Catalyzes the epimerization at C4 of 3-dehydro-D-gulosides leading to 3-dehydro-D-glucosides. Probably functions in a metabolic pathway that transforms D-gulosides to D-glucosides. Can use methyl alpha-3-dehydro-D-glucoside and methyl beta-3-dehydro-D-glucoside as substrates in vitro. However, the actual specific physiological substrates for this metabolic pathway are unknown. Cannot act on D-psicose, D-fructose, D-tagatose, D-sorbose, L-xylulose, or L-ribulose. a 3-dehydro-D-guloside = a 3-dehydro-D-glucoside Binds 1 Mn(2+) ion per subunit. kcat is 19.2 sec(-1) for the epimerization of methyl alpha-3-dehydro-D-glucoside (at pH 8.0 and 30 degrees Celsius). kcat is 2.2 sec(-1) for the epimerization of methyl beta-3-dehydro-D-glucoside (at pH 7.0 and 30 degrees Celsius). kcat is 4.0 sec(-1) for the epimerization of methyl beta-3-dehydro-D-glucoside (at pH 8.0 and 30 degrees Celsius). Belongs to the hyi family.