Interaction of apolipoprotein-alamine and apolipoprotein-glutamine-I with phosphatidylcholine. Effect on solute quenching of intrinsic fluorescence

Luminescence of Amphiphilic PtII Complexes Controlled by Confinement

The formation of hybrid silica-based systems to study the effect of the confinement on the emission properties of self-assembled platinum(II) complexes is reported. The complexes behave as surfactants since they possess a hydrophobic moiety and, on the ancillary ligand, a relatively long hydrophilic chain terminated with a positively charged group. The compounds, soluble in water, self-assemble, even at very low concentration, in supramolecular structures which display an orange luminescence. The properties of the assemblies have been studied in detail and in order to stabilize these supramolecular architectures and to enhance their emission properties hybrid silica porous nanoparticles have been prepared. In particular the Pt^II complexes have been employed as co-surfactant for the template formation of mesoporous silica nanoparticles (MSNs) using a sol gel synthesis. Interestingly, upon encapsulation in the silica pores, the platinum aggregates exhibit an emission profile similar in energy to the complexes assembled in solution, but the photoluminescence quantum yields of the hybrid systems are significantly higher (up to 45 %), and the excited state lifetimes much longer than those recorded in solution. Such enhancement of the photophysical properties together with the possibility to process the hybrid silica nanomaterials can pave the way to new type of emitters.

Chemical modification of lysine residues in cytochrome P450LM2 (P450IIB4): influence on heme liganding of arylamines

Treatment of cytochrome P450LM2 with fluorescein isothiocyanate to introduce up to two equivalents of fluorophore per polypeptide chain resulted in the selective derivatization of lysine residues. CD-spectral measurements revealed the overall conformation as well as the immediate heme environment of the hemoprotein to remain unaffected by attachment of the label. Modification caused decreased affinity of p-phenylenediamine and other 4-substituted anilines for the heme site, whereas there was a rise in the extent of substrate interaction. Experiments with pigment containing acetylated lysines gave analogous results, suggesting that the observed phenomenon was due to charge neutralization. There was linear correlation between the Hammett sigma P values and both the optical dissociation constants for arylamine binding to intact enzyme and the dipole moments of the anilines, indicating that basicity along with electronic factors controlled heme liganding; lipophilicity appeared to be of minor importance. Introduction of fluorescein isothiocyanate into the oxygenase was found to influence the bond-making process through modulating basicity of the nitrogenous compounds, but perturbation of optimal spacial orientation of the amine nitrogen toward the heme iron also might have been operative. The lysines studied seem to represent metabolically inactive elements of the substrate channel located on the cytosolic surface of the aggregates, as evidenced by steady-state fluorescence measurements. A hydrophilic segment in the cytochrome P450LM2 molecule that would accommodate the critical residues is discussed.

Non enzymatic glycation of apolipoprotein A-I. Effects on its self-association and lipid binding properties

In diabetic patients, hyperglycaemia results in the non enzymatic glycation of many proteins including apolipoprotein A-I. We purified glycated apo A-I and compared its lipid binding properties to those of normal apo A-I. Analysis of tryptophan fluorescence spectra and of fluorescence quenching in the presence of iodine showed that glycation of apo A-I induces a decrease in the stability of the lipid-apoprotein interaction and in that of the apoprotein self-association. Repetitive ultracentrifugations of High Density Lipoprotein (HDL) samples containing radioiodinated apo A-I or glycated apo A-I revealed that glycation of the apoprotein facilitates its dissociation from HDL. These results suggest that the non enzymatic glycation of apo A-I may affect the structural cohesion of HDL particles.

Hydrolysis of phosphatidylcholine by phospholipase A2 does not cause dissociation of apolipoprotein C from rat plasma very low density lipoprotein

To test the hypothesis that hydrolysis of glycerophosphatides causes displacement of apolipoprotein C from very low density lipoprotein, we have studied the effects of a snake venom phospholipase A2 on very low density lipoprotein labeled with [125I]apoC, [3H]cholesterol, [14C]palmitate and [32P]phospholipids. In spite of hydrolysis of 97% of the phosphatidylcholine, only small amounts of labeled apoC and labeled cholesterol were displaced from the very low density lipoprotein. With purified lipoprotein lipase in contrast, 80-90% of the labeled apoC and cholesterol were removed from the lipoprotein. It is concluded that hydrolysis of phosphatidylcholine does not cause an appreciable dissociation of apolipoprotein C from very low density lipoprotein.

