Do the spectra of maleimide spin-labelled whole blood platelets reflect the structure and conformation of membrane proteins?

The effects of in vivo and in vitro non-enzymatic glycosylation and glycoxidation on physico-chemical properties of haemoglobin in control and diabetic patients

The erythrocyte deformability, which is related to erythrocyte internal viscosity, was suggested to depend upon the physico-chemical properties of haemoglobin. In the present study we employed ESR spectroscopy on order to explore further the extent to which the in vivo or in vitro glycation and/or glycoxidation might affect haemoglobin structure on conformation. We revealed that under both in vivo and in vitro conditions the attachment of glucose induced a mobilization of thiol groups in the selected domains of haemoglobin molecules ( the increased h+1/h0 parameter of maleimide spin label, MSL; 0.277 +/- 0.021 in diabetics vs 0.338 +/- 0.017 in controls, n = 12, P < 0.0001). The relative rotational correlation time (tau c) of two spin labels, TEMPONE and TEMPAMINE, respectively, in erythrocyte insides (5.22 +/- 0.42 in diabetics, n = 21 vs 4.79 +/- 0.38, n = 16 in controls, P < 0.005) and in the solutions of in vitro glycated haemoglobin, were increased. Neither oxidation nor crosslinking of thiol groups was evidenced in glycated and/or oxidized haemoglobin. In addition, erythrocyte deformability was found to be reduced in type 2 diabetic patients (6.71 +/- 1.08, n = 28 vs 7.31 +/- 0.96, n = 21, P < 0.015). In conclusion, these observations suggest that: the attachment of glucose to haemoglobin might have decreased the mobility of the Lys-adjacent Cys residues, thus leading to the increased h+1/h0 parameter of MSL. Such structural changes in haemoglobin owing to non-enzymatic glycosylation may contribute to the increased viscosity of haemoglobin solutions (r = 0.497, P < 0.0035) and the enhanced internal viscosity of diabetic erythrocytes (r = 0.503, P < 0.003). We argue that such changes in haemoglobin, and consequently in red blood cells, might contribute to the handicapped oxygen release under tissue hypoxia in the diabetic state.

Microenvironmental changes in platelet membranes induced by the interaction of fibrinogen-derived peptide ligands with platelet integrins

A few studies have confirmed the influence of peptides containing either the RGD or dodecapeptide H-12-V (HHLGGAKQAGDV) sequence on cell membrane structure and function. In order to consider previous findings and to explore microenvironmental changes associated with the interaction of these two fibrinogen-derived peptides with platelet membranes, we employed fluorescence quenching and electron paramagnetic resonance techniques to monitor the possible alterations in platelet membrane dynamics induced by RGDS and H-12-V. The interaction of RGDS with platelet membranes resulted in reduced values of the h+1/ho parameter in both 5-doxylstearic acid and 16-doxylstearic acid spectra indicating a significant rigidification of the membrane lipid bilayer. Otherwise, the fibrinogen-derived peptide that contained the gamma chain C-terminal sequence H-12-V had a fluidizing effect on the platelet membrane lipid bilayer. The labelling of platelet membranes with 1-anilino-8-naphthalenesulphonate (ANS) enabled us to estimate the energy transfer efficiency and the apparent interchromophore distance between membrane protein tryptophan and ANS embedded into the membrane lipid bilayer. As RGDS interacts with platelet membrane this distance decreases, resulting in the relevant increase of energy transfer efficiency. The opposite alterations were recorded upon interaction of platelet membranes with H-12-V. Furthermore, a small shift towards longer wavelengths, which accompanies the spectra of ANS in control platelet membranes, vanishes during the interaction with the peptide H-12-V. This observation can be accounted for by a decrease in the polarity of the ANS environment, and may suggest an enhanced contact of the membrane tryptophan with phospholipid fatty acids. Thus, the data indicate that after the action of H-12-V on platelet membrane receptors, the membrane tryptophan residues become exposed to the external environment and the quenchable fraction of membrane tryptophan becomes smaller. The increase (a) in the relative rotational correlation time (tau c) of 4-(ethoxyfluorophosphinyloxy)-2,2,6,6-tetramethylpiperidine- 1-oxyl (ethoxyfluorophosphinyloxy-TEMPO) and (b) in the hw/hs ratio in the spectra of 4-maleimido-2,2,6,6-tetramethylpiperidine-1-oxyl (maleimido-TEMPO) indicate that under these conditions there is an effective immobilization of some domains located on the hydrated surface of membrane proteins and mobilization of those domains buried inside the membrane protein molecules. The interaction of RGDS with platelet membrane integrins resulted in contrary effects, as compared to H-12-V. In conclusion, our spectroscopic data indicate that these two fibrinogen-derived peptides induce opposite effects in the dynamics of platelet membrane components.