Catalyzes the conversion of L-arabinose to L-ribulose. beta-L-arabinopyranose = L-ribulose Binds 1 Mn(2+) ion per subunit. Carbohydrate degradation; L-arabinose degradation via L-ribulose; D-xylulose 5-phosphate from L-arabinose (bacterial route): step 1/3. Belongs to the arabinose isomerase family.