Detection of an unfolding intermediate in alpha-urease with enhanced affinity for ANSA

Protein aggregation is believed to be due to conformers that expose hydrophobic clusters that promote protein association. Such conformers can be detected using a fluorescent probe like 8-anilino 1-naphthalenesulfonic acid (ANSA). Here we show that urease exposed to 1.0 M guanidine-hydrogen chloride has a higher affinity for ANSA that native or denatured urease. The binding occurs over a narrow range of denaturant concentration, well below the concentration required to induce denaturation. The impact of ANSA on urease aggregation was further studied by fluorescence, light-scattering, and activity measurements. We found that ANSA modifies urease aggregates and can provide partial protection against inactivation arising from thermally induced aggregation. It seems that the well-known susceptibility of urease to aggregation is due to an intermediate that can be populated in the absence of denaturation. Such a rationale would explain why folding stability of urease is a poor indicator of long-term stabilization by various media.