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PubMed2003

Can black tea influence plasma total homocysteine concentrations?

BACKGROUND: Polyphenols can act as acceptors of methyl groups during the metabolism of methionine to homocysteine. This may result in elevations in plasma total homocysteine (tHcy) concentrations after ingestion of polyphenol-rich beverages such as tea. OBJECTIVES: Our major objective was to determine whether regular, moderate-to-high intakes of black tea alter tHcy concentrations. We also assessed the relation between the degree of O-methylation of tea-derived polyphenols and the change in tHcy with regular ingestion of tea. DESIGN: Twenty-two subjects completed a randomized, controlled crossover study. Subjects consumed 1250 mL black tea/d (5 cups each containing 2 g tea leaves in 250 mL boiled water) and 1250 mL hot water/d for 4 wk each. Fasting tHcy concentrations and 24-h urinary excretion of 4-O-methylgallic acid (4OMGA, the major O-methylated metabolite of gallic acid) were measured at the end of each period. 4OMGA was used as a marker of overall O-methylation of tea-derived polyphenols. RESULTS: Black tea did not significantly alter mean (+/- SEM) tHcy concentrations (9.9 +/- 0.5 and 10.0 +/- 0.5 micro mol/L for the hot water and black tea periods, respectively). However, the increased excretion of 4OMGA as a consequence of black tea consumption was positively associated with the change in tHcy from the hot water period to the black tea period (r = 0.55, P = 0.008). Subjects in the bottom quartile of increase in 4OMGA excretion had a significant decrease in tHcy (-0.28 +/- 0.10 micro mol/L; P = 0.046), and those in the top quartile had a significant increase in tHcy (0.78 +/- 0.16 micro mol/L; P = 0.005). CONCLUSIONS: Overall, regular ingestion of black tea did not alter mean tHcy concentrations. However, individual differences in O-methylation of polyphenolic compounds may influence the ultimate effects of black tea on tHcy.

Adult↗
PubMed1988

Iron, copper and zinc metabolism of rats fed various levels and types of tea.

The effects of tea and components in tea on mineral metabolism were investigated in a series of studies. In studies A and B weanling rats were fed diets containing various levels (0, 0.35, 1.17 or 3.50%) of a commercially prepared instant tea and diets containing 0 or 1.17% catechin. In study C anemic rats were fed diets containing 0 or 2.31% desiccated green or black tea infusions for 23 d. In study D rats were fed a diet containing 2.31% desiccated black tea infusions for 16 d, were fed the tea-containing diet in a single meal labeled with 59Fe or were given black tea infusions as their sole fluids. Rats fed diets containing 2.31% green or black tea or given fluid tea had elevated hematocrits but experienced minimal changes in tissue iron levels or in iron absorption; this is counter to conclusions from studies in which iron absorption was monitored indirectly. Ingestion of instant or black tea consistently elevated liver copper levels; this may be part of the mechanism by which hematological variables were affected. Although the green tea contained significant amounts of aluminum, rats fed the tea did not accumulate aluminum in their bones.

Absorption↗
PubMed2003

Antioxidant activity of tea polyphenols in vivo: evidence from animal studies.

Tea is particularly rich in polyphenols, including catechins, theaflavins and thearubigins, which are thought to contribute to the health benefits of tea. Tea polyphenols act as antioxidants in vitro by scavenging reactive oxygen and nitrogen species and chelating redox-active transition metal ions. They may also function indirectly as antioxidants through 1) inhibition of the redox-sensitive transcription factors, nuclear factor-kappaB and activator protein-1; 2) inhibition of "pro-oxidant" enzymes, such as inducible nitric oxide synthase, lipoxygenases, cyclooxygenases and xanthine oxidase; and 3) induction of phase II and antioxidant enzymes, such as glutathione S-transferases and superoxide dismutases. The fact that catechins are rapidly and extensively metabolized emphasizes the importance of demonstrating their antioxidant activity in vivo. Animal studies offer a unique opportunity to assess the contribution of the antioxidant properties of tea and tea polyphenols to the physiological effects of tea administration in different models of oxidative stress. Most promising are the consistent findings in animal models of skin, lung, colon, liver and pancreatic cancer that tea and tea polyphenol administration inhibit carcinogen-induced increases in the oxidized DNA base, 8-hydroxy-2'-deoxyguanosine. In animal models of atherosclerosis, green and black tea administration has resulted in modest improvements in the resistance of lipoproteins to ex vivo oxidation, although limited data suggest that green tea or green tea catechins inhibit atherogenesis. To determine whether tea polyphenols act as effective antioxidants in vivo, future studies in animals and humans should employ sensitive and specific biomarkers of oxidative damage to lipids, proteins and DNA.

