Effect of divalent cations on structure-function relationships of the antitumor protein alpha-sarcin

alpha-Sarcin binds one Zn(II) cation per protein molecule, with a Kd value of 0.9 mM, determined by equilibrium dialysis experiments. Ca(II), Mg(II), and Mn(II) do not bind to alpha-sarcin. Cd(II) and Co(II) also behave as Zn(II). The binding produces local modifications on the protein conformation affecting the microenvironment of tryptophan residues. The three cations modify the fluorescence emission of the protein. The near-u.v. circular dichroism spectrum of the protein is also altered. The binding of Zn(II) and related cations does not modify the secondary structure of the protein. The ribonucleolytic activity of alpha-sarcin is inhibited upon Zn(II) binding, but no alteration of the ability of the protein to aggregate phospholipid vesicles has been observed.