Complex formation between indole-3-acetic acid and phospholipid membrane components in aqueous media. 1. Parameters of the system

Anomalous solubility behavior of the antibiotic ciprofloxacin encapsulated in liposomes: a 1H-NMR study

Many drugs are weak bases and can be accumulated into liposomes in response to a pH gradient to achieve high internal drug concentrations. This study is aimed at gaining an understanding of the relationship between the retention of the fluoroquinolone antibiotic ciprofloxacin in liposomes and the intraliposomal form and location of this drug. 1H-NMR spectroscopy was used to probe the interactions experienced by ciprofloxacin following uptake into large unilamellar liposomes (LUV). It is shown that ciprofloxacin is located in the aqueous interior of the liposomes and is self-associated in the form of small stacks. It does not precipitate out of solution even though the intraliposomal ciprofloxacin concentration can exceed its solubility in aqueous solutions by almost two orders of magnitude. The results also indicate that little entrapped ciprofloxacin partitions into the inner monolayer of the LUV. As a result of the lack of precipitation and rapid exchange properties, ciprofloxacin can respond quickly to changes in electrochemical equilibria such as depletion of the pH gradient. This provides a rationale for the rapid leakage of this drug in response to serum destabilization or depletion of the pH gradient.

Indole-3-acetic acid-mediated transport of Mn2+ and other ions across phosphatidylinositol vesicular membranes as determined by 31P-NMR

The indolic plant hormone, indole-3-acetic acid (IAA), mediated the transport of Mn2+ and other ions into small unilamellar vesicles prepared from soybean phosphatidylinositol (PI) and this process has been studied using 31P nuclear magnetic resonance spectroscopy (NMR). The rate of Mn2+ movement into PI vesicles is dependent on IAA concentration and temperature with an IAA stoichiometry of 4.1 and an activation energy of 16.8 kcal mol-1 derived for the rate-determining process. These values are altered by low concentrations of endogenous ions (which can be removed by treatment with EDTA) present in the PI. With non-EDTA-treated PI, values of 2.3 and 23.0 kcal mol-1 were obtained for the stoichiometry and activation energy, respectively. These values indicate that (a) IAA interacts with PI membranes; (b) IAA-induced changes in membrane permeability can be substantially modulated by ions and (c) IAA very significantly influences the rate of movement of some (but possibly not all) cations across PI membranes. Such effects are also modified by the oxidation state of the PI.

Interaction of the plant hormone, indole-3-acetic acid, with phosphatidylcholine model membranes: effects of acyl chain length

The interaction between the plant hormone, indole-3-acetic acid (IAA), and phosphatidylcholines (PC) of varying acyl chain length has been studied by monitoring the IAA-induced changes in 1H-NMR chemical shifts of lipid headgroup -+N(CH3)3 protons. For PCs in both micellar and vesicle bilayer systems these shifts increase with chain length although for the latter the magnitude of the shifts decreases with an increase in chain unsaturation. In systems composed of mixtures of pure PCs the headgroup -+N(CH3)3 resonance for each phospholipid is shifted by IAA to different extents, indicating that IAA is able to distinguish between individual PCs in mixtures. In di-C12PC and di-C14PC, but not di-C10PC vesicle systems, the -+N(CH3)3 resonance is split into two components reflecting differences in packing of the inside and outside lamellae. This splitting is altered by IAA indicating that IAA interacts differently with the inside and outside PC molecules.

Infrared spectroscopic evidence for a conformational alteration of plant plasma membranes upon exposure to the growth hormone analog, 2,4-dichlorophenoxyacetic acid

Infrared spectroscopy of highly purified fractions of plasma membrane vesicles from hypocotyls of etiolated soybean (Glycine max L.) seedlings revealed changes in bands assigned to proteins and phospholipids upon exposure to the growth hormone analog, 2,4-dichlorophenoxyacetic acid (2,4-D). The changes included a concentration dependent broadening of amide I absorbance and a change in the absorbance ratios of amide I and amide II indicative of a change in protein conformation. Band broadening of amide I was observed at 2,4-D concentrations as low as 10(-8) M, and the optimal 2,4-D concentration to evoke the change was 1 microM whereas the amide peak ratios (amide II/amide I) declined steadily over the range of concentrations (10(-8) to 10(-3)M) tested. An alteration in hydrocarbon chains (CH2 scissoring) was seen only at 1 mM (10(-3) M) 2,4-D. In contrast, the vibrational frequency of the choline stretch declined proportionally over the range 10(-6) to 10(-3). The findings provide evidence for a conformational change in the plasma membrane in response to the hormone demonstrable in a cell-free system.

Membrane-directed effects of the plant hormones abscisic acid, indole-3-acetic acid and 2,4-dichlorophenoxyacetic acid

This study examines two ways plant hormones might influence membrane processes, effects on overall permeability and modifications of specific ion channels. Abscisic acid (ABA) and indole-3-acetic acid (IAA) greatly enhanced erythritol permeability in mixed egg lecithin bilayers. In single component dioleoylphosphatidylcholine bilayers ABA was less effective than IAA, while 2,4-dichlorophenoxyacetate (2,4-D) did not affect either system or alter their ABA response. In Myxicola axons ABA and IAA had no effect, while 2,4-D (10 uM) caused a depolarizing shift of voltage-dependent Na+ and K+ activation by 25 +/- 4 mV and 15 +/- 3 mV, consistent with internal negative surface charge changes of -0.002 e-/A2 and -0.0007 e-/A2. We conclude that both generalized and ion channel-directed effects may link plant hormones and intracellular regulation.

Abscisic acid increases lipid bilayer permeability to cations as studied by phosphorus-31 NMR

Using a 31P-NMR lanthanide shift technique, abscisic acid is shown to enhance the permeability to praeseodymium of lipid bilayers composed of 80 mol% phosphatidylcholine and 20 mol% phosphatidylethanolamine. Praeseodymium permeability is immeasurably slow in the absence of the hormone whether or not phosphatidylethanolamine is present in the bilayers. Only in the presence of abscisic acid is praeseodymium permeability observed, the effect being significantly greater when phosphatidylethanolamine is present. These results substantiate prior reports from nonelectrolyte permeability studies that abscisic acid interacts with phosphatidylethanolamine in lipid bilayers.

Auxin-polyamine interaction with the calcium-controlled secretion of peroxidase by sugarbeet cells

Spermidine and ornithine given to normal auxin-requiring cell suspensions of sugarbeet inhibited peroxidase secretion in the absence of Ca(2+). Habituated (organogenic or not) cells did not respond. Both compounds counteracted the Ca(2+) - promoted enzyme secretion by three cell lines. Auxins (2,4-D and BSAA) did not modify the extracellular level of peroxidase activity in the absence of Ca(2+) When Ca(2+) was added, auxins increased its effect in normal cells and had practically no effect in habituated cells. The inhibitory effect of spermidine and ornithine was somewhat reduced by auxins in normal cells and increased in habituated cells. It was hypothesized that the effect of auxins did not involve the mediation of polyamines and that both types of compounds directly interacted with Ca(2+) at the membrane level.