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 Co(2+), Fe(2+), Mn(2+) and Zn(2+). Carbohydrate degradation; pentose phosphate pathway; D-xylulose 5-phosphate from D-ribulose 5-phosphate (non-oxidative stage): step 1/1. Belongs to the ribulose-phosphate 3-epimerase family.