Search PubMed⌕ Search

Biomedical subjects

R Y Chan

Publications and source records attributed to R Y Chan.

33 records · Page 2Linked to original sources
PubMed1993

Expression of ferrochelatase mRNA in erythroid and non-erythroid cells.

Ferrochelatase, which catalyses the last step in haem biosynthesis, i.e. the insertion of Fe(II) into protophorphyrin IX, is present in all cells, but is particularly abundant in erythroid cells during haemoglobinization. Using mouse ferrochelatase cDNA as a probe two ferrochelatase transcripts, having lengths of 2.9 kb and 2.2 kb, were found in extracts of mouse liver, kidney, brain, muscle and spleen, the 2.9 kb transcript being more abundant in the non-erythroid tissues and the 2.2 kb transcript more predominant in spleen. In mouse erythroleukemia cells the 2.9 kb ferrochelatase transcript is also more abundant; however, following induction of erythroid differentiation by dimethyl sulphoxide there is a preferential increase in the 2.2 kb transcript, which eventually predominates. With mouse reticulocytes, the purest immature erythroid cell population available, over 90% of the total ferrochelatase mRNA is present as the 2.2 kb transcript. Since there is probably only one mouse ferrochelatase gene, the occurrence of two ferrochelatase transcripts could arise from the use of two putative polyadenylation signals in the 3' region of ferrochelatase DNA. This possibility was explored by using a 389 bp DNA fragment produced by PCR with synthetic oligoprimers having sequence similarity with a region between the polyadenylation sites. This fragment hybridized only to the 2.9 kb ferrochelatase transcript, indicating that the two transcripts differ at their 3' ends and suggesting that the 2.2 kb transcript results from the utilization of the upstream polyadenylation signal. The preferential utilization of the upstream polyadenylation signal may be an erythroid-specific characteristic of ferrochelatase gene expression.

5-Aminolevulinate Synthetase↗
PubMed1992

Transferrin-receptor-independent but iron-dependent proliferation of variant Chinese hamster ovary cells.

The purpose of this study is to clarify the role of iron, transferrin, an iron-binding protein in vertebrate plasma, and transferrin receptors in cell proliferation. Transferrin, which is indispensable for most cells growing in tissue culture, is frequently referred to as a "growth factor". Proliferating cells express high numbers of transferrin receptors, and the binding of transferrin to their receptors that is needed for cells to initiate and maintain their DNA synthesis is sometimes regarded as analogous to other growth factor-receptor interactions. Although numerous previous experiments strongly indicate that the only function of transferrin in supporting cell proliferation is supplying cells with iron, they did not completely rule out some direct or signaling role transferrin receptors could play in cell proliferation. To address this issue, we exploited transferrin-receptor-deficient mutant Chinese hamster ovary (CHO) cells (McGraw, T. E., Greenfield, L., and Maxfield, F. R., 1987, J. Cell. Biol. 105, 207-214) in which various aspects of iron and transferrin metabolism in relation to their capacity to proliferate were investigated. Variant cells neither specifically bind transferrin nor do their extracts contain any detectable functional transferrin receptors, yet they proliferate and synthesize DNA with rates comparable to those observed with parent CHO cells. Desferrioxamine, an iron chelating agent, inhibits growth and DNA synthesis of both variant and control CHO cells. This inhibition can be fully alleviated, in both cell types, by ferric pyridoxal isonicotinoyl hydrazone, which can supply cells with a utilizable form of iron by a pathway not requiring transferrin and their receptors. Studies of 59Fe uptake and 125I-transferrin binding revealed that parent cells can take up iron by at least three mechanisms: from transferrin by receptor-dependent and -independent (nonspecific, nonsaturable, not requiring acidification) pathways and from inorganic iron salts (initially present in the medium as FeSO4). Although variant CHO cells are unable to acquire transferrin iron via the receptor pathway, two remaining mechanisms provide these cells with sufficient amounts of iron for DNA synthesis and cell proliferation. In conclusion, although transferrin receptors are dispensable in terms of their absolute requirement for proliferating cells, a supply of iron is still needed for their DNA synthesis. Transferrin-receptor-deficient CHO cells may be a useful model for investigating receptor-independent iron uptake from transferrin and nontransferrin iron sources.

