Lactose and D-galactose metabolism in Staphylococcus aureus. II. Isomerization of D-galactose 6-phosphate to D-tagatose 6-phosphate by a specific D-galactose-6-phosphate isomerase.

The inducible D-galactose-6-phosphate isomerase that functions in the metabolism of lactose and D-galactose in Staphylococcus aureus was partially purified from extracts of D-galactose-grown cells. It was shown to catalyze specifically the reversible isomerization of D-galactose 6-phosphate to D-tagatose 6-phosphate, the apparent Km values being 9.6 mM and 1.9 mM, respectively. At equilibrium, the ratio of D-galactose 6-phosphate to D-tagatose 6-phosphate was 9.0. The enzyme was not simulated by mono- or divalent cations and was not inhibited by EDTA, but it was inactivated reversibly by the thiol reagent N-ethylmaleimide. Its molecular weight was estimated to be about 100,000 both by gel filtration and by sedimentation in a sucrose density gradient. Data on stability, pH optimum, and inducibility of the enzyme are also presented. An improved procedure for the chemical synthesis of D-tagatose 6-phosphate is described, and resolution of the anomers of D-tagatose 6-phosphate by gas-liquid chromatography is reported.