Characterization of β-amylase from Sinapis alba cotyledons

Abstract

β-Amylase from mustard ( Sinapis alba) cotyledons was purified to homogeneity by affinity chromatography on a starch column. M r determination by sodium dodecyl sulphate polyacrylamide gel electrophoresis and gel filtration revealed that the native enzyme is a monomer of 58 000. The analysis of products of enzyme action on amylose by paper chromatography and optical rotation revealed the exclusive formation of β-maltose as a product confirming the molecular nature of the purified enzyme as β-amylase. The substrate specificity of mustard β-amylase was akin to other plant β-amylases. The K m value for β-amylase with amylose as substrate was 0.24%. The enzyme was fairly stable at or below ambient temperature, to repeated freezing and thawing and exposure to a wide pH range (3–8). The inclusion of dithiothreitol in storage buffer improved the stability of enzyme. The enzyme was susceptible to denaturation (70–90% inactivation) by heavy metal ions (Cu 2+, Pb 2+, Ag +) and sulphydryl reagents such as p-chloromercuribenzoate, indicating the sulphydryl nature of the enzyme. In contrast, α-cyclodextrin, a competitive inhibitor of β-amylase caused only a mild reduction in enzyme activity.