PURIFICATION AND PROPERTIES OF L-GALACTONO-GAMMA-LACTONE DEHYDROGENASE, A KEY ENZYME FOR ASCORBIC-ACID BIOSYNTHESIS, FROM SWEET-POTATO ROOTS

L-Galactono-gamma-lactone dehydrogenase (L-galactono-gamma-lactone:ferricytochrome c oxidoreductase [EC 1.3.2.3], GLDHase) which catalyzes the terminal step in the biosynthesis of L-ascorbic acid (AsA) has been purified from roots of sweet potato (Ipomoea batatas L., cv. Kintoki). Highly purified preparation of the GLDHase was obtained by three column chromatography steps with a recovery of ca. 1%, after solubilization from mitochondria in sweet potato roots. SDS-PAGE exhibited a single band at 56 kDa. In the native state, the apparent molecular mass of the enzyme was 58 kDa, based on a Sephadex G-100 gel filtration. The pI and optimum pH values were 5.8 and 7.9, respectively. The K-m value for L-galactono-gamma-lactone was 0.12 mM. Substrate inhibition was obtained at concentrations greater than 4.2 mM. The enzyme was inhibited by p-chloromercuribenzoate (PCMB) and acriflavine, and the inhibition of acriflavine was diminished by the addition of FAD or FMN. The only effective substrate for the GLDHase was L-galactono-gamma-lactone.