Bifidobacterium longum NRRL B-41409 l-arabinose isomerase (l-AI) was cloned and overexpressed in Lactococcus lactis using a phosphate-depletion-inducible expression system. The purified B. longum l-AI was characterized using d-galactose and l-arabinose as the substrates. The enzyme was active and stable at acidic pH with an optimum at pH 6.0-6.5. The enzyme showed the highest activity at 55 A degrees C during a 20-min incubation at pH 6.5. The K (m) value was 120 mM for l-arabinose and 590 mM for d-galactose. The V (max) was 42 U mg(-1) with l-arabinose and 7.7 U mg(-1) with d-galactose as the substrates. The enzyme had very low requirement for metal ions for catalytic activity, but it was stabilized by divalent metal ions (Mg2+, Mn2+). The enzyme bound the metal ions so tightly that they could not be fully removed from the active site by EDTA treatment. Using purified B. longum l-AI as the catalyst at 35 A degrees C, equilibrium yields of 36 % d-tagatose and 11 % l-ribulose with 1.67 M d-galactose and l-arabinose, respectively, as the substrates were reached.