Animals↗
PubMed2005

Proline-rich proteins moderate the inhibitory effect of tea on iron absorption in rats.

Tea inhibits iron absorption in studies in which tea is given with radiolabeled iron to humans as a single dose. Our objective was to test the hypothesis that proline-rich proteins (PRPs) may act as a defense against this effect by forming complexes with tannins, thereby preventing them from inhibiting iron absorption. Two studies were conducted. In study 1, rats were given test solutions containing (59)FeCl(3) in water, tea, or tea + gelatin (T/G). In study 2, the rats were divided into 3 groups and assigned to one of 3 nutritionally complete diets: control, tea (5 g tea tannin/kg diet), or T/G (5 g tea tannin + 60 g gelatin/kg diet). Rats were fed the respective diets for 5 d and then given a single (59)Fe-labeled meal of the diet. Iron absorption was measured by whole-body retention of the (59)Fe over a 2-wk period. Iron absorption in study 1 was lower in the tea group (24 +/- 9.6%, P < 0.05) than in the T/G (42 +/- 19.4%) or water groups (50 +/- 7.5%). In study 2, iron absorption did not differ among the groups. Rats fed the tea diet had dramatic hypertrophy of the parotid salivary glands. Adding gelatin as a proxy for salivary PRPs to the tea eliminated the inhibitory effect of tea on iron absorption. The results suggest that PRPs, whether from salivary glands or diet, can protect against the inhibition of iron absorption by tea.

Animal Feed↗
PubMed2001

Multielement determination and speciation of major-to-trace elements in black tea leaves by ICP-AES and ICP-MS with the aid of size exclusion chromatography.

A multielement determination of major-to-trace elements in black tea leaves and their tea infusions was carried out by ICP-AES (inductively coupled plasma atomic emission spectrometry) and ICP-MS (inductively coupled plasma mass spectrometry). Tea infusions were prepared as usual tea beverage by brewing black tea leaves in boiling water for 5 min. About 40 elements in tea leaves and tea infusions could be determined over the wide concentration range in 8 orders of magnitude. The extraction efficiency of each element was estimated as the ratio of its concentration in tea infusions to that in tea leaves. From the experimental results for the extraction efficiencies, the elements in black tea leaves were classified into three characteristic groups: (i) highly-extractable elements (>55%): Na, K, Co, Ni, Rb, Cs and Tl, (ii) moderately-extractable elements (20-55%): Mg, Al, P, Mn and Zn, and (iii) poorly-extractable elements (<20%): Ca, Fe, Cu, Sr, Y, Zr, Mo, Sn, Ba and lanthanoid elements. Furthermore, speciation of major-to-trace elements in tea infusions was performed by using a combined system of size exclusion chromatography (SEC) and ICP-MS (or ICP-AES). As a result, many diverse elements were found to be present as complexes associated with large organic molecules in tea infusions.

Chromatography, Gel↗
PubMed1999

Traditional tea drinking in Senegal. A real source of fluoride intake for the population.

