Submitted to: Journal of Sugarbeet Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/30/2001
Publication Date: N/A
Interpretive Summary: Acid invertase is one of three major enzymes responsible for sucrose metabolism in sugarbeets. This enzyme catalyzes the hydrolysis of sucrose to the two invert sugars, glucose and fructose. Although its function in sucrose metabolism is unclear, this enzyme has been implicated in sucrose losses during root development and postharvest storage. To better understand its function and its contribution to sucrose losses, the soluble acid invertase of sugarbeet roots was partially purified and some of its biochemical and physical properties characterized. The enzyme exhibits high activity at pH 5.0 to 5.5 and requires low sucrose concentrations to operate at its maximum velocity (Km=8.9). It, therefore, is highly active and efficient in hydrolyzing sucrose at the physiological conditions it encounters in the cell. The enzyme had little activity above pH 7.0 and was unstable at pH values greater than 7.5. Acid invertase was most active at 35 C. Temperatures above 40 C were damaging to the enzyme, and temperatures above 55 C caused the complete loss of enzyme activity. The stability of acid invertase to pH and temperature conditions is important to the sugarbeet industry since rapid and complete inactivation of sucrose degrading enzymes is critical for efficient processing of sugarbeet roots. These enzymes can lower sucrose yield not only by the direct loss of sucrose but also by the formation of invert sugars which interfere with the sucrose crystallization process. These studies also reveal that acid invertase is capable of degrading sucrose during postharvest storage of roots. Although enzyme activity declines with temperature, the enzyme retains 16% its activity at 5 C.
Technical Abstract: A soluble acid invertase from sugarbeet roots was partially purified and some of its biochemical and physical properties characterized. This invertase isoenzyme has a Km for sucrose of 8.9 mM and is not inhibited by fructose. The enzyme exhibits a plateau of activity at pH 5.0 to 5.5, and is activated 7.5-fold at pH 3.0, possibly due to the loss or decreased effectiveness of an inhibitor. The enzyme is unstable at pH values equal to or greater than 7.5 at high ionic strength. While short incubations at pH 3.0 and 4.7 caused minor losses in activity, prolonged exposures to these pH conditions seem to activate the enzyme. The enzyme exhibits a sharp temperature optimum at 35 C. At temperatures above or below this optimum, enzyme activity declines rapidly, although 16% of its activity is retained at 5 C. Rapid and irreversible inactivation occurs at 40 C and above. Partial inactivation occurs at 40 to 50 C, while complete inactivation occurs at 55 C and above.