Animals↗
PubMed1992

omega-6 and omega-3 fatty acids: monolayer packing and effects on bilayer permeability and cholesterol exchange.

It has been suggested that the polyunsaturated omega-3 fatty acid, docosahexaenoic acid (DHA), can adopt unique closely packed arrays in lipid bilayers (Glomset and Applegate. (1986) J. Lipid Res. 27, 658-680). These conformations are predicted on the basis of molecular dynamics calculations and are in contrast to the expanded conformations characteristic of omega-6 unsaturated fatty acids. It has also been suggested that close packing of omega-3 acyl chains could have a substantial affect on the physical properties of lipid bilayers (e.g. permeability). We report here some experimental tests of these predictions. Surface pressure-area experiments have been carried out on DHA and its mixtures with stearic and oleic acids. At low surface pressures DHA is more expanded than oleic acid. Extrapolation to the high surface pressures characteristic of lipid bilayers indicates that the area per molecule of DHA is only marginally less than that for oleic acid. Thus there is no compelling evidence to suggest that the average area per molecule of the omega-3 fatty acid is substantially different from the omega-6 fatty acid at high surface pressures. Experiments also show that the permeability of bilayers to glucose and the rates of dissociation of pyrenyl cholesterol from bilayers were similar for bilayers containing DHA compared to bilayers containing oleic acid or linoleic acid.

Cholesterol↗
PubMed1991

Spectrofluorometric determination of lipase activity.

A fluorescent triglyceride, 1(3)-pyrenylbutanoyl-2,3(1,2)-dipalmitoyl-sn-glycerol, was synthesised, characterised by NMR and mass spectrometry, incorporated into a lipid emulsion and used as a fluorescent substrate for pancreatic lipase. It is shown that the product of the reaction, pyrene butyric acid, diffuses into the aqueous phase resulting in a decrease in the excimer fluorescence of the pyrene fluorophore in the emulsion and an increase in its monomer fluorescence. The phenomenon can be used to assay the enzyme thereby cirumventing the need to extract the fatty acid product.

Fluorescent Dyes↗
PubMed1988

Perturbation of intracellular pH by DIDS on endocytosis of transferrin and iron uptake in rabbit reticulocytes.

DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid) inhibited transferrin and iron uptake by rabbit reticulocytes in a dose-dependent manner. Measurement of intracellular pH shows that DIDS neutralized the pH in the intracellular vesicles and at the same time, rendered the cytoplasmic pH more acidic. The latter may be the result of a perturbance in the acid-extruding mechanism(s) by DIDS, a known anion exchange blocker. Examination of the single-cycle endocytosis of transferrin with pulse-chased technique suggests that DIDS retarded transferrin internalization, iron unloading and transferrin receptor recycling in the cells.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1986

The membrane effect of 1-anilino-8-naphthalene sulfonate on transferrin and iron uptake by rabbit reticulocytes.

Studies on the effect of 1-anilino-8-naphthalene sulfonate on uptake of transferrin and iron by rabbit reticulocytes show that the inhibitory action of ANS is localized at the membrane level. The intravesicular pH and cellular ATP level were not affected by this anionic probe. ANS shifted the transition temperature and reduced the enthalpy changes of iron uptake by rabbit reticulocytes. These suggested that the drug reduced the membrane fluidity. Hence, ANS disturbed the physicochemical environment of the receptor for transferrin resulting in the perturbation of receptor-mediated endocytosis.

Anilino Naphthalenesulfonates↗
PubMed1985

Effects of spermine on transferrin and iron uptake by reticulocytes: I. Action on the endocytic uptake of transferrin.

