Fibrinogen-β-Estradiol Binding Studied by Fluorescence Spectroscopy: Denaturation and pH Effects

Attenuation of Thrombin-Mediated Fibrin Formation via Changes in Fibrinogen Conformation Induced by Reaction with S-nitroso-N-acetylpenicillamine, but not S-nitrosoglutathione

Previous work in a 4 h rabbit thrombogenicity model has shown that a nitric oxide- (NO) generating polymer extracorporeal circuits (ECC) with infusion of S-nitroso-N-acetyl-penicillamine (SNAP) preserved platelets eventhough platelets were activated as shown by an increase in the glycoprotein, p-selectin. The platelet preservation mechanism was shown to be due to a changing fibrinogen structure leading to attenuation of platelet aggregation. Understanding the effects that SNAP, another RSNO, S-nitroso-glutathione (GSNO) as well as the non-RSNO, sodium nitroprusside (SNP), may have on human fibrinogen polymerization, this in vitro study evaluated the released NO effects on the thrombin-mediated fibrin formation and fibrinogen structure. Thrombin-induced fibrin formation at 300 μM SNAP (50 + 11% of baseline) was significantly reduced compared to SNAP's parent, N-acetyl-penicillamine (NAP) (95 + 13%) after 1 h of RSNO exposure. GSNO, its parent, glutathione (GSH) and 1000 ppm NO gas did not attenuate the thrombin-mediated fibrin formation. SNAP, NAP and SNP exposure for 1 h, however, did not decrease thrombin activity by directly inhibiting thrombin itself. Changes in fibrinogen conformation as measured by intrinsic tryptophan fluorescence significantly decreased in the 300 μM SNAP (38057 + 1196 mean fluorescence intensity (MFI) and SNP (368617 + 541 MFI) groups versus the NAP control (47937 + 1196 MFI). However, infused 1000 ppm NO gas had no direct effect on the ITF after 1 h incubation at 37°C. High performance liquid chromatography (HPLC) showed that fibrinogen degradation by 0.03 U/ml thrombin was concentration-dependently reduced after 1 h with SNAP but not with NAP or SNP. Western blotting showed RSNOs, SNAP, NAP and the non-RSNO, SNP-incubated fibrinogen solutions showed that the percent level of the Aγ dimer to total Aγ dimer + γ monomer was significantly reduced in the case of the SNAP group when compared to SNP group. These results suggest that NO donors such as SNAP and SNP induce fibrinogen conformational changes by potentially nitrosating fibrinogen tyrosine residues. These NO-mediated fibrinogen changes induced via NO donors may provide another mechanism of NO for improving thromboresistance in ECC.

Denaturing of single electrospun fibrinogen fibers studied by deep ultraviolet fluorescence microscopy

Deep ultraviolet (DUV) microscopy is a fluorescence microscopy technique to image unlabeled proteins via the native fluorescence of some of their amino acids. We constructed a DUV fluorescence microscope, capable of 280 nm wavelength excitation by modifying an inverted optical microscope. Moreover, we integrated a nanomanipulator-controlled micropipette into this instrument for precise delivery of picoliter amounts of fluid to selected regions of the sample. In proof-of-principle experiments, we used this instrument to study, in situ, the effect of a denaturing agent on the autofluorescence intensity of single, unlabeled, electrospun fibrinogen nanofibers. Autofluorescence emission from the nanofibers was excited at 280 nm and detected at ∼350 nm. A denaturant solution was discretely applied to small, select sections of the nanofibers and a clear local reduction in autofluorescence intensity was observed. This reduction is attributed to the dissolution of the fibers and the unfolding of proteins in the fibers.

Microcompartmentation in artificial cells: pH-induced conformational changes alter protein localization

