A superior drug carrier--aponeocarzinostatin in partially unfolded state fully protects the labile antitumor enediyne.

BackgroundNeocarzinostatin is a potent antitumor drug consisting of an enediyne chromophore and a protein carrier.MethodsWe characterized an intermediate in the equilibrium unfolding pathway of aponeocarzinostatin, using a variety of biophysical techniques including 1-anilino-8-napthalene sulfonate binding studies, size-exclusion fast protein liquid chromatography, intrinsic tryptophan fluorescence, circular dichroism, and 1H-15N heteronuclear single quantum coherence spectroscopy.ResultsThe partially unfolded protein is in molten globule-like state, in which approximately 60% and approximately 20% tertiary and secondary structure is disrupted respectively. Despite lacking a fully coordinated tertiary structure for assembling a functional binding cleft, the protein in molten globule-like state is still able to fully protect the labile chromophore. Titration of chromophore leads the partially denatured apoprotein to fold into its native state.ConclusionThese findings bring insight into conserving mechanism of neocarzinostatin under harsh environment, where even the partially denatured apoprotein exhibits protective effect, confirming the superiority of the drug carrier.