Catalyzes the isomerization of L-ribulose 5-phosphate to D-xylulose 5-phosphate. Is involved in the anaerobic L-ascorbate utilization. L-ribulose 5-phosphate = D-xylulose 5-phosphate Binds 1 zinc ion per subunit. Cofactor degradation; L-ascorbate degradation; D-xylulose 5-phosphate from L-ascorbate: step 4/4. Induced by L-ascorbate. Repressed by UlaR. Belongs to the aldolase class II family. AraD/FucA subfamily.