An interaction between apo C-III variants and protease inhibitors contributes to high triglyceride/low HDL levels in treated HIV patients

BACKGROUND

Long-term therapy with protease inhibitors (PI) is associated with hypertriglyceridaemia, low high-density lipoprotein (HDL) levels and accumulation of apolipoprotein (apo) E- and apo C-III-containing lipoproteins.

OBJECTIVES

To evaluate the impact, on this dyslipaemic phenotype, of three polymorphisms of the apo C-III gene: two on an insulin response element and one in the 3'-region. Apo E genotypes were evaluated also.

DESIGN

Sixty consecutive male patients attending the HIV follow-up consultation were included during a 3-month period. All patients received at least one PI. Apo C-III and apo E genotypes were determined. Besides routine bio-clinical examination, a detailed exploration of lipoproteins and of insulin secretion markers was carried out.

METHODS

Plasma lipoparticles, insulin, proinsulin and C-peptide were measured by specific immuno-assays. Determination of apo C-III genotypes (-455C/T, -482C/T and SstI) and of apo E alleles (epsilon2, epsilon3 and epsilon4) were performed by amplification and endonuclease digestion and were confirmed by allele-specific oligonucleotide hybridization.

RESULTS

Distribution of apo C-III alleles defined four major haplotypes. Carriers of the -455C variant had 30% lower levels of HDL-cholesterol than non-carriers. Plasma triglycerides increased according to the number of variant alleles. In multivariate analysis, a model including age, body mass index, clinical stage and treatment length, plasma insulin and apo C-III haplotypes explained around 43% of the HDL-cholesterol and triglycerides variability. Measurements of lipids before and after the use of PI demonstrated synergistic effects of the treatment and apo C-III variants on triglyceride levels.

CONCLUSIONS

Apo C-III polymorphisms might identify a genetic predisposition to develop dyslipidaemia under PI therapy.

Interaction between TOAC free radical and photoexcited triplet chromophores linked to peptide templates

The intramolecular quenching of photoexcited triplet states by free radicals linked to peptide templates was studied by time-resolved electron paramagnetic resonance (EPR) with pulsed laser excitation. The systems investigated are 3(10)-helix forming peptides, having in the amino acid sequence the free radical 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC) and a triplet precursor, such as Bin, Bpa, or Trp, incorporated at different relative positions. Upon interaction with the excited triplet the TOAC radical spin sublevel populations assume values that differ from the Boltzmann equilibrium values. This spin polarization effect produces EPR lines in emission whose time evolution reflects the triplet quenching rate. In particular, in a series of peptides labeled with Bpa and TOAC at successive positions in the 3(10)-helix, radical-triplet interaction was observed in all cases. However, for the peptide where Bpa and TOAC are at positions 2 and 4 the rate of triplet quenching is lower than for the other peptides in the series. In addition, the radical-excited triplet complex in the quartet spin state was observed in a peptide containing fullerene (C(60)) as a triplet precursor and TOAC.

CIDEP effects of intramolecular quenching of singlet and triplet excited states by nitroxide radicals in oligopeptides: a potentially useful new method for investigating peptide secondary structures in solution

Two hexapeptides, each bearing one photoactive alpha-amino acid (Bin or Bpa) and one nitroxide-containing TOAC residue, have been synthesized and fully characterized. FT-IR absorption measurements indicate that a 3(10)-helical conformation is adopted by these peptides in solution. As two amino acid units separate the photoactive residue from TOAC in the peptide sequences, the two moieties face each other at a distance of about 6 A after one complete turn of the ternary helix. Irradiation by a light pulse from an excimer laser populates the excited states localized on the chromophores. An intramolecular interaction between the singlet (Bin) or triplet (Bin and Bpa) excited states and the doublet state of the TOAC nitroxide makes a spin-selective decay pathway possible, that produces transient spin polarization. In addition, in order to determine whether the intramolecular exchange interaction occurs through-bond or through-space, we have prepared linear and cyclic TOAC-Bin dipeptide units. A CIDEP study revealed that a through-space intramolecular interaction is operative. The observation of spin polarization makes the two helical hexapeptides suitable models to test the possibility of application of this novel technique to conformational studies of peptides in solution.

Aluminum (III) induces alterations on the physical state of the erythrocytic membrane: an ESR evaluation

The action of aluminum [Al(III)] as Al(acac)3 on erythrocytes causes biophysical effects such as osmotic fragility and echino-acanthocytes formation. In this paper, we present these effects in terms of variation of membrane fluidity, together with findings regarding conformational modifications of membrane proteins consequent to Al(III) exposure, as well as the effects on the mobility of the membrane protein bound sialic acid. To this end, we utilized ESR measurements of rabbits and humans erythrocytic ghosts after probing or labeling with suitable stable radicals used as spin probes or labels. Our results show that the lipophilic, hydrolytically stable toxicant Al(acac)3 causes a remarkable reduction of membrane fluidity in rabbit erythrocytes, an appreciable structural compacting effect on cytoskeletal and transmembrane proteins, as well as a reduction of rotational mobility of cell-surface sialic acid of human erythrocytes.

Synthesis and conformational studies of peptides containing TOAC, a spin-labelled C alpha, alpha-disubstituted glycine

A variety of host L-alanine homo-peptides (to the pentamer) containing one or two spin-labelled TOAC (2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) residues were synthesized by solution methods and fully characterized. The conformational features of the terminally blocked, doubly spin-labelled TOAC-(Ala)2-TOAC-Ala-pentapeptide were examined in the crystal state by X-ray diffraction and in solution using a combination of techniques (Fourier transform infrared, circular dichroism, cyclic voltammetry and electron spin resonance) in comparison with singly labelled shorter peptides. The 3(10)-helical structure of the pentapeptide, promoted by the two C alpha, alpha-disubstituted glycines under favourable experimental conditions, allows an interaction to take place between the two nitroxide TOAC side chains spaced by one turn of the helix. Taken together, these results suggest that TOAC is an excellent probe for exploring bends and helices in doubly labelled peptides.