We report artificial cells in which protein localization in a primitive synthetic model for the cytoplasm is controlled by pH. Our model cells are giant lipid vesicles (GVs, ca. 5-30 microm diameter) with two coexisting aqueous compartments generated by phase separation of an encapsulated poly(ethylene glycol) (PEG) and dextran solution. Proteins are localized to a microcompartment by partitioning between the phases. We quantified the local concentration of fluorescently labeled human serum albumin (HSA) via confocal fluorescence microscopy. At pH 6.5, the labeled HSA was more concentrated in the dextran-rich phase, but at partially/fully denaturing pH (4.1 or 12) it was localized in the PEG-rich phase. This partitioning behavior is consistent with a more expanded, hydrophobic conformation at low and high pH. Labeled HSA could be relocalized from the PEG-rich into the dextran-rich phase domain by increasing the pH from 4.1 to 6.5 to renature the protein. This approach to controlling protein localization does not require extensive reorganization of the vesicle interior; coexisting PEG-rich and dextran-rich compartments are maintained throughout the experiments. It is also quite general; we demonstrated that several other proteins varying in size and isoelectric point also relocalized within compartmentalized artificial cells in response to external pH change. This work presents stimulus-responsive protein relocalization between compartments in an artificial cell; such experimental models can provide a framework for investigating the consequences of protein localization in cell biology.

Fibrinogen-dependent signaling in microvascular erythrocyte function: implications on nitric oxide efflux

Experimental evidence has shown that plasma fibrinogen plays a key role as a major cardiovascular risk factor, acting directly to trigger erythrocyte aggregation in occlusive vascular disease. However, due to the complex and hitherto unclear interaction between fibrinogen and the erythrocyte membrane, no study has yet evaluated the effects of fibrinogen, under physiological range values, on the erythrocyte nitric oxide (NO) mobilization. Taking into consideration the potential NO-derived molecules, we have raised the hypothesis that fibrinogen, under physiological conditions, may act to influence blood flow via erythrocyte NO modulation. In this in vitro study whole-blood samples were harvested from healthy subjects, erythrocyte suspensions were incubated in the absence (control aliquots) and presence of different fibrinogen concentrations and levels of NO, nitrite, nitrate and S-nitroglutathione (GSNO) were determined. Our results showed, when compared with control aliquots, that the presence of fibrinogen modulates the NO mobilization in erythrocytes by (1) decreasing erythrocyte NO efflux levels (P < 0.001); (2) increasing levels of intraerythrocytic NO oxidative metabolites, namely, nitrite (P < 0.0001) and nitrate (P < 0.0001); and (3) enhancing the formation of GSNO (P < 0.001). In conclusion, this study provides new insights into an unknown mechanism by which fibrinogen modulates the erythrocyte capacity to supply NO, the effects of which on inflammation profiles (generally associated with blood hyperviscosity and hyperaggregation) still need to be elucidated. Also, increased erythrocyte GSNO levels may be associated with platelet NO metabolism, its activation status and hypotension, which may be extremely relevant in the clinical setting as biomarkers.

Haemostatic actions of the folkloric medicinal plant extract Ankaferd Blood Stopper

Ankaferd Blood Stopper (ABS), a standardized mixture of five plants, has been used historically as a haemostatic agent but its mechanism of action remains unknown. This study investigated the in vitro effects of ABS on haemostatic parameters. When added to plasma or serum, ABS induced the very rapid formation of a protein network and erythrocyte aggregation. The levels of coagulation factors II, V, VII, VIII, IX, X, XI, and XIII were not affected by ABS. Plasma fibrinogen activity and antigen levels were decreased following the addition of ABS, in parallel with the prolonged thrombin time. Total protein, albumin, and globulin levels decreased after the addition of ABS. Our findings suggest that ABS stimulates the formation of an encapsulated protein network that provides focal points for erythrocyte aggregation. ABS has the therapeutic potential to be used for the management of haemorrhage and this agent should be investigated further in clinical trials.

Fluorescence spectroscopy evaluation of fibrinogen–β-estradiol binding

Fluorescence spectroscopy experiments were performed in order to study conformational changes induced by the binding of beta-estradiol to fibrinogen at different ligand concentrations. The association constant (Ka) obtained for the fibrinogen-beta-estradiol binding was 6.47x10(6)M(-1), indicating a high affinity interaction. Fluorescence quenching experiments showed that approximately 30% of the tryptophan residues in the protein quaternary structure are accessible to ionic quenchers. The extent of quenching in the absence and presence of beta-estradiol was maximum for cesium ions and minimum for iodide, suggesting the presence of negatively charged residues in the vicinity of the tryptophan residues. The quenching parameters obtained at different beta-estradiol concentrations show alterations that confirm a conformational change, possibly due to a discrete reorganization of tryptophan residues during fibrinogen-beta-estradiol binding. This binding may be responsible for the effects of beta-estradiol on the decrease of erythrocyte aggregation and on cardiovascular risk reduction.