Beta-D-phosphogalactoside galactohydrolase from Streptococcus cremoris HP: purification and enzyme properties

beta-d-phosphogalactoside galactohydrolase (beta-PGal) was isolated and purified from cell-free extracts of Streptococcus cremoris HP to apparent homogeneity to gel electrophoresis. Using the chromogenic o-nitrophenol-beta-d-galactopyranoside-6-phosphate as substrate, the purified enzyme exhibited a specific activity of 18.71 U/mg of protein and K(m) and V(max) values of 5.88 x 10(-4) M and 23.8 mumol of o-nitrophenol liberated per min per mg of protein, respectively. d-Galactose-6-phosphate was a weak competitive inhibitor of beta-PGal. Activity was relatively heat resistant and was maximal from pH 5.0 to 8.0 and over a temperature range of 45 to 52 C. Dithiothreitol, ethylenediaminetetraacetic acid, and citrate stimulated beta-PGal activity, whereas Mg(2+), Li(1+), and p-hydroxymercuribenzoate were inhibitory. Molecular weight of the enzyme was estimated at 6.76 x 10(4). Amino acid composition was similar to other beta-phosphogalactosidases previously investigated, with the exception that the S. cremoris enzyme contains a small amount of half cystine.