Biomedical subjects

S Akoka

Publications and source records attributed to S Akoka.

40 records · Page 3Linked to original sources

PubMed1987

Interactions between biliary lipid micelles and intestinal brush border membranes investigated by 1H and 31P nuclear magnetic resonance.

The effect of taurocholate and lecithin-cholesterol-taurocholate mixed micelles on the structure of isolated intestinal brush border membranes was investigated by nuclear magnetic resonance (NMR). Rabbit brush border membranes isolated by a Mg2+ precipitation step were chosen for this study because of their stability and integrity as revealed by 31P NMR. Incubation of taurocholate with the brush border membranes does not induce significant solubilization of these membranes even when the taurocholate/phospholipid ratio reaches 3.0. 1H NMR studies indicate that taurocholate is included in the membrane bilayer at low concentration (3 mM). However this biliary salt produces a size diminution of the vesicles when its concentration increases. Incorporation of lecithin or lecithin-cholesterol in micelles of taurocholate and subsequent incubation with brush border membranes lead simultaneously to a decrease in the 31P NMR isotropic/bilayer line ratio, and to an increase in delta sigma. These results indicate a protective effect of these compounds against lytic damage of taurocholate. Furthermore the equilibrium distribution of lecithin between mixed micelles and the membrane bilayer is strongly in favour of complete integration of micellar components in the bilayer. These data suggest that uptake of lipids from the micellar phase by isolated brush border membranes involves an interaction of the micelles with membranes followed by a fusion process.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed1986

Molecular order, dynamics, and ionization state of phosphatidylethanolamine bilayers as studied by 15N NMR.

Dipalmitoylphosphatidylethanolamine (DPPE) and dipalmitoylphosphatidylcholine (DPPC), 15N-labeled in the polar head group, were synthesized. The proton-decoupled 15N spectra of DPPC and DPPE in aqueous dispersion have exactly the form anticipated for powder line shapes governed by an axially symmetric shielding tensor. The chemical shift anisotropy (delta sigma) of DPPC is lower than 0.4 ppm at 30 degrees C and vanished when the temperature or the half-height line width is increased; DPPE always exhibits an asymmetric line shape, and 15N NMR spectra of DPPE are obtained at various temperatures and simulated to measure exactly the chemical shift anisotropy. At each temperature, the order parameter of the C-N bond segment is derivated from delta sigma and reveals that the average orientation of the C-N bond around the axis of rotation is near the "magic angle" (54.7 degrees). Isotropic correlation times are derived from T1, which are higher than values obtained for phosphatidylcholine by other nuclei. Arrhenius plots of T1 and T2 allowed us to calculate the activation energy for the motion of the DPPE and the DPPC C-N bond. The value of this activation energy for the DPPE (53 kJ/mol) is higher than the one found for the DPPC C-N bond (32 kJ/mol). These differences agree with the capacity of the ethanolamine head groups to bind noncovalently to their neighbors in the plane of the membrane surface. A direct titration curve of the amino group is achieved by the variation of the chemical shift with the bulk pH, and the interfacial pKa is calculated to be 11.1.(ABSTRACT TRUNCATED AT 250 WORDS)

1,2-Dipalmitoylphosphatidylcholine↗

PubMedDate not supplied

Diffusion imaging with a multi-echo MISSTEC sequence.

An imaging method is presented to measure the water-diffusion coefficient. The sequence (MISSTEC) uses the simultaneous acquisition of a spin echo and several stimulated echoes with the same intensity except for diffusion weighting. The optimal number of stimulated echoes was calculated to minimize the diffusion coefficient error (D). D values obtained in vitro and in vivo were in good agreement with those from the spin-echo sequence (IntraVoxel Incoherent Motion [IVIM] method). The total acquisition time is half that of the classic IVIM method.

Brain↗

PubMedDate not supplied

Superparamagnetic iron oxide nanoparticles as a liver MRI contrast agent: contribution of microencapsulation to improved biodistribution.

We have developed a new method of synthetizing superparamagnetic iron oxide nanoparticles, consisting in the modifications of Molday's method, which ensures high relaxivity (2.4 10(5) s-1.M-1.L), good chemical stability, singular biodistribution and a considerable safety margin. The ED (Efficace Dose) to LD50 ratio is 1/2400 instead of 1/50 for Gd-DTPA. In order to develop a magnetite-delivery system to the liver we have incorporated the nanoparticles into biodegradable synthetic microcapsules. Encapsulated 59Fe oxide nanoparticles are injected into rats; in these conditions the sequestration is 9-fold greater in liver and 6 and 5 times lower in blood and carcase, respectively. This modification of the biodistribution enables the use of magnetite containing microcapsules at only 0.3 mg/kg iron to obtain an improved contrast in liver.

Animals↗