Spin label reduction kinetics, a procedure to study the effect of drugs on membrane permeability: The effects of monosodium urate, dimethyl sulfoxide and amphotericin B

Abstract

A method is presented to study the effect of drugs on membrane permeability. It is based on the reduction of a spin label trapped in the internal aqueous compartment(s) of membranes by ascorbate ions added to the bulk aqueous phase. The decay of the electron spin resonance signal of the spin label as a function of time gives an indication of the effect of added agents on the permeability of membranes. To demonstrate the technique, the effect on model membranes of egg phosphatidylcholine of the gout-implicated compound monosodium urate, the aprotic solvent dimethyl sulfoxide and the polyene antibiotic amphotericin B were examined. Monosodium urate did not affect the permeability, casting doubt on a proposed mechanism whereby the agent disrupts the membranes via hydrogen bonding. Dimethyl sulfoxide promoted a gradual increase in rate of solute passage across cholesterol-containing model membranes. Amphotericin B had pronounced effect on the permeability of cholesterol-containing membranes, causing nearly total loss of paramagnetism immediately after addition. Some aspects of the mechanism of action of the drugs are discussed as well as the advantages and disadvantages of the method. The experiments also allow the evaluation of the effect of surface charge and cholesterol on the dimensions of model membranes.