Catalyzes the isomerization of L-xylulose-5-phosphate to L-ribulose-5-phosphate. Is involved in the anaerobic L-ascorbate utilization. L-ribulose 5-phosphate = L-xylulose 5-phosphate Cofactor degradation; L-ascorbate degradation; D-xylulose 5-phosphate from L-ascorbate: step 3/4. Induced by L-ascorbate. Repressed by UlaR. Belongs to the L-ribulose-5-phosphate 3-epimerase family.