The effect of nitric oxide on metal release from metallothionein-3: gradual unfolding of the protein

Metallothionein-3 (MT-3), or neuronal growth inhibitory factor, which exhibits growth inhibitory activity, is a brain-specific metallothionein. In this study, the effect of nitric oxide (NO) on metal release (using Cd²⁺ as a probe) from MT-3 was examined by ¹¹³Cd and 2D [¹H–¹⁵N] heteronuclear single-quantum coherence NMR spectroscopy. The exposure of human MT-3 to NO leads to a nonselective release of the three metals from the β-domain. In contrast to metallothionein-1 and metallothionein-2, two of the bound metals in the α-domain were also partially released, with the domain structure remaining almost unchanged. Further addition of NO resulted in the complete release of metals and concomitant unfolding of the protein. The preference of release of the two metals in the α-domain was attributed to the presence of two slightly different coordination environments for the four cadmium/zinc atoms.