Evidence for site-selective metal binding in calf liver metallothionein

Two isoproteins of calf liver metallothionein (MT) have been isolated, purified, and characterized by atomic absorption, ultraviolet absorption, electron spin resonance, and 113Cd nuclear magnetic resonance spectroscopy. Native calf liver MT was found to contain both Cu+ and Zn2+ in a mole ratio of approximately 0.75. Selective replacement of the native Zn2+ with 113Cd2+ can be accomplished in vitro by adding 113CdCl2 to the homogenate before chromatography. Both isoproteins of metallothionein thus prepared contain approximately 3.9 g atoms of Cd2+ and 2.6 g atoms of Cu+/mol of protein. No ESR signal was found, indicating that either Cu+ or antiferromagnetically coupled Cu2+ is the form of copper present. Arguments in support of the former state are presented. Unlike the native 113Cd,Zn MT from rabbit liver, calf liver 113Cd,Cu MT exhibits a remarkably simple 113Cd NMR spectrum. Four major resonances were found for each isoprotein, in the same positions as the resonances assigned to the metals in the four-metal cluster A of rabbit liver metallothionein. This conclusion was confirmed by homonuclear decoupling experiments. This result in conjunction with the stoichiometry of bound metal ions found in the native protein suggests that Cu+ is bound selectively to the three-metal cluster B sites, and that one homogeneous protein fraction predominates. Three minor resonances to higher field are observed in the 113Cd NMR spectrum of calf liver MT-1 and one in calf liver MT-2, which may be attributed to a small fraction of cluster B with one Cu+ replaced by 113Cd2+. The possible biological significance of the different metal ion specificities of cluster A versus cluster B is discussed.