Proteins with acid invertase activity have been identified from potato (Solanum tuberosum L.) leaves and tubers, and purified to near homogeneity. Under denaturing conditions (SDS-PAGE) the purified invertase is associated in each case with a 58 000 Mr polypeptide. A pI of pH 5.2 was determined for the tuber enzyme and both invertases had a pH optimum of 5. The Km (sucrose) was 7.9 mM for the tuber enzyme and 2.4 mM for the leaf enzyme. In addition, leaf invertase also hydrolysed lactose (Km 6.4 mM) at approximately 10% of the rate of sucrose. Tuber invertase also hydrolysed lactose (Km 9.2 mM), raffinose (Km 7.1 mM) and trehalose (Km 0.66 mM), again at a rate approximately 10% that of sucrose. Product inhibition by fructose was linear mixed for the leaf enzyme and competitive for the tuber enzyme. Glucose inhibition was uncompetitive for the leaf enzyme and non-competitive for tuber invertase.
- Solanum tuberosum