A β-Galactosidase from Radish ( Raphanus sativus L.) Seeds

Abstract

A basic beta-galactosidase (beta-Galase) has been purified 281-fold from imbibed radish (Raphanus sativus L.) seeds by conventional purification procedures. The purified enzyme is an electrophoretically homogeneous protein consisting of a single polypeptide with an apparent molecular mass of 45 kilodaltons and pl values of 8.6 to 8.8. The enzyme was maximally active at pH 4.0 on p-nitrophenyl beta-d-galactoside and beta-1,3-linked galactobiose. The enzyme activity was inhibited strongly by Hg(2+) and 4-chloromercuribenzoate. d-Galactono-(1-->4)-lactone and d-galactal acted as potent competitive inhibitors. Using galactooligosaccharides differing in the types of linkage as the substrates, it was demonstrated that radish seed beta-Galase specifically split off beta-1,3- and beta-1,6-linked d-galactosyl residues from the nonreducing ends, and their rates of hydrolysis increased with increasing chain lengths. Radish seed and leaf arabino-3,6-galactan-proteins were resistant to the beta-galase alone but could be partially degraded by the enzyme after the treatment with a fungal alpha-l-arabinofuranosidase leaving some oligosaccharides consisting of d-galactose, uronic acid, l-arabinose, and other minor sugar components besides d-galactose as the main product.