Influence of Antipsychotic Drug Risperidone on Human Serum Albumin Affinity to Organic Anions.

BACKGROUND: Risperidone is an antipsychotic drug. In blood, this drug binds mainly to human serum albumin (HSA) and is also transported by HSA. METHOD: To study certain details of the interaction between risperidone and HSA, a fluorescent dye CAPIDAN was used as a reporter. This dye specifically fluoresces from HSA in serum and is highly sensitive to structural changes in HSA including pathology-induced changes. Interaction of CAPIDAN with HSA has been studied using time-resolved fluorescence techniques. RESULT: The addition of phenylbutazone, a marker for the HSA drug-binding site I, leads to displacement of CAPIDAN from this site due to direct competition between phenylbutazone and the dye. The addition of risperidone induces a response of CAPIDAN fluorescence that is highly similar to its response to phenylbutazone. This response depends strongly on ionic strength and is very similar in both cases, phenylbutazone and risperidone. This similarity suggests that risperidone binds to HSA in the region of site I. In this site, the risperidone molecule probably covers the positive charge of Arginine 218 or Arginine 222 preventing their interaction with the CAPIDAN negatively charged carboxyl group. This effect was observed both in isolated HSA and in serum, suggesting similarity of the interaction. CONCLUSION: Thus, risperidone is able to prevent binding of organic anions (i.e. CAPIDAN as a drug-like molecule) to HSA.

A fluorescent reporter detects details of aromatic ligand interference in drug-binding sites of human serum albumin.

Human serum albumin (HSA) transports many ligands including small aromatic molecules: metabolites, drugs etc. Phenylbutazone is an anti-inflammatory drug, which binds to the drug-binding site I of HSA. Its interaction with this site has been studied using a fluorescent dye, CAPIDAN, whose fluorescence in serum originates from HSA and is sensitive to the changes in HSA site I in some diseases. Its fluorescence in HSA solutions is strongly suppressed by phenylbutazone. This phenomenon seems to be a basic sign of a simple drug-dye competition. However, a more detailed study of the time-resolved fluorescence decay of CAPIDAN has shown that phenylbutazone lowers fluorescence without changing the total amount of bound dye. In brief, the HSA-bound dye forms three populations due to three types of environment at the binding sites. The first two populations probably have a rather strong Coulomb interaction with the positive charge of residues Arginine 218 or Arginine 222 in site I and are responsible for approximately 90% of the total fluorescence. Phenylbutazone blocks this interaction and therefore lowers this fluorescence. At the same time the binding of the third population increases considerably in the presence of phenylbutazone, and, as a result, the actual number of bound dye molecules remains almost unchanged despite the ligand competition. So, time resolved fluorescence of the reporter allows to observe details of interactions and interference of aromatic ligands in drug binding site I of HSA both in isolated HSA and in serum.

Cholesteryl ester diffusion, location and self-association constraints determine CETP activity with discoidal HDL: excimer probe study.

The transfer of cholesteryl ester by recombinant cholesteryl ester transfer protein (CETP) between reconstituted discoidal high-density lipoprotein (rHDL) was studied. Particles contained apolipoprotein A-I, unsaturated POPC or saturated DPPC and cholesteryl ester as cholesteryl 1-pyrenedecanoate (CPD) or cholesteryl laurate (CL) in donor and acceptor rHDL, respectively. Probe dynamics fulfilled the quenching sphere-of-action model. The cholesteryl ester exchange between donor and acceptor particles was characterized by a heterogeneous kinetics; the fast exchanging CPD pool was much higher in a case of POPC compared to DPPC complexes. Probe fraction accessible to CETP increased with temperature, suggesting a more homogeneous probe distribution. Noncompetitive inhibition of probe transfer by acceptor particles was observed. The values of Vmax (0.063µMmin(-1)) and catalytic rate constant kcat (0.42s(-1)) together with a similarity of Km (0.9µM CPD) and KI (2.8µM CL) values for POPC-containing rHDL suggest the efficient cholesteryl ester transfer between nascent HDL with unsaturated phosphatidylcholine in vivo. The phospholipid matrix in discoidal HDL may underlie CETP activity through the self-association, diffusivity and location of cholesteryl ester in the bilayer, the accessibility of cholesteryl ester to cholesterol-binding site in apoA-I structure and the binding of cholesteryl ester, positionable by apoA-I, to CETP.

Electronically excited states of membrane fluorescent probe 4-dimethylaminochalcone. Results of quantum chemical calculations.

Quantum-chemical calculations of ground and excited states for membrane fluorescent probe 4-dimethylaminochalcone (DMAC) in vacuum were performed. Optimized geometries and dipole moments for lowest-lying singlet and triplet states were obtained. The nature of these electronic transitions and the relaxation path in the excited states were determined; changes in geometry and charge distribution were assessed. It was shown that in vacuum the lowest existed level is of (n, π*) nature, and the closest to it is the level of (π, π*) nature; the energy gap between them is narrow. This led to an effective (1)(π, π*) →(1)(n, π*) relaxation. After photoexcitation the molecule undergoes significant transformations, including changes in bond orders, pyramidalization angle of the dimethylamino group, and planarity of the molecule. Its dipole moment rises from 5.5 Debye in the ground state to 17.1 Debye in the (1)(π, π*) state, and then falls to 2 Debye in the (1)(n, π*) state. The excited (1)(n, π*) state is a short living state; it has a high probability of intersystem crossing into the (3)(π, π*) triplet state. This relaxation path explains the low quantum yield of DMAC fluorescence in non-polar media. It is possible that (3)(π, π*) is responsible for observed DMAC phosphorescence.

Composition, structure and substrate properties of reconstituted discoidal HDL with apolipoprotein A-I and cholesteryl ester.

To investigate the influence of lipid unsaturation and neutral lipid on the maturation of high density lipoproteins, the discoidal complexes of apoA-I, phosphatidylcholine and cholesteryl ester (CE) were prepared. Saturated dipalmitoylphosphatidylcholine (DPPC) and unsaturated palmitoyllinoleoylphosphatidylcholine (PLPC), palmitoyloleoylphosphatidylcholine (POPC), and fluorescent probe cholesteryl 1-pyrenedecanoate (CPD) that forms in a diffusion- and concentration-dependent manner short-lived dimer of unexcited and excited molecules (excimer) were used. The apoA-I/DPPC/CPD complexes were heterogeneous by size, composition and probe location. CPD molecules incorporated more efficiently into larger complexes and accumulated in a central part of the discs. The apoA-I/POPC(PLPC)/CPD were also heterogeneous, however, probe molecules distributed preferentially into smaller complexes and accumulated at disc periphery. The kinetics of CPD transfer by recombinant cholesteryl ester transfer protein (CETP) to human plasma LDL is well described by two-exponential decay, the fast component with a shorter transfer time being more populated in PLPC compared to DPPC complexes. The presence of CE molecules in discoidal HDL results in particle heterogeneity. ApoA-I influences the CETP activity modulating the properties of apolipoprotein-phospholipid interface. This may include CE molecules accumulation in the boundary lipid in unsaturated phosphatidylcholine and cluster formation in the bulk bilayer in saturated phosphatidylcholine.