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A T Tsin

Publications and source records attributed to A T Tsin.

At least 19 recordsLinked to original sources
PubMed2000

Substrate specificities and 13-cis-retinoic acid inhibition of human, mouse and bovine cis-retinol dehydrogenases.

Recent studies of the human, mouse and bovine genes for 11-cis-retinol dehydrogenase (11cRDH) and human and mouse 9-cis-retinol dehydrogenase (9cRDH) suggest that they are homologs of the same enzyme. This conclusion is inconsistent with earlier literature indicating that 11cRDH is expressed solely in the eye and does not utilize 9-cis-retinol as a substrate. We have compared directly the kinetic properties of recombinant human and mouse 9cRDH with those of bovine 11cRDH for 9-cis- and 11-cis-retinol and investigated the inhibitory properties of 13-cis-retinoic acid on each of these enzymes. Human and mouse 9cRDH and bovine 11cRDH have very similar kinetic properties towards 9-cis- and 11-cis-retinol oxidation and they respond identically to 13-cis-retinoic acid inhibition. Our biochemical data are consistent with the conclusion that 9cRDH and 11cRDH are the same enzyme.

Alcohol Oxidoreductases↗
PubMed2000

The metabolic availability of vitamin A is decreased at the onset of diabetes in BB rats.

Streptozotocin (STZ)-induced diabetic rats have been associated with an impaired metabolic availability of vitamin A (retinol). This study was undertaken to investigate whether Biobreeding (BB) rats, in which diabetes mellitus resembling human type I diabetes develops spontaneously, respond the same way at the onset of diabetes. Weaning diabetes-prone (BBdp) and normal (BBn) BB rats consumed NIH-07 nonpurified diet ad libitum until 120 d of age. Plasma and hepatic concentrations of retinol and its carriers, retinol-binding protein (RBP) and transthyretin (TTR) were lower in diabetic BB (BBd) rats than in BBn rats. In parallel with RBP, the abundance of mRNA was lower in the liver of BBd rats. Furthermore, the status of zinc, an important factor for the synthesis of RBP, was also disturbed in BBd rats, as indicated by lower circulatory levels and greater urinary excretion. To determine whether the biochemical evidence of vitamin A deficiency in BBd rats could be reversed, BBdp rats were fed a diet supplemented with vitamin A either alone or in combination with zinc. None of these treatments increased plasma vitamin A concentration. The hepatic abundance of RBP mRNA was significantly greater, whereas circulatory RBP concentrations were unaffected by vitamin A plus zinc supplementation. Overall, these results suggest that impaired metabolic availability of vitamin A, possibly caused by its decreased transport from hepatic stores, is another metabolic derangement associated with type I diabetes.

Animals↗
PubMed1998

Distribution of 11-cis LRAT, 11-cis RD and 11-cis REH in bovine retinal pigment epithelium membranes.

Our recent finding of the co-localization of 11-cis retinyl esters and 11-cis retinyl ester hydrolase (11-cis REH) activity in bovine retinal pigment epithelium (RPE) plasma membrane (PM) has led us to explore the possibility that the PM may provide 11-cis retinal for rhodopsin regeneration. In the RPE, visual chromophore is synthesized via a membrane associated 11-cis retinol dehydrogenase (11-cis RD). Accordingly, bovine RPE membranes enriched with either endoplasmic reticulum (ER) or plasma membrane (PM) enzyme markers were prepared and assayed for visual cycle enzyme activities. Pronounced 11-cis RD activity was associated with both ER- and PM-enriched membrane fractions. In contrast, 11-cis REH activity was mostly recovered in PM-enriched fractions while LRAT activity was found only in ER-enriched membranes. The finding that both 11-cis retinol and 11-cis retinal can be produced at the PM of the bovine RPE strongly suggests that 11-cis retinyl esters at this subcellular locale serve as a precursor of visual chromophore for pigment regeneration.

Alcohol Oxidoreductases↗
PubMed1998

Analysis of nutritional components of eight famine foods of the Republic of Niger.

