Purification and characterization of a soluble β‐fructofuranosidase from Daucus carota

Abstract

Soluble β‐fructofuranosidase with an intracellular location and an isoelectric point of 3.8 (isoenzyme I) was purified and characterized from dry seeds and seedlings of carrot (Daucus carota). The enzyme hydrolyzed sucrose with a K_m of 5 mM and a broad pH optimum around 5.0. The purified protein, which was N‐glycosylated with high‐mannose‐containing and high‐xylose‐containing complex glycans, eluted as a monomeric polypeptide with a molecular mass of 68000 from a gelfiltration column. On SDS/PAGE, the protein separated in the presence of SDS and 2‐mercaptoethanol into three polypeptides with molecular masses of 68, 43 and 25 kDa. The amount of the 68‐kDa polypeptide was highest in dry seeds and decreased with increasing age of carrot seedlings. Amino acid sequence analysis and immunological studies showed that the 43‐kDa and 25‐kDa polypeptides were N‐terminal and C‐terminal proteolytic fragments of the 68‐kDa polypeptide. A comparison of partial amino acid sequences of the soluble β‐fructofuranosidase with the complete sequence of carrot cell‐wall β‐fructofuranosidase showed that their N‐terminal sequences were different, whereas some of the internal tryptic peptide sequences were up to 70% identical.