Activation of pyridoxal kinase by metallothionein.

Brain pyridoxal kinase, which uses ATP complexed to either Zn(II) or Co(II) as substrates, displays high catalytic activity in the presence of Zn-thionein and Co-thionein. Several steps intervene in the process of pyridoxal kinase activation, i.e., binding of Zn ions to ATP and interaction between Zn-ATP and the enzyme. Equilibrium binding studies show that ATP mediates the release of Zn ions from the metal-thiolate clusters of the thioneins, whereas spectroscopic measurements conducted on Co-thionein reveal that the absorption transitions corresponding to the metal-thiolate of the protein are perturbed by ATP. The binding Zn-ATP to the kinase proceeds with a delta G = -6.3 kcal/mol as demonstrated by fluorometric titrations. Direct interaction between the kinase and derivatized-metallothionein could not be detected by emission anisotropy measurements, indicating that juxtaposition of the proteins does not influence the exchange of metal ions. Since the concentration of free Zn in several mammalian tissues is lower than 1 nM, it is postulated that under in vivo conditions the concentration of metallothionein regulates the catalytic activity of pyridoxal kinase.