In the western Sahel, indigenous plants become important staples when cereal harvests are inadequate to support populations inhabiting that region of Africa. The purpose of this study was to assess the nutrient content of several of these edible wild plants. The leaves of the following seven plant foods were analyzed: Ziziphus mauritiana, Cerathotheca sesamoides, Moringa oleifera, Leptadenia hastata, Hibiscus sabdarifa, Amaranthus viridis, and Adansonia digitata. The fatty acid, vitamin E, carotenoid, selected mineral and amino acid contents of these plant foods were determined. These same analyses were performed on the fruit of the Adanisonia digitata. In quantitative and qualitative terms, Amaranthus viridis was found to be an excellent source of protein. Its amino acid composition compared favorably to that of a World Health Organization (WHO) protein standard. It also contained considerable amounts of the two fatty acids that are essential in humans (linoleic and alpha-linolenic) and a number of minerals including iron, magnesium. calcium and zinc. The leaves of Hibiscus sabdarifa contained an appreciable quantity of protein the composition of which was comparable to the WHO standard. The mineral content of the leaves of this plant was also exceptionally high; noteworthy was its high zinc content. H. sabdarifa also contained significant quantities of the two essential fatty acids. Ziziphus mauritiana was an excellent source of the essential fatty acid linoleic acid and several of the metals including iron, calcium, magnesium and zinc. Its content of other essential nutrients, however, was rather low. In general, Adansonia digitata leaves were nutritionally superior to the fruit of the tree; however, the fruit did contain useful quantities of potassium, phosphorus, zinc and alpha-linolenic acid. The Leptadenia hastata leaves were an especially good source of lutein and beta-carotene. These data should be useful to the people who inhabit the western Sahel in helping them devise healthy diets during times when cereal staples are in short supply.

Amino Acids↗
PubMed1998

Substrate specificity of retinyl ester hydrolase activity in retinal pigment epithelium.

In the eye, hydrolysis of stored retinyl esters is catalyzed by retinyl ester hydrolase (REH) activities in retinal pigment epithelium (RPE) membranes. In the present study, biochemical analyses were conducted to determine the substrate specificity of these activities. Specific activities determined for hydrolysis of various retinol isomers of retinyl palmitate (9-cis-, 11-cis-, 13-cis-, and all-trans-retinyl palmitates) indicated that 11-cis-retinyl palmitate is preferentially hydrolyzed (1.7 nmol/min/mg) compared to the other isomers (0.1-0.3 nmol/min/mg). Examination of the specificity of REH activity for 11-cis-retinyl esters of varied acyl chain length (-myristate, -palmitate, and -stearate) and degree of saturation (-oleate and -linoleate) further demonstrated that palmitate is the preferred fatty acyl moiety. Notably, retinyl esters possessing chain lengths which more closely approximate that of the palmitate ester exhibited higher rates of hydrolysis. Similar results were obtained in retinyl ester-plasma membrane fusion studies in which hydrolysis took place within the membrane domain rather than at the lipid-water interface. REH substrate specificity was further assessed in competition studies in which 11-cis-retinyl palmitate hydrolysis was monitored in the presence of 13-cis-, 9-cis-, or all-trans-retinyl palmitate. Results show that addition of these retinyl palmitate isomers does not affect the rate of hydrolysis of 11-cis-retinyl palmitate. However, the hydrolytic rates associated with other retinyl palmitate isomers were significantly reduced in the presence of 11-cis-retinyl palmitate. Finally, cholesterol ester hydrolase activity was found to be distinct from the observed 11-cis-REH activity and the presence of cholesterol oleate did not affect the rate of 11-cis-retinyl palmitate hydrolysis. Collectively, these data support the hypothesis that a distinct, membrane-associated, 11-cis-retinyl palmitate-specific retinyl ester hydrolase activity exists in the retinal pigment epithelium.

Animals↗
PubMed1998

Colocalization of 11-cis retinyl esters and retinyl ester hydrolase activity in retinal pigment epithelium plasma membrane.

