4-didehydroretinyl palmitate, which was also measured by HPLC. The amount of 3,4-didehydroretinyl palmitate formed by this reaction decreased in proportion to increased retinol concentration in the incubation mixture. This further confirms that a competition exists between the esterification of retinol and 3,4-didehydroretinol by retinal pigment epithelium of the frog.

Animals↗
PubMed1986

Effects of glycosylation inhibitors on the frog retina.

Effects of the intraocular injection of three inhibitors of glycosylation (tunicamycin, castanospermine, and swainsonine) on the rhodopsin content and the integrity of disc membranes in frog retina were studied. The administration of 10 or 100 micrograms of tunicamycin resulted in a 78% loss of rhodopsin in the frog retina which also exhibited a significant reduction in the length of photoreceptor outer segments (as examined under light microscope). This suggests that the synthesis and/or insertion of rhodopsin into the disc membrane is inhibited by tunicamycin. In contrast, injections of up to 250 micrograms of castanospermine and swainsonine resulted in neither a decrease in rhodopsin content nor a change in the length of photoreceptor outer segments. Examination of retinal tissue homogenates for alpha Mannosidase and alpha-Glucosidase activities revealed homogenates for alpha Mannosidase and alpha-Glucosidase activities revealed these hydrolase activities to be significantly decreased (70%). We suggest the possibility that specific oligosaccharide processing reactions may not be required for the insertion of rhodopsin and subsequent assembly of disc membranes in frog photoreceptors.

Alkaloids↗
PubMed1986

The in vivo regeneration of goldfish rhodopsin and porphyropsin.

Goldfish with retinas rich in either rhodopsin or porphyropsin were illuminated with bright light and then placed in the dark room to allow visual pigment regeneration. The kinetics of this in vivo pigment regeneration were followed by sampling these animals at regular time intervals. The first-order kinetic rate constant for the initial period of porphyropsin regeneration at 20 degrees C was 8.3 X 10(-3) nmol kg-1 body weight min-1 and the half-life of this reaction was 83 min. At 30 degrees C, the rate constant was increased to 1.4 X 10(-2) nmol kg-1 body weight min-1, yielding a reduced half-life of 49 min. This suggests that the Q10 of porphyropsin regeneration is about 1.7. In goldfish retinas enriched with rhodopsin (62% rhodopsin and 38% porphyropsin), the initial phase of visual pigment regeneration (at 30 degrees C) proceeded at a slower rate (first-order rate constant: 6.5 X 10(-3) nmol kg-1 body weight min-1; half-life of reaction = 106 min) than the rate of porphyropsin regeneration. This suggests that the high proportion of rhodopsin in the retina of goldfish held at 30 degrees C is not a direct result of a faster rate of regeneration of rhodopsin than of porphyropsin.

Adaptation, Ocular↗
PubMed1986

Distribution of ascorbate in the retina, subretinal fluid and pigment epithelium.

The posterior segment of the eye was divided into four compartments: retinal cytosol (R), subretinal fluid on the retinal surface (S/R), retinal pigment epithelial (RPE) cytosol, and subretinal fluid on the RPE surface (S/RPE). The volume of each compartment was estimated from the dilution of creatinine (in the extraction buffer) by the endogenous tissue fluid. The ascorbate concentrations in R, S/R, S/RPE, and RPE were 20.6, 12.3, 3.7, and 5.8 mg/dl respectively. Dehydroascorbate was observed only in the RPE and S/RPE. The decreasing ascorbate concentration from the retina to RPE, and the distribution of dehydroascorbate suggest a movement of ascorbate from the vitreous cavity into the subretinal space. The permeability of retinal cell layers to ascorbate was confirmed by the high radioactivity observed in the subretinal space after an intravitreal injection of C14-ascorbate. The occurrence of dehydroascorbate in the RPE and the S/RPE indicates the presence of oxidative reaction of ascorbate in these compartments, where light induced free radicals are located.

Animals↗
PubMed1986

Interstitial retinol-binding protein (IRBP) in subretinal fluid.

Antibodies against bovine interstitial retinol-binding protein (b-IRBP) were used to detect human IRBP (h-IRBP) on immunoblots of eight samples of subretinal fluid (SRF) from patients with retinal detachments of between 2 days' and more than 2 years' duration. Using this sensitive technique, it was found that seven of the samples contained h-IRBP in concentrations estimated to range from below 5% up to 19% of normal human IPM. One of these samples displayed two immunoreactive bands of roughly equal intensity, one at a molecular weight of 135,000 (h-IRBP), the other at 115,000. The latter may have been generated by proteolytic cleavage. No h-IRBP could be detected in an eighth sample from a patient with retrolental fibroplasia. It is concluded that the reduced concentration of h-IRBP in SRF may be due to a number of factors that include dilution, proteolytic degradation, and metabolic inactivation of photoreceptors at the detachment site.

Adolescent↗
PubMed1985

The 3, 4-didehydroretinal chromophore of goldfish porphyropsin.

The isomeric configuration of the 3,4-didehydroretinal chromophore of goldfish porphyropsin was determined by high performance liquid chromatography (HPLC) and by the regeneration of this visual pigment with authentic isomers of 3,4-didehydroretinal. A nonisomerizing, quantitative method using hydroxylamine and methylene chloride was employed to extract the 3,4-didehyroretinal chromophore from the rod outer segment membrane (containing the porphyropsin). When this extracted chromophore was injected into the HPLC, only a single major peak was observed and this peak coeluted with the authentic 11-cis 3,4-didehydroretinyl oxime. This suggests that the chromophore of goldfish porphyropsin is 11-cis 3,4-didehydroretinal. When the bleached rod outer segments (containing the opsin) were incubated with different 3,4-didehydroretinal isomers (13-cis, 11-cis, 9-cis, and all-trans), only the 11-cis isomer resulted in the degeneration of porphyropsin. This also suggests that the porphyropsin chromophore exists in the 11-cis configuration.

Animals↗
PubMed1985

Conversion of retinol to 3,4-didehydroretinol in the tadpole.

The conversion of retinol to 3,4-didehydroretinol in bullfrog tadpoles was studied by injecting [3H] all-trans retinol into the peritoneal cavity. The specific activities of retinoids in the eye and the rest of the body at various time intervals after the injection were then determined by HPLC (high-performance liquid chromatography). Radioactivity was observed in ocular 3,4-didehydroretinyl esters after 2 days and their specific activity increased throughout the 2 weeks of experiment. This demonstrates that tadpoles can convert retinol to its 3,4-didehydro derivative. In vitro experiments performed on isolated eye cups also suggested that the ocular tissues could convert retinol to 3,4-didehydroretinol. In the eye, the specific activity of porphyropsin or all-trans 3,4-didehydroretinal (extracted by the denaturing solvent acetone) exceeded that of the all-trans 3,4-didehydroretinyl esters in storage. This suggests that the main ocular store of 3,4-didehydroretinyl esters does not constitute a precursor pool for porphyropsin synthesis.

Animals↗