Stoichiometry of the polyene-sterol interaction

The stoichiometry of the polyene antibiotic—cholesterol interaction has been measured in various systems with different techniques. The filipin—cholesterol interaction has been the subject of extensive study and the results suggest this interaction is equimolar. The data on other polyenes is not so definite, but suggests that only a small number of sterol molecules interact with one polyene antibiotic molecule. 

Fluorometric and spectrophotometric studies of filipin-cholesterol interaction showed that the stoichiometry of the interaction was 1 1 [150] or 1 1.5 [146,147]. UV spectrophotometry changes have been used to monitor the stoichiometry of the interaction between filipin and free or liposome-bound cholesterol. Analysis of aqueous dispersions suggested that the stoichiometry was 1 1 [171]. Lecithin, dicetyl phosphate-cholesterol liposomes only produced maximal spectral changes of filipin when the sterol polyene ratio was 1 1 [172]. Filipin released trapped ion markers from sterol—phospholipid liposomes. The rate of release was dependent upon cholesterol content of the liposome membrane (maximum at sterol phospholipid ratio of 1 1) and upon the molar fllipin sterol ratio (maximum at fllipin sterol ratio of 1 1). 

Estimates of the stoichiometry of polyene—sterol complex can be made from differential scanning calorimetry (DSC) of lecithin—sterol dispersions [146]. When the values obtained by DSC [146] were corrected for the amount of cholesterol which would be unavailable for polyene binding [154], it was shown that the number of cholesterol molecules that could be complexed or associated with fllipin, etniscomycin, pimaridn, nystatin and amphotericin B were 1.2,0.6, 1.7, 1.2 and 3.9, respectively. Permeability studies using cholesterol containing A. laidlawii cells [173] indicated a similar stoichiometry (fllipin 0.7, etrusco-mycin, 0.3, nystatin, 1.6 and amphotericin B, 3.3). Measurement of the binding of various polyene antibiotics to cholesterol-containing A. laidlawii membranes also gave an estimate of the number of cholesterol molecules that combine with fllipin, etruscomycin, pimaricin and amphotericin B. The values obtained were 1.2,1.6,1.5 and 0.7, respectively [153]. 

Schaffner [145] showed that addition of cholesterol or other 3 3-OH, A-5 sterols to aqueous ethanolic solutions of polyene antibiotics produced changes in the UV absorption spectrum suggestive of molecular interaction. In aqueous 

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