PURPOSE: To identify the subcellular locale of 11-cis retinyl esters in bovine retinal pigment epithelium (RPE) and to characterize the enzymic mechanism responsible for liberation of 11-cis retinoids in this compartment. METHODS: Endoplasmic reticulum (ER)- enriched and plasma membrane (PM)-enriched protein fractions were prepared from bovine RPE microsomes using sequential discontinuous sucrose and Percoll gradient fractionation. Enzyme markers for ER (such as carboxylesterase), and PM (such as 5'-nucleotidase [5'-ND]; alkaline phosphatase [AP]; and ouabain-sensitive Na+,K+-ATPase [ATPase]) were used to identify the subfractions. Membrane-associated retinoids were quantified by high-performance liquid chromatography (HPLC) and retinyl ester hydrolase (REH) activities were determined by radiometric and chromatographic (HPLC) means. RESULTS: Chromatographic analyses of membrane-associated retinoids showed that 11-cis retinyl esters are localized mainly in PM-enriched fractions, whereas all-trans retinyl esters are associated predominantly with ER-enriched membranes; profiles of the distribution of 11-cis- and all-trans REH activities were consistent with the retinyl ester distribution. Further purification of the crude PM fraction yielded a fraction (P2) that was significantly enriched with 5'-ND (fivefold), ATPase (15-fold), AP (10-fold), and 11-cis retinyl ester hydrolase (11-cis REH; threefold) activities, but was relatively devoid of carboxylesterase and all-trans REH activities. Apparent kinetic constants (Km(app) and Vm(app)) for 11-cis REH activity in P2 were 18 microM and 1800 picomoles/min per mg, respectively. CONCLUSIONS: This is the first identification of an 11-cis-specific REH activity in RPE plasma membrane. Results from these studies demonstrate the capacity of RPE plasma membranes to accommodate and hydrolyze 11-cis retinyl esters. Plasma membrane storage and mobilization of 11-cis retinyl esters represents a novel compartmentalization of retinoid metabolism that is distinct from the sites where 11-cis retinoids are produced. The implication of these findings for present theories of visual chromophore biosynthesis are discussed.

5'-Nucleotidase↗
PubMed1996

Serum vitamin A, vitamin E, and beta-carotene levels in preeclamptic women in northern nigeria.

We compared the serum levels of beta-carotene, vitamin A (retinol), and vitamin E (alpha-tocopherol) in healthy pregnant women and their counterparts who exhibited the signs and symptoms of preeclampsia or eclampsia, including: systolic blood pressure greater than 160 mm Hg, edema, and proteinuria. The study was conducted in the cities of Maiduguri and Bauchi, which are located in the semi-arid northeastern region of Nigeria. Most of the pregnant subjects: (1) were teenagers, though they ranged in age from 14 to 25 years; (2) had 2 or fewer prior pregnancies; and (3) were predominantly of the Muslim faith and members of the Hausa, Fulani, or Kanuri ethnic groups. Few of the women had received prenatal care. Serum levels of vitamins A and E and betacarotene were quantified using high pressure liquid chromatography. The serum vitamin A levels of the 9 preeclamptic women (15.3 mg/dL) and the 7 eclamptic women (8.3 mg/dL) were significantly reduced (p < 0.01) relative to the serum vitamin A levels of healthy women in the third trimester (24.2 mg/dL). For the healthy pregnant controls, the levels of vitamins A and E and beta-carotene were relatively constant throughout pregnancy. The mean serum beta-carotene levels for both the preeclamptic and eclamptic groups of subjects were half as high as those of healthy control women in the third trimester (p = 0.004). The serum vitamin E levels of the preeclamptic and eclamptic women were 15% and 30% lower, respectively, than those of the corresponding controls (p < 0.01). The serum levels of these three lipids in the healthy pregnant and non-pregnant women we studied are similar to values reported by others for North American and European women of childbearing age. These results support the hypothesis that preeclampsia-eclampsia deplete natural lipid antioxidants and suggest that the reduced levels of vitamin A in such women experiencing hypertension of pregnancy, if they happen to be infected with the HIV-1 virus, may place them at increased risk for mother-child transmission of the virus.

Adolescent↗
PubMed1996

Streptozotocin-induced diabetes in rats is associated with impaired metabolic availability of vitamin A (retinol).

Using streptozotocin-induced diabetic Wistar rats, studies were carried out to examine the metabolic availability of vitamin A in the plasma, liver and the retina of the eye. Control and diabetic rats were fed ad lib. on a semi-purified diet either with or without (basal) vitamin A supplementation, or pair-fed on the basal diet for 4 weeks. Despite the fact that diabetic rats consumed 48% more feed, they had lower plasma concentrations of retinol (P < 0.003). The decrease in plasma retinol concentration was a response to diabetes (or diabetes-induced trauma), since neither pair-feeding (P < 0.01) nor vitamin A supplementation altered this effect (P < 0.05). Furthermore, the hepatic concentrations of the vitamin in these animals remained elevated and this increase was greater in the supplemented diabetic group (P < 0.001). Decreases in 11-cis retinal (a component of rhodopsin) concentrations in the retina were also observed in diabetic animals. The increased hepatic and the decreased plasma and retina vitamin A levels suggest a defect in the transport of the vitamin from the liver.

