A detergent-activated tyrosinase from Xenopus laevis. II. Detergent activation and binding

Tyrosinase purified from Xenopus is enzymatically inactive in aqueous buffers but is activated for both of its substrates by exposure to a variety of anionic detergents. Cationic and nonionic detergents, as well as a variety of other agents are ineffective. This stimulation by detergents is observed at all stages of the purification (Wittenberg, C., and Triplett, E. L. (1985) J. Biol. Chem. 260, 12535-12541). Sodium dodecyl sulfate (NaDodSO4) is the most effective activator, and it was chosen for further characterization. Activation of both activities by NaDodSO4 is rapid and concentration dependent, resulting in maximal activity after 4 min at 1 mM NaDodSO4. NaDodSO4 treatment also results in both long and short term stabilization of the enzyme. The activation and stabilization are separable but stoichiometrically related. Both effects occur well below the critical micelle concentration suggesting that the interaction of NaDodSO4 monomers with the enzyme is involved in these processes. In support of this suggestion, the enzyme is shown to bind NaDodSO4 with high affinity, as determined by equilibrium dialysis. The isotherm for this binding correlates well with the requirement of NaDodSO4 for both activation and stabilization. All three effects are observable at 3 X 10(-5) M NaDodSO4 in the presence of 0.1 M sodium chloride. Activation and stabilization are maximal at 6 X 10(-4) M NaDodSO4, the critical micelle concentration of NaDodSO4 under these conditions.