Tryptophan fluorescence of mitochondrial complex III reconstituted in phosphatidylcholine bilayers

Characterization of phenoloxidase activity from spider Polybetes pythagoricus hemocyanin

Hemocyanin of the spider Polybetes pythagoricus, in addition to its typical role as an oxygen transporter, also exhibits a phenoloxidase activity induced by micellar concentrations of SDS. In the present work, we found the kinetic parameters Km and Vmax of Polybetes pythagoricus hemocyanin (PpHc) PO activity to be 0.407 mM and 0.081 µmolmin(-1) mg protein(-1) , respectively. Dopamine was used as the substrate with SDS at a final concentration of 10 mM and a 30-min incubation at 25°C. Conformational changes in Hc associated with the SDS treatment were analyzed using far-UV circular dichroism, intrinsic fluorescence and absorption spectroscopy. The secondary and tertiary structural changes of PpHc induced by SDS led to increases in α-helical content and tryptophan fluorescence intensity. A reduction in the absorption spectrum at 340 nm in the presence of SDS was also observed. These results suggest that the SDS-induced PO activity of PpHc can be ascribed to conformational changes in the local environment of the typer-3 copper active site.

Contribution of the C-terminal end of apolipoprotein AI to neutralization of lipopolysaccharide endotoxic effect

It is well known that high density lipoprotein (HDL) binds bacterial lipopolysaccharide (LPS) and neutralizes its toxicity. The aim of this work was to study changes in the apolipoprotein (apo) AI structure after its interaction with LPS as well as to determine the protein domain involved in that interaction. The presented data indicate that LPS does not lead to major changes in the structure of apoAI, judging from Trp fluorescence spectra. However, analysis of denaturation behavior and binding of ANS show that LPS induces a loosened protein conformation. Further evidence for an apoAI—LPS specific interaction was obtained by incubation of the protein with 125I-ASD-LPS. The results show that multiple regions of the protein were able to interact with LPS, according to its amphiphatic nature. Finally, the contribution of the purified C-terminal fragment of the protein in the endotoxin neutralization was evaluated in comparison with the effect of apoAI. In both cases, the same decrease in tumor necrosis factor-α released was observed. This result suggests that the C-terminal half of apoAI is the main domain responsible of the neutralization effect of this protein. Our data may provide innovative pharmacological tools in endotoxin neutralization therapies.

Role of lipopolysaccharide on the structure and function of alpha-hemolysin from Escherichia coli

alpha-Hemolysin (HlyA) is a protein toxin (107 kDa) secreted by some pathogenic strains of E. coli. Several studies suggested the relationship between HlyA and lipopolysaccharide (LPS). We have studied experimentally the role of LPS on the stability and function of this toxin. The HlyA conformation in both, LPS-free and LPS-bound forms was investigated by tryptophan fluorescence. Studies about HlyA thermal and chemical denaturation indicated that its stability increased in the presence of LPS. On the other hand, the presence of negative and polar residues on the LPS reduced the tendency of HlyA to self-aggregation, and they may be the reservoir of calcium, cation essential for the lytic action of this toxin on red blood cells. These results suggest that HlyA and LPS are combined mainly via hydrophobic force to form an active toxin which stability is favored by the LPS.

Location of tryptophan residues in free and membrane bound Escherichia coli alpha-hemolysin and their role on the lytic membrane properties

alpha-hemolysin (HlyA) is an extracellular protein toxin secreted by Escherichia coli that acts at the level of plasma cell membranes of target eukaryotic cells. Previous studies showed that toxin binding to the bilayers occurs in at least two ways, a reversible adsorption and an irreversible insertion. Studies of HlyA insertion into bilayers formed from phosphatidylcholine show that insertion is accompanied by an increase in the protein intrinsic fluorescence. In order to better define structural parameters of the membrane-bound form, the location of tryptophan residues was studied by means of quenchers of their intrinsic fluorescence located at 7, 12 and 16 positions of the acyl chain of phosphatidylcholine. The quenching was progressively weaker suggesting an interfacial location of the Trp. In parallel, HlyA was subjected to oxidation with N-bromosuccinimide to study the role of Trp residues exposed to aqueous media in its structure-function relationship. In the folded toxin molecule, a single residue was susceptible to oxidation with NBS, whereas incubation with LUV of the toxin prior modification prevents its oxidation, suggesting that Trp residue(s) are directly involved in toxin binding and insertion. Finally, the modification of residues exposed to solvent resulted in a complete impairment of the lytic activity. It was concluded that the modification-sensitive Trp residues are essential for the structure and function of native HlyA. These results are consistent with the model proposed by Soloaga et al. (Mol. Microbiol. 31 (1999) 1013-1024) according to which HlyA is bound to a single monolayer through a number of amphipathic instead of inserted transmembrane helices.

Fluorescence quenching at interfaces and the permeation of acrylamide and iodide across phospholipid bilayers

Studies of fluorescence quenching in membrane proteins are complicated by the fact that the barrier effect of the bilayer towards the quenchers is not known with precision. Our studies show that (a) both acrylamide and iodide can permeate the membrane at comparable rates, (b) when quenchers are added externally to a vesicle suspension, the apparent Stern-Volmer quenching constants for the same fluorophores are lower in the inner than in the outer aqueous compartments, and (c) at least some non-polar fluorophores embedded in the bilayer are quenched by iodide, but not by acrylamide.