Relative avidity of etruscomycin to cholesterol and ergosterol.

Two methods, biological and spectroscopic, were used to determine the avidity of the polyene antibiotic Etruscomycin for cholesterol and ergosterol. The biological method consisted of measuring the inhibitory potency of both sterols on the Etruscomycin-induced damage to erythrocytes and fungi. The spectroscopic method consisted of recording of series of differential spectra in a number of solvents of different composition. The results obtained showed that cholesterol protected erythrocytes and candida albicans against the damaging action of Etruscomycin more efficiently than ergosterol did and that Etruscomycin-cholesterol complexes were more resistant to interruption by organic solvents than Etruscomycin-ergosterol complexes. These results and their comparison with the results obtained with other polyene antibiotics indicate that Etruscomycin resembles filipin in that it binds more avidly to cholesterol than to ergosterol. This implies that the length of the hydrophobic chain rather than the presence of the amino sugar determines sterol preference. The spectral method that we used can have general application for the quantitative measurement of complex formation between polyenes and sterols.