Activity and stability of a soluble acid invertase from sugarbeet roots

Abstract

Soluble acid invertase catalyzes the hydrolysis of sucrose to the invert sugars, glucose and fructose, and has been implicated in sucrose loss during sugarbeet root development, postharvest storage and processing. To better understand the ability of this enzyme to degrade sucrose during sugarbeet root production and processing, the activity and stability of the major sugarbeet root soluble acid invertase were determined under different physical conditions. Maximum enzyme velocity was observed at sucrose concentrations above 20 mM, with a K m for sucrose of 8.9 mM. The enzyme exhibited a plateau of activity at pH 5.0 to 5.5, a pH range in which most plant soluble acid invertases exhibit high activity. Enzyme activity increased at pH values less than 5.0, possibly due to the loss of an inhibitor. Irreversible inactivation of the enzyme occurred at pH values of 7.5 or greater. Enzyme activity was greatest at 35°C and declined rapidly at temperatures above or below this optimum. At 5°C, the enzyme retained 16% of its maximal activity. Partial inactivation of the enzyme occurred at 40° to 50°C; complete inactivation was observed at 55°C and above. Inactivation at elevated temperatures was rapid and irreversible. This study indicates that the major soluble acid invertase is likely to operate at or near its maximum velocity in sugarbeet roots, is capable of hydrolyzing sucrose at the temperatures typical of sugarbeet root storage, and is active at the pH conditions typical of sugarbeet root processing.