Kinetic properties of sweet potato beta-amylase

Sweet potato tuber is a rich source of beta-amylase. Large variations in activity, ranging from 90 to 960 units, were noticed among varieties. The enzyme was purified using ammonium sulphate fractionation. SDS-PAGE of the protein showed an average molecular weight of 50 KDa. The enzyme has a broad pH optimum from 4.6 to 6.0. No feed-back inhibition was noticed with end product maltose or with fructose or glucose. However, 1 mM CuSO4 or 0.1 mM HgCl2 totally inhibited the activity. Sweet potato beta-amylase was found to be a heat-stable enzyme. The optimum temperature of activity in vitro was 60 degrees C. However, it retained about 50 per cent activity even when the temperature in vivo rose to 80 degrees C. This remarkable property is primarily responsible for the large increases in sugar content on processing of sweet potato tubers.