Catalyzes the reversible epimerization of D-ribulose 5-phosphate to D-xylulose 5-phosphate. D-ribulose 5-phosphate = D-xylulose 5-phosphate Binds 1 divalent metal cation per subunit. Active with Fe(2+), and probably also with Mn(2+), Zn(2+) and Co(2+). Carbohydrate degradation. Homodimer. Belongs to the ribulose-phosphate 3-epimerase family. Extended C-terminus.