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β-galactosidase (EC 22.214.171.124) is important in the formation of a medicinal plant tomato (Lycopersicom esculentum). The plant extracts of tomato (Lycopersicom esculentum) were used to characterize the enzyme in the term of pH, temperature, enzyme kinetic and effects of some heavy metals on its activity. The enzyme activity was measured by its ability to hydrolyze the substrate 2-nitrophenyl β-D-galactopyranoside (ONPG). the
isoelectric point for enzyme was 4.4. Carbohydrate concentration of enzyme was found to be 19.5 % by employing phenol-sulfuric acid method. The Km and Vmax values of the enzyme were 3.65 mM and 0.18μmol/min, respectively. The activities of β-galactosidase from tomato (Lycopersicom esculentum)is completely inhibited by HgCl2 and KCN. After 4h incubation, lactose in milk was reduced by 38.5 and 70% by β-galactosidase from tomato (Lycopersicom esculentum), respectively. The study showed that Hg+2 was the most potent inhibitor while Cu+2 exhibited the least inhibition degree on β-galactosidase activity in the tomato (Lycopersicom esculentum). These finding indicated that the enzyme β-galactosidase in the leaves extract of tomato (Lycopersicom esculentum) can be used in industrial and medical applications.