Tea drinking after meals is a traditional practice in Senegal where more than 80% of the population from 15 to 60 years old drink tea. According to the tradition, in one session, each tea consumer has to drink 3 cups of decocted tea. The content of a cup is about 30 ml of liquid. Some people drink tea three times daily, that is to say after each meal. Tea plant is rich in fluoride. To determine the effective intake of the Senegalese population from this source, we measured the fluoride concentration not only for each component of the prepared tea but also for each cup of prepared tea. For this study, we used the two main kinds of tea existing locally. The analyses have been done at Rochester, NY Eastman Dental Center, Oral Biology Dept Fluoride Laboratory using the Taves Microdiffusion Method and the fluoride Ion Specific Electrode. The results so that the mean total fluoride concentration of each cup, from the first to the third one, is: 4.0 mg F-/L, 7.436 mg F-/L and 1.230 mg F-/L. It means that on an average in one session, a Senegalese tea consumer has a daily fluoride intake of 1.139 mg F-/L when taking in count the total fluoride and drinking only 90 ml of tea. If we consider the ionic fluoride the amount of daily ingested fluoride for someone who takes only 3 tea-pots of 30 ml each, is 0.830 mg. To conclude, we state that this traditional practice may have a caries preventive effect. Further studies will be grateful for that practice when setting up a caries preventive program in our country. We will also be careful in extending that practice to children less than 8 years old because it might cause dental fluorosis as in Senegal the optimal dose of fluoride is 0.8 mg F-/L.

Adolescent↗
PubMed1998

[Induction of NAD(P)H: quinone reductase by anticarcinogenic ingredients of tea].

By assaying the activity of NAD(P)H: quinone reductase (QR) in Hep G2 cells exposed to inducing agents, a variety of ingredients in tea, we compared their abilities on inducing QR and preventing cancer. The results showed that tea polyphenols, tea pigments and mixed tea were all able to induce the activity of QR significantly. The single-component ingredients of tea polyphenols and tea pigments, including thearubigens, EGCG and ECG, also enhanced the activity of QR. But EGC, EC, theaflavins, tea polysaccharide and tea caffeine, showed no apparent induction of QR. We found that among those tea ingredients studied, the multi-component ingredients were more effective than the single-component ones. So we thought that the abilities of antioxidation and cancer prevention of tea depended on the combined effects of several kinds of active ingredients, which mainly include tea polyphenols and tea pigments.

Animals↗
PubMed2003

Oolong tea increases energy metabolism in Japanese females.

Oolong tea is a traditional Chinese tea that has long been believed to be beneficial to health such as decreasing body fat. We were interested in this assertion and tried to evaluate the effect of oolong tea on energy expenditure (EE) in comparison with green tea. The subjects were eleven healthy Japanese females (age 20+/-1 y; body mass index (BMI) 21.2+/-2.5 kg/m2) who each consumed of three treatments in a crossover design: 1) water, 2) oolong tea, 3) green tea. Resting energy expenditure (REE) and EE after the consumption of the test beverage for 120 min were measured using an indirect calorimeter. The cumulative increases of EE for 120 min were significantly increased 10% and 4% after the consumption of oolong tea and green tea, respectively. EE at 60 and 90 min were significantly higher after the consumption of oolong tea than that of water (P<0.05). In comparison with green tea, oolong tea contained approximately half the caffeine and epigallocatechin galate, while polymerized polyphenols were double. These results suggest that oolong tea increases EE by its polymerized polyphenols.

Adult↗
PubMed1989

[Fluoride contents in tea and Sakura-shrimp in relation to other inorganic constituents].

The purpose of this study is to determine the fluoride contents of shrimp and of tea samples obtained from Shizuoka prefecture which is famous for tea products area and fishery market and to examine the relations between fluoride and other mineral contents, such as calcium, magunesium and phosphorus. Fluoride contents in tea produced in this area have been reported by Matsuura and Kokubu and other investigators, but more recent data are scarce. Samples were taken from a kind of shrimp known as Sakura-shrimp and from 4 kinds of commercial tea (coarse tea, 2 kinds of green tea and refined green tea). After having been dried and powdered, 1g of each sample was reduced to ashes at 550 degrees C for 10 hours with Ca(OH)2 as a fluoride fixative. Fluoride was distilled from each ash sample at about 140 degrees C with HClO4, and about 200 ml of distillate was collected from each sample. Fluoride contents were determined by means of ion-specific electrode. Calcium and magnesium contents were determined by atomic absorption spectrophotometry, and phosphorus contents were determined by Chen, Toribara and Warner's method. Total fluoride contents of the samples were as follows: 61.73ppm in shrimp, 180.16ppm in coarse tea, 72.62ppm and 89.02ppm in the 2 kinds of green tea, and 71.11ppm in refined green tea. More than 99% of the total fluoride was obtained from 150ml distillate of each sample. Calcium contents were extremely high in shrimp (21,822ppm) and 2,106-2,693ppm in tea samples. Magnesium contents were highest in shrimp (3,088ppm) and lowest in coarse tea (1,333ppm).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1983

Folacin content of tea.

