Solubility and Metastability of the Amyloidogenic Core of Tau.

Intracellular deposits of neurofibrillary tau tangles and extracellular Aβ plaques are closely associated with Alzheimer's disease. The mapping of thermodynamic parameters, including solubility limits, indicates when a protein forms amyloid fibrils or remains in solution. This reveals the direction of change of the system and may help in understanding drift and steady states in living systems. Here we have developed methodology for tau solubility quantification and determined the solubility of the amyloidogenic core fragment of tau in vitro. We monitored the concentration of free tau304-380C322S fragment at 37 °C in phosphate buffer at pH 8.0 using three separate methods: HPLC-UV, derivatization with ortho-phthalaldehyde and scintillation counting. The measurements were repeated over time until a stable value was reached, implying that an equilibrium with fibrils had been established. The solubility measurements converged on a free monomer concentration of 6.1 ± 3.5 nM, which represents the solubility of the fragment under the current experimental conditions.