Interaction of adriamycin with negatively charged model membranes: evidence of two types of binding sites.

The interaction of the antitumor compound adriamycin with negatively charged unilamellar phospholipid vesicles was studied. The negative charges were provided by cardiolipin or phosphatidic acid. By analyzing the changes in the circular dichroism spectrum of adriamycin, we demonstrated the presence of two different spectral patterns corresponding to two different binding sites (I and II) on the vesicles. In site I, the amino sugar of adriamycin is bound to the ionized phosphate of either cardiolipin or phosphatidic acid, and the dihydroxyanthraquinone lies outside the bilayer. In site II, the amino sugar is still bound to the phosphate, but the dihydroxyanthraquinone moiety is embedded in the bilayer. This has been shown by measuring spectroscopically the binding of the aglycon part to an external probe and by measuring the susceptibility of bound adriamycin to reduction by NADH dehydrogenase.