Total folacin and free folacin activities of 22 commercial tea samples, involving 5 types and 14 brands, were determined by a microbiological assay, using Lactobacillus casei (ATCC 7469). Substantially higher amounts of folacin were found in tea leaves than had been reported previously, due to an improved assay method with added ascorbic acid to protect the labile folate forms. Tea brews made from green and black teas contained approximately three to four times more folacin than the brews made from oolong tea, herb tea, and instant tea powders. Tea brews made from loose leaf teas and tea bags did not differ appreciably in folacin content. However, the tea bag itself inhibited folacin extraction. A range of 3% to 25% of the RDA for folacin can be met by consuming five servings (5 to 6 oz. per serving) of tea daily.

Folic Acid↗
PubMed2001

Differential effect of TEA on long-term synaptic modification in hippocampal CA1 and dentate gyrus in vitro.

The effectiveness of tetraethylammonium (TEA) and high-frequency stimulation (HFS) in inducing long-term synaptic modification is compared in CA1 and dentate gyrus (DG) in vitro. High-frequency stimulation induces long-term potentiation (LTP) at synapses of both perforant path-DG granule cell and Schaffer collateral-CA1 pyramidal cell pathways. By contrast, TEA (25 mM) induces long-term depression in DG while inducing LTP in CA1. The mechanisms underlying the differential effect of TEA in CA1 and DG were investigated. It was observed that T-type voltage-dependent calcium channel (VDCC) blocker, Ni2+ (50 microM), partially blocked TEA-induced LTP in CA1. A complete blockade of the TEA-induced LTP occurred when Ni2+ was applied together with the NMDA receptor antagonist, D-APV. The L-type VDCC blocker, nifidipine (20 microM), had no effect on CA1 TEA-induced LTP. In DG of the same slice, TEA actually induced long-term depression (LTD) instead of LTP, an effect that was blocked by D-APV. Neither T-type nor L-type VDCC blockade could prevent this LTD. When the calcium concentration in the perfusion medium was increased, TEA induced a weak LTP in DG that was blocked by Ni2+. During exposure to TEA, the magnitude of field EPSPs was increased in both CA1 and DG, but the increase was substantially greater in CA1. Tetraethylammonium application also was associated with a large, late EPSP component in CA1 that persisted even after severing the connections between CA3 and CA1. All of the TEA effects in CA1, however, were dramatically reduced by Ni2+. The results of this study indicate that TEA indirectly acts via both T-type VDCCs and NMDA receptors in CA1 and, as a consequence, induces LTP. By contrast, TEA indirectly acts via only NMDA receptors in DG and results in LTD. The results raise the possibility of a major synaptic difference in the density and/or distribution of T-type VDCCs and NMDA receptors in CA1 and DG of the rat hippocampus.

2-Amino-5-phosphonovalerate↗
PubMed1979

Effect of TEA on light emission from aequorin-injected aplysia central neurons.

Aplysia central neurons were injected with the calcium-sensitive photoprotein aequorin and stimulated with trains of identical depolarizing voltage-clamp pulses. The light emissions grew and the outward currents declined in successive pulses. Tetraethylammonium (TEA) enhanced the light emissions to single depolarizing pulses and suppressed the outward current. The remaining net inward current is carried primarily by calcium ions and does not facilitate. The aequorin emissions were larger at all amplitudes of depolarizing pulses that elicited emissions, and the facilitation of emissions in a train of pulses was reduced. The effect of TEA on outward current was nearly maximal when sodium ions were partially replaced with 0.1 M TEA, while the aequorin emissions were further enhanced by increasing the TEA concentration to 0.459 M. TEA enhanced the aequorin emissions at all voltages. These observations suggest that the action of TEA on aequorin emissions is not strictly a consequence of its better known outward current blocking action. The effects of TEA could be partly due to the lowered sodium concentration of these solutions. Replacement of sodium by Tris, sucrose or mannose, however, all produced no enhancement of emissions. Tetramethylammonium (TMA) replacement of sodium had effects similar to those of TEA. Thus TEA and TMA appear to have a specific effect. Part of the enhancement of light emissions by TEA is due to the removal of a series resistance error in the voltage clamp, and this may also account partly for the reduced facilitation of aequorin emissions in TEA. The remainder of the action of TEA on aequorin emissions evidently reflects a specific but previously unrecognized action on the cellular metabolism of calcium ions or on the voltage-dependent calcium channels.

