Plagiochilines C,D, E and F,four novel secoaromadendrane-type sesquiterpene hemiacetals from Plagiochila asplenioides and Plagiochila semidecurrens

Four novel secoaromadendrane-type sesquiterpene hemiacetals, plagiochilines C, D, E and F, together with the previously known (?)-bicyclogermacrene, have been isolated from the liverwort, Plagiochila asplenioides and their structures have been spectroscopically elucidated. From Plagiochila semidecurrens plagiochilines A and C have been obtained along with (?)-bicyclogermacrene and its related sesquiterpene hydrocarbons.     

Glucuronidation of the green tea catechins, (-)-epigallocatechin-3-gallate and (-)-epicatechin-3-gallate, by rat hepatic and intestinal microsomes

The flavonoids (-)-epigallocatechin-3-gallate (EGCg) and (-)-epicatechin-3-gallate (ECg) are major components of green tea and show numerous biological effects. We investigated the glucuronidation of these compounds and of quercetin by microsomes. Quercetin was almost fully glucuronidated by liver microsomes after 3 h, whereas ECg and ECGg were conjugated to a lesser extent ([Formula: See Text] and [Formula: See Text] respectively). The intestinal microsomes also glucuronidated quercetin much more efficiently than ECg and EGCg. Although the rates were lower than quercetin, intestinal microsomes exhibited higher activity on the galloyl group of ECg and EGCg compared to the flavonoid ring, whereas hepatic glucuronidation was higher on the flavonoid ring of EGCg and ECg compared to the galloyl groups. The low glucuronidation rates could partially explain why these flavanols are present in plasma as unconjugated forms.     

EGCG attenuates AMPA-induced intracellular calcium increase in hippocampal neurons

We have investigated the protective effect of (-)-epigallocatechin gallate (EGCG) on alpha-amino-3-hydroxy-5-methyl-4-isoxazolo propionate (AMPA)-induced toxicity in cultured rat hippocampal neurons. Treatment of 24 h AMPA (10 microM) reduced the neuronal viability in both survival neuron counting and MTT reduction assay compared with control, with increase in cellular concentrations of hydrogen peroxide and malondialdehyde. These responses to AMPA were significantly blocked by co-treatments with EGCG (10 microM), which effect was very similar to the protective ability of a known antioxidant catalase (2000 U/ml). AMPA (50 microM) elicited the increase in intracellular calcium concentration ([Ca(2+)]i) on which EGCG significantly attenuated both peak amplitude and sustained nature of that [Ca(2+)]i increase in a dose-dependent manner. These data suggest that EGCG has a neuroprotective effect against AMPA through inhibition of AMPA-induced [Ca(2+)]i increase and consequent attenuation of reactive oxygen species production and lipid peroxidation as an antioxidant and a radical scavenger.     

表没食子儿茶素没食子酸酯研究进展

近些年发现,茶叶中的表没食子儿茶素没食子酸脂具有降低血脂、除去胆固醇、抑制细菌和病毒、抗氧化、抗突变、抗肿瘤等多方面的重要生理功能。     

Green tea epigallocatechin gallate binds to and inhibits respiratory complexes in swelling but not normal rat hepatic mitochondria

Epigallocatechin gallate (EGCG), the major flavonoid in green tea, is consumed via tea products and dietary supplements, and has been tested in clinical trials. However, EGCG can cause hepatotoxicity in humans and animals by unknown mechanisms. Here EGCG effects on rat liver mitochondria were examined. EGCG showed negligible effects on oxidative phosphorylation at 7.5–100 μM in normal mitochondria. However, respiratory chain complexes (RCCs) were profoundly inhibited by EGCG in mitochondria undergoing Ca²⁺ overload-induced mitochondrial permeability transition (MPT). As RCCs are located in mitochondrial inner membranes (IM) and matrix, it was reasoned that EGCG could not readily pass through IM to affect RCCs in normal mitochondria but may do so when IM integrity is compromised. This speculation was substantiated in three ways. (1) Purified EGCG-bound proteins were barely detectable in normal mitochondria and contained no RCCs as determined by Western blotting, but swelling mitochondria contained about 1.5-fold more EGCG-bound proteins which included four RCC subunits together with cyclophilin D that locates in mitochondrial matrix. (2) Swelling mitochondria consumed more EGCG than normal ones. (3) The MPT blocker cyclosporine A diminished the above-mentioned difference. Among four subunits of RCC II, only SDHA and SDHB which locate in mitochondrial matrix, but not SDHC or SDHD which insert into the IM, were found to be EGCG targets. Interestingly, EGCG promoted Ca²⁺ overload-induced MPT only when moderate MPT already commenced. This study identified hepatic RCCs as targets for EGCG in swelling but not normal mitochondria, suggesting EGCG may trigger hepatotoxicity by worsening pre-existing mitochondria abnormalities.     

