[Relationship between the molecular composition of C1027, a new macromolecular antibiotic with enediyne chromophore,...

The molecule of C1027, an antitumor antibiotic with extremely potent cytotoxicity against cultured cancer cells, is composed of an enediyne chromophore and an apoprotein of 10.5 kDa. These two parts of the molecule, connecting each other through non-covalent binding, can be dissociated and reconstituted. As determined by clonogenic assay, the chromophore served as the active part of the molecule, displaying potent cytotoxicity similar to that of intact C1027. The activity of free chromophore decreased more rapidly than that of intact C1027, indicating that apoprotein played a role in protecting the chromophore from inactivation. By incubating together in phosphate buffer, the chromophore and apoprotein were reconstituted to form an intact C1027. The ratio of chromophore and apoprotein remained 1 : 1 in the reconstituted molecule, even though extra amount of chromophore was added. The optimal condition for the reconstitution was pH 7.0, at 20 degrees C for 12 h. When the disulfide bond of the apoprotein was reduced by DTT, the activity of C1027 decreased more rapidly. C1027 was digested by pronase and the produced fragments of various molecular weights were examined by capillary electrophoresis. The cytotoxicity of 3-5 kDa fragment approximated that of intact C1027 and its IC50 value was 0.07 fmol.L-1. The results indicate that the intactness of the apoprotein is not indispensable for stabilizing the chromophore and a smaller molecule of 3-5 kDa consisting of a peptide fragment and a chromophore may retain full C1027 activity.