THE G.A-GALACTOSIDASE OF GRACILARIA-TENUISTIPITATA AND G SORDIDA (GRACILARIALES, RHODOPHYTA)

An alpha-galactosidase (alpha-galactopyranoside galactohydrolase, EC 3.2.1.22) has been isolated from the red seaweeds Gracilaria tenuistipitata Zhang et Xia from China and Gracilaria sordida W. Nelson from New Zealand, and its properties have been investigated. The isolated enzyme displayed high activity towards p-nitrophenyl-alpha-D-galactopyranoside and floridoside (O-alpha-D-galactopyranosyl-(1-2)-glycerol), and low activity towards isofloridoside (O-alpha-D-galactopyranosyl-(1-1)-glycerol). It showed little activity towards melibiose, raffinose and other glycosides tested. The enzymes from G. tenuistipitata and G. sordida had K(m) values of 2.47 and 2.23 mM, respectively, towards p-nitrophenyl-alpha-D-galactopyranoside, and exhibited no substrate inhibition at the concentrations tested. The optimal pH range for enzyme activity was 7.4-7.9, and 2-mercaptoethanol was essential for both activity and stability. Stability was also improved at pH 8.2-8.5 and in the presence of 15% glycerol. The molecular weight of the enzyme, as estimated by Sephacryl S-300 HR, was 596 and 570 kDa for G. tenuistipitata and G. sordida, respectively. Activity of alpha-galactosidase was also demonstrated in a number of other species of red algae.