Polyphenol‐solubility alters amyloid fibril formation of α‐synuclein

Amyloid fibril formation is associated with various amyloidoses, including neurodegenerative diseases such as Alzheimer''s and Parkinson''s diseases. Amyloid fibrils form above the solubility of amyloidogenic proteins or peptides upon breaking supersaturation, followed by a nucleation and elongation mechanism, which is similar to the crystallization of solutes. Many additives, including salts, detergents, and natural compounds, promote or inhibit amyloid formation. However, the underlying mechanisms of the opposing effects are unclear. We examined the effects of two polyphenols, that is, epigallocatechin gallate (EGCG) and kaempferol‐7─O─glycoside (KG), with high and low solubilities, respectively, on the amyloid formation of α‐synuclein (αSN). EGCG and KG inhibited and promoted amyloid formation of αSN, respectively, when monitored by thioflavin T (ThT) fluorescence or transmission electron microscopy (TEM). Nuclear magnetic resonance (NMR) analysis revealed that, although interactions of αSN with soluble EGCG increased the solubility of αSN, thus inhibiting amyloid formation, interactions of αSN with insoluble KG reduced the solubility of αSN, thereby promoting amyloid formation. Our study suggests that opposing effects of polyphenols on amyloid formation of proteins and peptides can be interpreted based on the solubility of polyphenols.     

GUV preparation and imaging: Minimizing artifacts

The components of biological membranes are present in a physical mixture. The nonrandom ways that the molecules of lipids and proteins mix together can strongly influence the association of proteins with each other, and the chemical reactions that occur in the membrane, or that are mediated by the membrane. A particular type of nonrandom mixing is the separation of compositionally distinct phases. Any such phase separation would result in preferential partition of some proteins and lipids between the coexisting phases, and thus would influence which proteins could be in contact, and whether a protein could find its target. Phase separation in a plasma membrane would also influence the binding of molecules from outside the cell to the membrane, including recognition proteins on viruses, bacteria, and other cells. The concept of these and other events associated with membrane phase separation are sometimes grouped together as the “raft model” of biological membranes. Several types of experiments are aimed at detecting and characterizing membrane phase separation. Visualizing phase separation has special value, both because the immiscibility is so decisively determined, and also because the type of phase can often be identified. The fluorescence microscope has proven uniquely useful for yielding images of separated phases, both in certain cell preparations, and especially in models of cell membranes. Here we discuss ways to prepare useful model membranes for image studies, and how to avoid some of the artifacts that can plague these studies.     

Differences in binding behavior of (−)‐epigallocatechin gallate to β‐lactoglobulin heterodimers (AB) compared to homodimers (A) and (B)

The lipocalin β‐lactoglobulin (β‐LG) exists in different natural genetic variants—of which β‐LG A and B are predominant in bovine milk. At physiological conditions the protein dimerizes—building homodimers of β‐LG A and β‐LG B and heterodimers of β‐LG AB. Although β‐LG is one of the most intensely characterized lipocalins, the interaction behavior of ligands with hetero‐ and homodimers of β‐LG is largely unknown. The present findings revealed significant differences for hetero‐ and homodimers regarding ligand binding capacity as tested with a model ligand (i.e. surface binding (?)‐epigallocatechin gallate (EGCG)). These findings were confirmed using FT‐IR, where the addition of EGCG influenced the β‐sheet backbone of homodimer A and B with significantly higher intensity compared to heterodimer AB. Further, shape analysis by SAXS revealed oligomerization of both types of dimers upon addition of EGCG; however, homodimer A and B produced significantly larger aggregates compared to the heterodimer AB. In summary, the present study revealed that EGCG showed significantly different interaction reactivity (binding sites, aggregation size and conformational changes) to the hetero and homodimers of β‐LG in the order β‐LG A > B > AB. The results suggest that conformational differences between homodimers and heterodimers strongly influence the EGCG binding ability. This may also occur with other polyphenols and ligands of β‐LG and gives not only important information for β‐LG binding studies, but may also apply for polymorphisms of other self‐aggregating lipocalins. Copyright © 2015 John Wiley & Sons, Ltd.