Animals↗
PubMed1996

Comparison of retinyl ester hydrolase activities in bovine liver and retinal pigment epithelium.

Various properties of retinyl ester hydrolysis in the liver and the retinal pigment epithelium (RPE) have been studied, yet the relationship between the retinyl ester hydrolase (REH) activities in these tissues of the same species is not known. In the present study, REH activities in bovine liver and RPE microsomes were compared to explore potential biochemical relationships of retinyl ester metabolism in these tissues. Rates of [3H]all-trans retinyl palmitate hydrolysis by liver and RPE were comparable (i.e., Vmaxapp approximately 300 pmol/min per mg; K(m)app approximately 30 microM), while hydrolysis of [3H]11-cis retinyl palmitate by RPE was significantly higher (Vmaxapp = 1,667 pmol/min per mg). When equimolar amounts (10 microM) of either [14C]triolein or unlabeled 11-cis retinyl palmitate were added to [3H]all-trans REH assays, all-trans REH activities in liver and RPE demonstrated similar time-dependent inhibition profiles. In contrast, hydrolysis of [3H]11-cis retinyl palmitate by RPE was relatively unaffected by addition of either [14C]triolein or unlabeled all-trans retinyl palmitate. Additionally, modification of the microsomal proteins with N-ethylmaleimide produced profound, dose-dependent alterations in K(m)app values for all-trans retinyl ester hydrolysis, whereas K(m)app for 11-cis REH in the RPE was not significantly altered. These results have elucidated common biochemical features of all-trans retinyl ester hydrolysis in liver and RPE. In contrast, hydrolysis of 11-cis retinyl ester in RPE is characterized by a distinctive substrate preference and unique biochemical properties.

Animals↗
PubMed1995

Retinyl ester hydrolase and the visual cycle in the chicken eye.

The ability of chicken retina and retinal pigment epithelium (RPE) membrane to hydrolyze vitamin A esters ([9,10(-3)H]all-trans- and 11-cis-retinyl palmitate) was studied. Hydrolytic activity within the retina was optimal at acidic pH of 5.0, whereas in the RPE significant hydrolytic activity was exhibited over a broad range of hydrogen ion concentrations. The highest rate of hydrolysis was associated with the all-trans-isomer and located within retina and RPE membranes [the apparent maximal velocity (Vmax) and Michaelis-Menten constant (Km) were 770 pmol.min.-1.mg-1 and 45 microM and 300 pmol.min-1.mg-1 and 3.6 microM, respectively[. Retinyl ester hydrolase activities for 11-cis-retinyl palmitate in the retina and RPE were correspondingly lower (apparent Vmax of 204 pmol.min.-1.mg-1 and Km of 18.5 microM in the retina; apparent Vmax of 131 pmol.min.-1.mg-1 and Km of 4 microM in the RPE). Together with results from other laboratories, results from the present study suggest that chicken retina contains important enzymes to complete the visual cycle.

Animals↗
PubMed1993

Detection of sub-nanogram quantities of Mojave toxin via enzyme immunoassay with light addressable potentiometric detector.

Sensitive detection of Mojave toxin (MoTX), a potent neurotoxin isolated from the venom of Crotalus scutulatus scutulatus, was developed using an enzyme immunoassay (EIA) and light addressable potentiometric detection. This EIA utilizes both biotin- and fluorescein-labeled anti-MoTX antibodies to immobilize and detect sub-nanogram to nanogram quantities of toxin. Labeled mono- and polyclonal anti-MoTX antibodies were used alone and/or in combination to determine maximum assay sensitivity. Assays performed using a combination of biotinylated poly- and fluoresceinated monoclonal antibodies produced an assay with a lower detection limit near 2.5 ng. Assays performed using labeled polyclonals alone, or in combination with biotinylated mono- and fluoresceinated polyclonal antibodies, indicated increased sensitivity with a detection limit near 300 pg. In conclusion, we describe an enzyme immunoassay using different labeled antibody schemes which detects sub-nanogram quantities of MoTX via light addressable potentiometric detection.