Aequorin↗
PubMed2000

An investigation on the antimutagenic properties of South African herbal teas.

The antimutagenic properties of South African herbal teas were investigated using the Salmonella typhimurium mutagenicity assay. Aqueous extracts of fermented and unfermented rooibos tea (Aspalathus linearis) and honeybush tea (Cyclopia intermedia) both possess antimutagenic activity against 2-acetylaminofluorene (2-AAF) and aflatoxin B(1) (AFB(1))-induced mutagenesis using tester strains TA98 and TA100 in the presence of metabolic activation. A far less inhibitory effect was noticed against the direct acting mutagens, methyl methanesulfonate (MMS), cumolhydroperoxide (CHP), and hydrogen peroxide (H(2)O(2)) using TA102, a strain designed to detect oxidative mutagens and carcinogens. Depending on the mutagen used, the unfermented tea exhibited the highest protective effect. A similar response regarding the protection against mutagenesis was obtained when utilising different variations of the double layer Salmonella assay. The double layer technique proved to be more effective to detect the protective effect of the different tea preparations against the direct acting mutagens. With respect to indirect mutagens, the highest protection was noticed when the carcinogen was metabolically activated in the presence of the tea extract as compared with when the tea extract was incubated in a separate layer with the bacteria. The current data suggest that two mechanisms seem to be involved in the antimutagenicity of the tea extracts towards carcinogens that require metabolic activation: (i) the tea components may interfere with cytochrome P450-mediated metabolism of these mutagens and (ii) the direct interaction between the tea constituents, presumably the polyphenolic compounds, with the promutagens and/or the active mutagenic metabolites. However, the mild and/or lack of protection and in some cases even enhancement of mutagenesis induced by direct acting or oxidative mutagens, provide new perspectives regarding the role of the polyphenolic compounds known to exhibit antioxidant properties, in the protection against mutagenesis in the Salmonella assay. The present study provides the first evidence on the antimutagenic activity of honeybush tea and further evidence on the antimutagenicity of rooibos tea.

2-Acetylaminofluorene↗
PubMed2001

Extracellular blockade of K(+) channels by TEA: results from molecular dynamics simulations of the KcsA channel.

TEA is a classical blocker of K(+) channels. From mutagenesis studies, it has been shown that external blockade by TEA is strongly dependent upon the presence of aromatic residue at Shaker position 449 which is located near the extracellular entrance to the pore (Heginbotham, L., and R. MacKinnon. 1992. Neuron. 8:483-491). The data suggest that TEA interacts simultaneously with the aromatic residues of the four monomers. The determination of the 3-D structure of the KcsA channel using X-ray crystallography (Doyle, D.A., J.M. Cabral, R.A. Pfuetzner, A. Kuo, J.M. Gulbis, S.L. Cohen, B.T. Chait, and R. MacKinnon. 1998. Science. 280:69-77) has raised some issues that remain currently unresolved concerning the interpretation of these observations. In particular, the center of the Tyr82 side chains in KcsA (corresponding to position 449 in Shaker) forms a square of 11.8-A side, a distance which is too large to allow simultaneous interactions of a TEA molecule with the four aromatic side chains. In this paper, the external blockade by TEA is explored by molecular dynamics simulations of an atomic model of KcsA in an explicit phospholipid bilayer with aqueous salt solution. It is observed, in qualitative accord with the experimental results, that TEA is stable when bound to the external side of the wild-type KcsA channel (with Tyr82), but is unstable when bound to a mutant channel in which the tyrosine residue has been substituted by a threonine. The free energy profile of TEA relative to the pore is calculated using umbrella sampling simulations to characterize quantitatively the extracellular blockade. It is found, in remarkable agreement with the experiment, that the TEA is more stably bound by 2.3 kcal/mol to the channel with four tyrosine residues. In the case of the wild-type KcsA channel, TEA (which has the shape of a flattened oblate spheroid) acts as an ideal plug blocking the pore. In contrast, it is considerably more off-centered and tilted in the case of the mutant channel. The enhanced stability conferred by the tyrosine residues does not arise from Pi-cation interactions, but appears to be due to differences in the hydration structure of the TEA. Finally, it is shown that the experimentally observed voltage dependence of TEA block, which is traditionally interpreted in terms of the physical position of the TEA along the axis of the pore, must arise indirectly via coupling with the ions in the pore.

