Studies on the competition of polyene antibiotics for sterols.

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Abstract
The fractional change in the corrected fluorescence of pimaricin or filipin in the presence of a limiting amount of sterol and a competing polyene antibiotic has been used to estimate the relative affinity of amphotericin B, nystatin, filipin, and pimaricin for stigmasterol and for cholesterol. The relative affinities for cholesterol were filipin>amphotericin B>pimaricin >nystatin, while the relative affinities for stigmasterol were filipin>pimaricin>amphotericin B>nystatin. The data indicate that pimaricin and filipin can both interact simultaneously with about 30% of the cholesterol or stigmasterol. However, the stoichiometry of filipin and pimaricin alone for cholesterol and for stigmasterol in dilute aqueous solutions is 1 : 1. In the experiments which indicated both pimaricin and filipin interact with the same sterol molecule changes in corrected fluorescence and the absorbance spectra were monitored; and these criteria indicated that both pimaricin and filipin had interacted with the sterols. Light-scattering measurements indicate large aggregates were not formed. Although the data shows in dilute aqueous solutions the stoichiometry of filipin and/or pimaricin for sterols is 1 : 1, in more complex solutions, other combinations or interactions are indicated especially forpimaricin.