The mechanism of adsorption of Ga-67 citrate to cultured cells

Protein binding onto surfactant-based synthetic vesicles

Synthetic vesicles were prepared by mixing anionic and cationic surfactants, aqueous sodium dodecylsulfate with didodecyltrimethylammonium or cetyltrimethylammonium bromide. The overall surfactant content and the (anionic/cationic) mole ratios allow one to obtain negatively charged vesicles. In the phase diagram, the vesicular region is located between a solution phase, a lamellar liquid crystalline dispersion, and a precipitate area. Characterization of the vesicles was performed by electrophoretic mobility, NMR, TEM, and DLS and we determined their uni-lamellar character, size, stability, and charge density. Negatively charged vesicular dispersions, made of sodium dodecylsulfate/didodecyltrimethylammonium bromide or sodium dodecylsulfate/cetyltrimethylammonium bromide, were mixed with lysozyme, to form lipoplexes. Depending on the protein/vesicle charge ratio, binding, surface saturation, and lipoplexes flocculation, or precipitation, occurs. The free protein in excess remains in solution, after binding saturation. The systems were investigated by thermodynamic (surface tension and solution calorimetry), DLS, CD, TEM, 1H NMR, transport properties, electrophoretic mobility, and dielectric relaxation. The latter two methods give information on the vesicle charge neutralization by adsorbed protein. Binding is concomitant to modifications in the double layer thickness of vesicles and in the surface charge density of the resulting lipoplexes. This is also confirmed by developing the electrophoretic mobility results in terms of a Langmuir-like adsorption isotherm. Charges in excess with respect to the amount required to neutralize the vesicle surface promote lipoplexes clustering and/or flocculation. Protein-vesicle interactions were observed by DLS, indicating changes in particle size (and in their distribution functions) upon addition of LYSO. According to CD, the bound protein retains its native conformation, at least in the SDS/CTAB vesicular system. In fact, changes in the alpha-helix and beta-sheet conformations are moderate, if any. Calorimetric methods indicate that the maximum heat effect for LYSO binding occurs at charge neutralization. They also indicate that enthalpic are by far the dominant contributions to the system stability. Accordingly, energy effects associated with charge neutralization and double-layer contributions are much higher than counterion exchange and dehydration terms.

Influence of various factors on binding of 67Ga to polymorphonuclear leukocytes

We have studied in vitro, factors that influence the uptake of 67Ga-citrate by human polymorphonuclear leukocytes. Citrate at 20 mM concentration decreased the uptake to 1% of control values. Uptake increased as a function of increased microCi of 67Ga/10(7) cells added and incubation time from 0 to 120 min. Uptake decreased somewhat as the incubation pH was lowered from 7.4 to 6.0. Our results suggest that, in vivo, these cells would accumulate 67Ga as the inflammatory lesion progresses while the acidic milieu would modestly reduce uptake.

In vitro uptake of gallium and chlorpromazine by mouse tumour cells

Primary cell suspensions were prepared from mouse sarcoma by enzymatic digestion with pronase. The cells were incubated with gallium citrate Ga 67 or the basic drug 14C-chlorpromazine (CPZ) for up to 1 h at 37 degrees C, and the label uptake was determined. The Ga uptake was proportional to time (0-30 min), whilst the CPZ uptake rapidly reached apparent saturation (less than 10 min). Metabolic inhibitors did not affect label uptake; however, disrupting the cell membrane with N-ethylmaleimide or heating at 56 degrees C significantly reduced CPZ accumulation but did not inhibit Ga uptake. Ga accumulation was decreased by adding human transferrin (0.5 mg/ml). Both gallium and chlorpromazine are fixed in the lysosomes of cells; however, in this system, they appeared to enter mouse sarcoma cells by different energy-independent mechanisms. The Ga uptake may reflect adsorption to cell components, whilst CPZ uptake required an intact cell and may be due to passive diffusion.