Iron uptake by rabbit reticulocytes was inhibited by spermine in a concentration-dependent manner. Examination of the single-cycle endocytosis of 125I-transferrin showed that a graded reduction in the rate of exocytosis of transferrin was related to increasing extracellular spermine concentrations. This reduction could affect the recycling of transferrin receptors and resulted in the loss of membrane binding sites in spermine-treated cells. As large vacuoles were observed in cells treated with spermine, the endotubular function of these cells was probably affected. Spermine also enhanced the binding affinity of transferrin to membrane receptors. The mechanism for this enhancement was not clear.

Animals↗
PubMed1985

Effects of spermine on transferrin and iron uptake by reticulocytes: II. Changes in intravesicular pH.

An increase in extracellular spermine concentration brought about a progressive rise in intralysosomal pH in rabbit reticulocytes. Since intracellular release of iron from transferrin is believed to involve the protonation of the iron-transferrin complex, the rise in intralysomal pH could account for the inhibitory effect of spermine on iron uptake. The inhibition could be reversed if spermine was removed by washing. As a result of spermine treatment, more acid-labile N-terminal monoferric transferrin and less apotransferrin were released from the cell. These results are consistant with the protonation theory of iron release.

Animals↗
PubMed1981

Anterior segment configuration correlated with Shaffer's grading of anterior chamber angle.

The central corneal thickness, peripheral corneal thickness, central anterior chamber depth, and peripheral anterior chamber depth were measured from 110 phakic eyes using attachments 1 and 2 to the slitlamp (Haag-Streit). The Shaffer grading system was used for the width of the anterior chamber angle. The results showed that both the average central anterior chamber depth and peripheral anterior chamber depth increased in a linear manner with the Shaffer grading system of the width of the anterior chamber angle. In addition, our "scattergram" showed that each Shaffer grading of the angle width could almost be uniquely determined by the combination of central and peripheral anterior chamber depths. However, the peripheral anterior chamber depth is the best single factor in predicting the Shaffer grading of the width of the anterior chamber angle.

Anterior Chamber↗
PubMed1980

Spatial organizations of catfish retinal neurons. II. Circular stimulus.

1. Temporal dynamics of receptive-field components were identified by use of circular stimuli whose diameter was modulated in white-noise fashion. 2. A thin ring of light evoked a complex response from the horizontal-cell soma but not from its axon. 3. All bipolar cells had a biphasic receptive field whose incremental sensitivities were comparable for the field's center and surround. 4. Type-N cells had a biphasic field whose two components were segregated both in time and space. 5. Type-C cells showed nonlinearities, which did not depend on the types of light inputs and which were intrinsic to the cell. 6. Ganglion cells could be classified roughly into two classes: small- and large-field cells. Their main nonlinearity was rectification. 7. Our studies on the catfish retna have shown that the distal cells are essentially linear in time and space and can be identified functionally by any type of input. The proximal cells, however, have a complex functional repertoire whose identification poses considerable difficulty.

Animals↗
PubMed1979

Adaptation in catfish retina.

1. We define absolute sensitivity as (voltage/illuminance) and incremental sensitivity as the peak-to-peak amplitude of the first-order (Wiener) kernels. 2. Incremental sensitivity of the horizontal cells is the local slopes of the Michaelis-Menten equation and that of more proximal neurons is the Fechner slope. In a log-log plot, the former has a slope of -2, whereas the latter a slope of -1, as predicted by Williams and Gale (39). 3. During a moderate to strong steady illumination, absolute sensitivity decreases but incremental sensitivity increases. The reverse occurs during dark adaptation. 4. The presence of a steady illumination did not prevent signal transmission from horizontal to ganglion cells. 5. From these results we conclude that: adaptation in the catfish retina includes two components: a) a lateral shift of the voltage-intensity curve along the intensity axis, and b) changes in the time course of light-evoked response. We argue that the latter phenomenon is related to the presumed horizontal cell-to-receptor cell negative feedback.

Adaptation, Physiological↗
PubMed1976

The amacrine cell.

Explore the source record for details and available documents.

Action Potentials↗