Cadmium-113 nuclear magnetic resonance studies of cadmium-substituted derivatives of bovine superoxide dismutase.

We have prepared the following cadmium-113-substituted derivatives of bovine superoxide dismutase and recorded the nuclear magnetic resonance (NMR) spectrum of the cadmium: 2Cd(II), in which Cd(II) is presumed to bind to the Zn(II) site and the copper site is unoccupied, and 2Cd(II)--2Cu(I), which is analogous to the reduced form of the native protein. NMR transitions were observed at 310 ppm downfield from Cd(ClO4)2 for 2Cd(II) and at 320 ppm for the 2Cd(II)--2Cu(I)-containing proteins. In each case the observed line width was 27 +/- 2 Hz. The following conclusions were drawn. (a) The very small chemical-shift difference between the two derivatives indicates that the Cd(II) binding site is very similar in both samples. It follows from this result and previous work that the imidazolato bridge is protonated on the Cu side upon reduction of the Cu ion from the II to I valence state. (b) The extremely narrow line width of the resonance in both forms suggests a virtual identity of Cd(II) bound to both subunits of the molecule. (c) The relaxation time, T1 = 1.2 s, is caused by approximately equal contributions from chemical-shift anisotropy and dipolar interactions with nearby protons.