Lipid-protein interactions in the plasma lipoproteins

The purpose of this review has been to discuss new information about the mechanism of lipid and apoprotein interaction in the plasma lipoproteins. A special form of the amphipathic helix has been identified as a major structural element of the apolipoproteins sequenced to date. Evidence is reviewed concerning the role of the amphipathic helix in the binding to phospholipids. Several different models for the organization of the components of HDL, LDL and LP-X have evolved from extensive structural studies. Resolution of the differences among these models will require additional experimental testing. Verification of models based on the study of reconstituted HDL will require rigorous proof of native structure in these particles. A detailed description of the molecular organization of the lipid and protein constituents of the plasma lipoproteins is still lacking. Further structural and sequence studies with apoB and the "arginine-rich" protein are needed. Crystallization of an apoprotein or lipoprotein and determination of the three-dimensional structure would be a major achievement. With such further detailed structural information, it may then be possible to correlate changes in structure with determinants of metabolism.

Vertical displacement of membrane proteins mediated by changes in microviscosity

Membrane proteins of intact human erythrocytes were labeled with two fluorescent sulfhydryl reagents. The tagged cells were then subjected to simultaneous liposome treatments for either depletion or enrichment of membrane cholesterol content. Cholesterol depletion, which reduces membrane microviscosity, was followed by a series of fluorescence changes all indicating masking of the membrane proteins. Conversely increasing the membrane microviscosity by cholesterol enrichment resulted in an appreciable increase of the protein exposure to the aqueous surrounding. These findings strongly suggest that membrane proteins may be vertically displaced upon changes in lipid fluidity, a mechanism that may play a significant role in modulation of antigens and receptors in vivo.

An investigation of the electronic and steric environments of tyrosyl residues in ribonuclease A and Erwinia carotovora L-asparaginase through fluorescence quenching by caesium, iodide and phosphate ions

The fluorescence lifetimes and relative quantum yields of several derivatives of tyrosine are reported. The quenching of the fluorescence of these compounds by phosphate, caesium and iodide ions has been investigated; the encounter rate constants, calculated from the quenching parameters and lifetimes, show a clear dependence on the charges borne by the quenchers and fluorophores. The ratio of the Stern-Volmer constants of iodide and caesium, ions of similar size, defines an electrostatic parameter sensitive to the charge of the fluorophore which can be evaluated without knowledge of the fluorescent lifetimes. The mean of the encounter rate constants for caesium and iodide ions defines a rate constant which is largely charge-independent and is used to establish a steric parameter. The two parameters are used to investigate the tyrosine environment in bovine ribonuclease A (EC 3.1.4.23) and Erwinia carotovora L-asparaginase (EC 3.5.1.1). The quantum yield of L-asparaginase (0.12) is very high for a class A protein and may be associated with the absence of disulphide bridges. There was no evidence for more than one type of tyrosine residue from the quenching experiments with either enzyme, an observation which is attributed to efficient energy transfer amongst tyrosine residues. At pH values close to the isoelectric points of the enzymes the electrostatic parameter suggests that the environment of the quenchable tyrosines in L-asparaginase is somewhat more positive than in ribonuclease. In 1% sodium dodecyl sulphate the tyrosine environment of L-asparaginase becomes markedly negative as expected. The steric parameter indicates a lower accessibility of the tyrosine residues in L-asparaginase than in ribonuclease; an illustrative calculation is provided linking the steric parameter with the number of exposed tyrosine residues by taking into account the greater collision frequency of the larger protein molecules and the encounter distance for quenching determined from charge effects on the quenching of the model compounds. The calculation suggests that three tyrosyl residues are accessible in ribonuclease, in good agreement with other studies, but in L-asparaginase the number increases from 0.4 at pH 5.73 to 0.8 at pH 9.16 suggesting a loosening of the enzyme structure at high pH.

Interaction of apoprotein from porcine high-density lipoprotein with dimyristoly lecithin. 2. Nature of lipid-protein interaction

The detailed molecular structure of the complex formed by the apoprotein from porcine high density lipoprotein and dimyristoly phosphatidylcholine (lecithin) has been investigated by a range of physical techniques. The complex, an oblate ellipsoid with major axis 11.0 nm and minor axis 5.5 nm (see the accompanying paper), is comprised of a section of lecithin bilayer with apoprotein at the surface. The main site of interaction between protein and lipid is in the lipid glycerophosphorylcholine group region; as with native high density lipoprotein the surface of the particle consists of a mosaic of lecithin polar groups and protein. The formation of this mosaic reduces the cooperativity of the lecithin chain motions and changes the curvature of the lipid-water interface, as compared to a bilayer. Otherwise, there are no major changes in lecithin motions indicating that no strong binding of lipid to protein occurs. The interaction involves the intercalation of amphipathic, 60% alpha-helical, apoprotein molecules among the lecithin molecules so that the protein residues at the lipid-water interface. The apoprotein has a high affinity for the lipid-water interface but specific lipid-protein interactions are not involved.