PubMed Health⌕ Search

Biomedical subjects

J A Magnuson

Publications and source records attributed to J A Magnuson.

7 recordsLinked to original sources
PubMed1979

Light-induced, carrier-mediated transport of tetracycline by Rhodopseudomonas sphaeroides.

Tetracycline accumulation by the phototrophic bacterium Rhodopseudomonas sphaeroides has been studied, using the fluorescence properties of the antibiotic and measuring uptake of [7- 3H]tetracycline. Accumulation was carrier mediated, with a Km of approximately 300 micronM. Efflux also appeared to be carried mediated, with a Km of 25 mM. Chlorotetracycline competitively inhibited tetracycline transport. The transport was energy dependent. Efflux occurred during the influx process, and an energy-requiring steady state was reached when influx balanced efflux. Transport was inhibited by metabolic inhibitors such as antimycin A, cyanide, and iodoacetate. Proton conductors such as carbonylcyanide m-chlorophenyl hydrazone were strongly inhibitory. Efflux was not energy dependent. Efflux is partially blocked by mercuric ions and completely blocked by an external pH of 9 to 11. Although efflux rates increased continuously with lowering of the pH, influx rates have a sharp maximum at pH 7.

Biological Transport↗
PubMed1978

Membrane lateral phase separations and chlortetracycline transport by Bacillus megaterium.

Chlortetracycline, a fluorescent probe of its own active transport, has been used to study lateral phase separations of membrane lipid in Bacillus megaterium cells. Arrhenius plots of initial accumulation rates are triphasic, with transitions or characteristic temperatures of 20 degrees and 9.5 degrees . At the higher temperature, the mobility of the chloretracycline, as measured by fluorescence polarization, is markedly altered. Chlortetracycline transport exhibits saturation kinetics, and fluorescence energy transfer from protein to bound antibiotic can be observed. N-Phenyl-1-naphthylamine, a lipophilic fluorescent probe, responds to changes in the hydrophobic regions of the membrane that are distinct from membrane protein. The fluorescent properties of N-phenylnaphthylamine in partitioning and polarization experiments are altered most significantly at the lower characteristic temperature. No fluorescence energy transfer between N-phenylnaphthylamine and membrane protein or bound tetracycline can be detected. In correlative electron spin resonance experiments on the partitioning of a lipid-soluble spin label, the same characteristic temperatures detected in the fluorescence studies were measured. These data suggest that different probes may respond to either or both of the characteristic temperatures describing the lateral phase separation. Between these characteristic temperatures the chlortetracycline transport system is most intimately associated with relatively immobile lipids that are surrounded by a more mobile lipid phase.

Bacillus megaterium↗
PubMed1977

Calcium-induced cooperativity of manganese binding to concanavalin A.

Titrations employing electron spin resonance spectroscopy and equilibrium dialysis studies have revealed that Mn2+ binding to concanavalin A is cooperative in the presence and noncooperative in the absence of Ca2+. The degree of cooperativity increases with increasing pH. Hill coefficients range from 1.4 at pH 5.0 to 1.8 at pH 6.85. In addition to inducing cooperativity in Mn2+ binding, Ca2+ influences the pH dependence and increases the affinity of Mn2+ binding. In contrast to previous suggestions based mostly on work conducted near pH 5, demetallized concanavalin A does bind Ca2+ with an appreciable binding constant. These observations indicate that at physiological pH the role of metal ions in determining functional properties of concanavalin A is different from that suggested by metal binding studies conducted at lower pH values.

Allosteric Site↗