Alanine Racemase from an Acidophile, Acidiphilium organovorum: Purification and Characterization

An alanine racemase (EC 5.1.1.1) from an acidophilic heterotrophic bacterium, Acidiphilium organovorum 13H, was purified and characterized. The enzyme had a dimeric structure with identical subunits of M r 33,000 each. Although A. organovorum 13H is an acidophile, the enzyme had its maximum velocity at pH 9, corresponding to its location in the cytoplasm. Activity was maximum between 50 and 60°C. For an enzyme from a mesophile, it was stable to heat, showing no loss of activity after a 30-min incubation at 65°C. The enzyme needed pyridoxal 5'-phosphate (PLP) as a cofactor for its activity, as seen from the loss of activity upon dialysis against PLP-free buffer containing hydroxylamine and its absorption maximum at 420 nm. Activity was ihhibited by common inhibitors of PLP-dependent enzymes. PLP content studies found that 1 mole of enzyme contained 2 moles of PLP. The enzyme catalyzed the symmetric reversible racemization of alanine exclusively.