Bacterial Proteins↗
PubMed1995

Relation of membrane potential to basolateral TEA transport in isolated snake proximal renal tubules.

We measured the effects of changes in bath K+ concentration ([K+]) on basolateral membrane potential difference (PD) and [3H]tetraethylammonium (TEA) transport in isolated snake (Thamnophis) proximal renal tubules (25 degrees C; pH 7.4). Increasing bath [K+] from 3 to 65 mM decreased PD from -60 mV (inside of cells negative) to -20 mV and 2-min uptake of [3H]TEA by approximately 25%, indicating that PD influences TEA entry into the cells. Uptake of [3H]TEA was inhibited similarly at both K+ concentrations by unlabeled TEA, indicating that uptake is carrier mediated. Kt (approximately 18 microM) for 2-min uptake of [3H]TEA in 3 mM K+ increased significantly in 65 mM K+, suggesting that the decrease in PD or the increase in [K+] alters the affinity of the transporter for TEA. The steady-state cell-to-bath ratio for [3H]TEA with 3 mM K+ (-60 mV PD) was approximately 16, significantly above the ratio of 10 predicted for passive distribution at electrochemical equilibrium. With 65 mM K+ (-20 mV PD) this ratio decreased to approximately 6, again significantly above the predicted ratio of 2. These data suggest that the PD can account for much, but not all, of the steady-state uptake of TEA. Efflux of [3H]TEA across the basolateral membrane was identical with either 3 or 65 mM K+ in the bath but was almost completely inhibited in either case by tetrapentylammonium, a potent inhibitor of TEA uptake. These data indicate that virtually all TEA transport across the basolateral membrane is carrier mediated and that transport out of the cells is unaffected by PD.

Animals↗
PubMed2004

Alpha-TEA plus cisplatin reduces human cisplatin-resistant ovarian cancer cell tumor burden and metastasis.

A novel nonhydrolyzable ether-linked acetic acid analog of vitamin E, 2,5,7,8-tetramethyl-2R-(4R,8R,12-trimethyltridecyl)-chroman-6-yloxyacetic acid (alpha-TEA) in combination with cisplatin, reduces tumor burden of A2780/cp70 (cp70) cisplatin-resistant human ovarian cancer cells xenografted into immune compromised nude mice. Two xenograft studies were conducted using cp70 cells stably expressing green fluorescent protein (cp70-GFP) subcutaneously transplanted into NU/NU mice. For studies 1 and 2, alpha-TEA was formulated in liposomes and delivered by aerosol such that approximately 36 microg and 72 microg of alpha-TEA were deposited in the respiratory tract of each mouse each day, respectively. Cisplatin at 5 mg/kg was administered by intraperitoneal injections once weekly for the first 3 weeks in Study 1 and on the third and 10th days following treatment initiation in Study 2. The combination alpha-TEA + cisplatin treatment reduced tumor burden and metastasis of cp70-GFP cells in comparison to control mice or mice treated with alpha-TEA or cisplatin singly. A significant reduction (P < 0.001) in growth of subcutaneous transplanted tumors was obtained with alpha-TEA + cisplatin for both studies. Visible metastases were observed in the lungs of animals from control and cisplatin-treated groups but not in animals from the alpha-TEA- or alpha-TEA + cisplatin-treated groups. The alpha-TEA + cisplatin significantly reduced the total number of lung and axillary lymph node micrometastasis (P < 0.03 and P < 0.0001, respectively). Analyses of tumor sections showed the alpha-TEA + cisplatin treatment group, in comparison to control, to have a significantly lower level of cell proliferation (Ki-67 staining; P < 0.0001) and a significantly higher level of apoptosis (terminal deoxynucleotidyl transferase-mediated nick end labeling [TUNEL]; P < 0.0001). In summary, combinations of alpha-TEA + cisplatin significantly reduced tumor burden and metastases in a xenograft model of cisplatin-resistant human ovarian cancer cells. These data show promise for combination alpha-TEA + cisplatin chemotherapy for ovarian cancer.