Antibodies↗
PubMed1993

Phototransduction-related circadian changes in indoleamine metabolism in the chick pineal gland in vivo.

The purpose of this study was to examine the day/night levels of pineal melatonin and its rate limiting enzyme N-acetyltransferase (NAT) in relationship to the ratio of 11-cis- to all-trans-retinal. Three-week-old chicks were placed in 12:12 light:dark (LD 12:12) cycle for one week, pineals were collected during the light phase at 1500 (i.e., after 10 hr light), during the dark phase at 1900 (i.e., 2 hr after dark), at 2100 (i.e., 4 hr after dark), and at 2300 (i.e., 6 hr after dark) and after light extension to 1900. The results show that light-sensitive 11-cis-retinal in the chick pineal has the same diurnal rhythm as NAT and melatonin; all constituents increased within 2 hr of darkness onset (at 1900) and reached their peak after 4 hr of dark. All values were lowest during the light phase at 1500. Low values for 11-cis-retinal, NAT, and melatonin were also seen in the group of chicks which experienced light extension to 1900. The data indicate that in vivo light plays a major role in triggering rhodopsin-bound 11-cis-retinal production within 2-4 hr after darkness onset; this change likely serves as the signal for the subsequent formation of the hormonal product of the pineal gland, melatonin.

Acetylserotonin O-Methyltransferase↗
PubMed1992

Hydrolysis of 11-cis- and all-trans-retinyl palmitate by retinal pigment epithelium microsomes.

A partial characterization of the enzymatic hydrolysis of 11-cis- and all-trans-retinyl palmitate by bovine retinal pigment epithelium microsomes was carried out using a micro-radiometric method to quantitate liberated palmitic acid. Retinyl ester hydrolase (REH) activity was examined in the absence of detergent. Hydrolysis of 11-cis- and all-trans-retinyl palmitate was protein- and time-dependent. Optimal enzyme activity occurred at slightly alkaline pH (8-9). Apparent kinetic constants (Vmax and Km) for the 11-cis-REH were 2.1 nmol/min/mg protein and 66 microM, respectively. All-trans-REH demonstrated a lower maximum velocity of 0.3 nmol/min/mg protein and a slightly higher substrate affinity of 27 microM. Further characterization of 11-cis-retinyl palmitate hydrolysis involved monitoring formation of reaction products, 11-cis retinol and palmitic acid, which were found to be released in essentially a 1:1 stoichiometry. Addition of all-trans retinyl bromoacetate, a known inhibitor of lecithin:retinol acyltransferase reduced both 11-cis and all-trans-REH activities but to significantly different degrees (50 and 76%, respectively). Although the microsomal preparation exhibited LRAT activity, acyl transfer was not readily reversible as labeled palmitic acid was not transferred to added acyl acceptor compounds. These findings suggest that hydrolysis of 11-cis-retinyl palmitate by bovine retinal pigment epithelium microsomes may occur at a catalytic site distinct from that for the all-trans isomer and that this hydrolysis is not representative of a reverse transesterification reaction.

Animals↗
PubMed1992

Bile salt independent retinyl ester hydrolases in the bovine eye.

Homogenates of bovine neuroretina and retinal pigment epithelium (RPE) were incubated with 11-cis and all-trans retinyl palmitate to study retinyl ester hydrolysis. The highest activity was found in RPE when 11-cis retinyl palmitate served as substrate (Km = 7.8 microM and Vmax = 44.8 pmol/min/mg). This retinyl ester hydrolase (REH) had an optimum activity at acidic pH (pH 5), which is in contrast to the neutral hydrolase (pH 8) found in the neuroretina. Similar to REH in the liver, REH activities in the bovine eye are not stimulated by bile salts because sodium cholate, taurocholate and deoxycholate did not enhance retinyl ester hydrolysis. Most REH activities in retinal homogenate were soluble, whereas in the RPE, these activities were membrane-bound. Divalent cations such as zinc and cadmium completely inhibited REH activities in the neuroretina. Our results show that bovine ocular tissues contain several retinyl ester hydrolases with distinct biochemical properties.

Animals↗
PubMed1991

Identification of 11-cis-retinal and demonstration of its light-induced isomerization in the chicken pineal gland.

