A Comparison of DNA Cleavage by Neocarzinostatin Chromophore and Its Aglycon:  Evaluating the Role of the Carbohydrate Residue

Through a comparative analysis of the reactivity and DNA cleaving activity of neocarzinostatin (NCS) chromophore ( 1 ) and the corresponding aglycon ( 2 ), we show that the carbohydrate residue (aminoglycoside) of 1 both accelerates the rate and improves the efficiency of DNA cleavage versus the aglycon ( 2 ), but does not appear to be a major determinant of the base specificity of DNA cleavage by 1 . This stands in contrast to earlier findings with another enediyne antibiotic, calicheamicin γ 1, where the carbohydrate residue was found to be a major determinant of the sequence specificity of DNA cleavage, in addition to playing a functional role in the reductive activation step. Thiol addition experiments with NCS aglycon ( 2 ) provide further evidence that the carbohydrate amino group of 1 functions as an internal base in the first step leading to DNA cleavage, thiol activation. Evidence is also presented supporting the proposal that NCS aglycon ( 2 ) is bound tightly and reversibly by the neocarzinostatin binding protein (apo-NCS) and that this binding stabilizes the aglycon in solution.