Toxic and/or carcinogenic consequences may result from metal ion substitution for the Zn-(II) in transcription factors containing zinc fingers, and the small Cys-rich metal-binding protein metallothionein (MT) may play a role in this metal substitution. To begin to evaluate this hypothesis, with regard to the carcinogenic metal ion Ni(II), a peptide corresponding to the third finger of the transcription factor Sp1 (Sp1-3) has been synthesized and its metal binding and redox reactions have been studied. The peptide binds Zn(II), Co(II), and Ni(II), with spectroscopic data indicating a tetrahedral coordination for the latter two; metal ion affinities have been quantified (Kd = 6 (+/- 3) x 10(-10), 3 (+/- 1) x 10(-7), and 4 (+/- 1) x 10(-6), respectively) and found to be less than those of an optimized zinc finger peptide (Krizek, B. A., Merkle, D. L., and Berg, J. M. (1993) Inorg. Chem. 32, 937-940) but greater than those of the second finger of transcription factor IIIA (Berg, J. M., and Merkle, D. L. (1989) J. Am. Chem. Soc. 111, 3759-3761). Reactions of the peptide and its metal-bound forms with dioxygen or hydrogen peroxide did not produce oxygen radical species; however, oxidation of the two Sp1-3 cysteines was modulated by metal ions (Zn < Co = apo < Ni), suggesting a protective role for Zn(II) but an enhancing role for Ni(II). Metal binding competition between Sp1-3 and the alpha domain of human liver MT-2 (alpha-hMT2) indicates a similar affinity for Zn(II). However, alpha-hMT2 has a higher affinity for Ni(II), suggesting that MT may play a protective role by ensuring Zn(II), rather than Ni(II), coordination to zinc finger sequences of transcription factors.