Direct evidence is not available that (1) rhodopsin-like photopigment exists in the chicken pineal and that (2) the visual pigment is responsible for the light sensitivity of the gland. Therefore, the objective of this study was to test for the existence of visual pigment in the chicken pineal by means of the identification of 11-cis-retinal in this organ. 11-cis- and all-trans retinoids were extracted from light- and dark-adapted chicken pineals and analyzed by high performance liquid chromatography (HPLC) using the formaldehyde method. 11-cis-Retinal was initially identified by coelution with an authentic standard. Further characterization was carried out by collecting the retinal from the HPLC eluant, subjecting it to reduction by sodium borohydride and then identifying the derived 11-cis-retinol using HPLC. Proportions of 11-cis-retinal to total pineal retinals were also studied from decapitated heads after light and dark adaptation. Analyses of dark-adapted, pooled chicken pineals revealed equal proportions of 11-cis and all-trans retinals at 2 h after dark and at night. Two hours of light adaptation resulted in the reduction of the 11-cis proportion (from 50%) to 26% of total retinals. These observations prove that 11-cis-retinal exists in the chicken pineal and that it undergoes light-induced cis to trans isomerization in a manner similar to the visual pigment chromophores in the vertebrate retina.

Adaptation, Physiological↗
PubMed1990

Comparison of retinal pigment epithelium cell preparations from the bovine eye.

Retinal pigment epithelium (RPE) cells were collected from bovine eyes using a new method. The cells were harvested by vortexing the RPE and underlying choroid in 0.05 M citrate phosphate buffer, pH 5. RPE cells recovered by this method were compared to a standard method by microscopic examination of cell integrity, estimation of total protein, and assay of 11-cis and all-trans retinyl ester hydrolase (REH) activities. Results suggest that this method collects RPE cells of good integrity and with a significantly higher protein yield than the conventional method. Additionally, a much higher retinyl ester hydrolase activity was noted. Therefore we propose that this procedure offers a new and convenient method in the collection of RPE proteins for certain purposes such as enzyme purification.

Animals↗
PubMed1990

Partial characterization of a high affinity [Ca2+ + Mg2+]-dependent adenosinetriphosphatase from bovine retina.

Examination of retinal tissue homogenates indicated the presence of a [Ca2+ + Mg2+]-dependent adenosinetriphosphatase activity that exhibited high affinity for Ca2+ (K0.5 = 0.17 microM) and moderately high affinity for Mg2+ and ATP (K0.5 = 12.5 microM and Km = 22.8 microM, respectively). Maximum ATP hydrolysis occurred at pH 7.4. Under conditions of optimal substrate, cation and hydrogen ion concentrations, specific activity ranged from 15 to 18 nmol phosphate released min-1 mg-1 protein. Although the retinal [Ca2+ + Mg2+] adenosinetriphosphatase hydrolyzes both ATP and dATP, other nucleotides (CTP, GTP, ITP and UTP) were not hydrolyzed to any great extent. The monovalent cations, Li+, K+ and Na+, had no effect upon hydrolysis of ATP; whereas Cs+ and NH4+ ions were moderately (approximately 30%) inhibitory. All divalent cations tested were stimulatory. With the exception of rotenone which inhibited ATP hydrolysis approximately 25%; retinal adenosinetriphosphatase activity was insensitive to mitochondrial inhibitors (NaN3, KCN, ruthenium red and oligomycin). Adenosinetriphosphatase activity was observed to be very sensitive to low concentrations (I50 approximately 2 microM) of vanadate; whereas, lanthanum administration resulted in no inhibition. Removal of calmodulin (80%) resulted in reducing adenosinetriphosphatase activity 60% but addition of exogenous calmodulin back to calmodulin deficient membranes did not restore activity to starting levels. Calmodulin antagonists trifluoperazine and calmidazolium reduced significantly Ca2+ stimulated, Mg2+ dependent ATP hydrolysis. We conclude that the [Ca2+ + Mg2+]-dependent adenosinetriphosphatase of bovine retina is a non-mitochondrial protein exhibiting very high affinity for Ca2+ and appears to require calmodulin for maximum activity. Because of its high affinity for Ca2+, this protein may play an important role in reducing intracellular Ca2+ to nanomolar levels.

Adenosine Triphosphate↗