PURIFICATION AND CHARACTERIZATION OF BETA-AMYLASE FROM LEAVES OF POTATO (SOLANUM-TUBEROSUM)

Amylolytic activity is widely distributed in plants. In potato leaves (Solanum tuberosum L.) the abundant amylolytic activity was found to b e beta-amylase (EC 3.2.1.2, a-1,4-D-glucan maltohydrolase). beta-Amyla se from potato leaves was purified to homogeneity for study of enzyme characteristics. The purification steps included ammonium sulphate pre cipitation, anion exchange chromatography, affinity chromatography and gel filtration. The end product of a-1,4-glucan degradation was malto se. The protein is a 111-kDa homo-dimer with a subunit molecular mass of 56 kDa and a pI of 5.6. The pH-optimum is 6.5 using p-nitrophenylma ltopentaoside (PNPG5) as substrate. The optimal temperature for hydrol ysis is at 40 degrees C. The enzyme is unstable at temperatures above 40 degrees C. The K-m-value for PNPG5 is 0.73 mM and the activity is i nhibited by cyclodextrins. At a concentration of 1 mM, beta-cyclodextr in is a stronger inhibitor than a-cyclodextrin (68 and 20% inhibition, respectively). Branched glucans (e.g, starch and amylopectin) are sup erior substrates as compared to long, essentially unbranched glucans ( e.g. amylose). This study of the catalytic properties of beta-amylase from potato leaves indicates the importance of beta-amylase as a starc h degrading enzyme.