Administration, Inhalation↗
PubMed2004

The external TEA binding site and C-type inactivation in voltage-gated potassium channels.

The location of the tetraethylammonium (TEA) binding site in the outer vestibule of K+ channels, and the mechanism by which external TEA slows C-type inactivation, have been considered well-understood. The prevailing model has been that TEA is coordinated by four amino acid side chains at the position equivalent to Shaker T449, and that TEA prevents a constriction that underlies inactivation via a foot-in-the-door mechanism at this same position. However, a growing body of evidence has suggested that this picture may not be entirely correct. In this study, we reexamined these two issues, using both the Kv2.1 and Shaker potassium channels. In contrast to results previously obtained with Shaker, substitution of the tyrosine at Kv2.1 position 380 (equivalent to Shaker 449) with a threonine or cysteine had a relatively minor effect on TEA potency. In both Kv2.1 and Shaker, modification of cysteines at position 380/449 by 2-(trimethylammonium)ethyl methanethiosulfonate (MTSET) proceeded at identical rates in the absence and presence of TEA. Additional experiments in Shaker demonstrated that TEA bound well to C-type inactivated channels, but did not interfere with MTSET modification of C449 in inactivated channels. Together, these findings rule out the possibility that TEA binding involves an intimate interaction with the four side chains at the position equivalent to Shaker 449. Moreover, these results argue against the model whereby TEA slows inactivation via a foot-in-the-door mechanism at position 449, and also argue against the hypothesis that the position 449 side chains move toward the center of the conduction pathway during inactivation. Occupancy by TEA completely prevented MTSET modification of a cysteine in the outer-vestibule turret (Kv2.1 position 356/Shaker position 425), which has been shown to interfere with both TEA binding and the interaction of K+ with an external binding site. Together, these data suggest that TEA is stabilized in a more external position in the outer vestibule, and does not bind via direct coordination with any specific outer-vestibule residues.

Binding Sites↗
PubMed2000

Herbal tea in the treatment of diabetes mellitus.

OBJECTIVE: To evaluate the effects of a native herbal tea in patients with type 2 diabetes. DESIGN: Randomized, placebo-controlled, single-blind study. SETTING: The Metabolic Centre at the University of Alberta Hospitals. SUBJECTS: Forty volunteers with type 2 diabetes. INTERVENTIONS: After a 1 month "run-in" period, subjects drank 250 mL/d of either the herbal tea or a placebo tea for 10 days, and were followed up for a further 4 weeks. OUTCOME MEASURES: A responder analysis defined as a 10% change in mean blood glucose levels based on 4 capillary glucose readings daily. Secondary end points included changes in HbA1c, fructosamine and response to a meal challenge using Ensure. RESULTS: The responder analysis showed no benefit from the herbal tea. Fructosamine levels before and after tea therapy decreased significantly in both study groups. Mean HbA1c levels and incremental areas under the glucose curve (AUC) in the meal challenge did not change in either study group. These data were reanalysed in hyperglycemic subjects with HbA1c levels greater than 120% of normal. The responder analysis and HbA1c levels did not change in either group. Mean (and standard deviation) fructosamine levels, before and after tea therapy, were significantly lower in the herbal tea group than in the placebo tea group (361 [98] versus 338 [100] micromol/L, p < 0.01 compared with 338 [60] versus 323 [49] micromol/L, p = 0.08). In the hyperglycemic subgroup the mean AUC during the meal challenge, before versus after tea therapy, was 776 (369) versus 639 (331) mmol/L (p = 0.22) in the herbal tea group and 433 (125) versus 420 (173) mmol/L (p = 0.90) in the placebo group. CONCLUSIONS: Although the responder analysis failed to show an effect of the herbal tea, the data suggest there may be a short-term benefit from the tea in subjects with poor glycemic control.

Aged↗