A grape invertase was purified from Muscat Bailey A juice by salting out with ammonium sulfate and successive chromatographies on Sephadex G-100 and Con A-agarose to a homogeneous state as confirmed by polyacrylamide gel electrophoresis. The molecular weight of the purified enzyme was 72 kDa in gel filtration chromatography. However, SDS polyacrylamide gel electrophoresis revealed three bands with molecular weights of 56 kDa, 25 kDa and 24 kDa. Affinity staining of Western blots with lectins indicated that the enzyme was a glycoprotein. The optimum pH for the enzyme reaction was 3.5 and the optimum temperature 80 degrees C. The enzyme was stable from pH 2.7 to 6.4, and up to 80 degrees C. The transfructosylation reaction could not be observed. The K-m value of this enzyme for sucrose was 4.4 mM at pH 4.0, but as the pH of the reaction mixture increased, the K-m value decreased sharply. From the dependence of the V-max and K-m values on pH, the ionization constant (pK(e)) of one of the two essential ionizable groups of the free enzyme was determined to be 2.7, suggesting that this essential ionizable group was a carboxyl.