The Cu(II) complex of the ligand all-cis-2,4,6-triamino-1,3,5-trihydroxycyclohexane (TACI) is a very efficient catalyst of the cleavage of plasmid DNA in the absence of any added cofactor. The maximum rate of degradation of the supercoiled plasmid DNA form, obtained at pH 8.1 and 37 degrees C, in the presence of 48 mu M TACI(.)Cu(II), is 2.3 x 10(-3) s(-1), corresponding to a half-life time of only 5 min for the cleavage of form I (supercoiled) to form 11 (relaxed circular). The dependence of the rate of plasmid DNA cleavage from the TACI-Cull complex concentration follows an unusual and very narrow bell-like profile, which suggests an high DNA affinity of the complexes but also a great tendency to form unreactive dimers. The reactivity of the TACI-Cull complexes is not affected by the presence of several scavengers for reactive oxygen species or when measured under anaerobic conditions. Moreover, no degradation of the radical reporter Rhodamine B is observed in the presence of such complexes. These results are consistent with the operation of a prevailing hydrolytic pathway under the normal conditions used, although the failure to obtain enzymatic religation of the linearized DNA does not allow one to rule out the occurrence of a nonhydrolytic oxygen-independent cleavage. A concurrent oxidative mechanism becomes competitive upon addition of reductants or in the presence of high levels of molecular oxygen: under such conditions, in fact, a remarkable increase in the rate of DNA cleavage is observed.