Stability of fungal alpha-amylase in sodium dodecylsulfate

Unfolding of a fungal alpha-amylase in aqueous sodium dodecylsulfate (SDS) solution was examined by SDS-polyacrylamide gel electrophoresis (PAGE). When the alpha-amylase was incubated with 1% SDS at room temperature and subjected to SDS-PAGE, it showed a much higher mobility than expected from the molecular weight. Circular dichroic and gel filtration analyses indicated that the protein is apparently in the native conformation upon incubation with 1% SDS. When the protein was heated in the presence of 1% SDS at 90 degrees C for 10 min, it had a lower mobility in SDS-PAGE and showed characteristics of an unfolded protein by circular dichroism and gel filtration. The melting temperatures of the protein were determined in the absence and presence of SDS by incubating it for 10 min at various temperatures. The melting temperatures were 70, 55, and 49 degrees C in the presence of 0, 1, and 2% SDS, respectively. The observed small shift of the melting temperatures by SDS suggests that the destabilizing action of SDS on the alpha-amylase is weak. However, the unfolding in SDS is not reversible process, since prolonged incubation of the protein with 1% SDS at 50 degrees C gradually increased the amount of unfolded protein. This indicates that the SDS-induced unfolding of the alpha-amylase is a slow process.