Reversible unfolding of pyridoxal kinase.

The unfolding of brain pyridoxal kinase by guanidinium HCl has been investigated at equilibrium. The overall process was reversible as judged from the complete recovery of catalytic activity after removal of guanidinium HCl. Unfolding of pyridoxal kinase was monitored by circular dichroism and fluorescence spectroscopy. The steepness of the spectroscopic changes between 0.2 and 1.5 M guanidinium HCl, and the lack of any discernible plateau suggests that unfolding of the monomer is a cooperative process. A compact intermediate on the unfolding pathway of pyridoxal kinase could not be detected by the method of denaturant gel filtration. The fluorescent analogs of the substrates ATP and pyridoxal were used to assess differences in stability among the domains of the protein. Based on fluorescence and steady emission anisotropy results, it is postulated that the nucleotide domain is more stable than the pyridoxal domain of the kinase.