Purification and properties of alanine racemase from crayfish Procambarus clarkii

Fresh water crayfish Procambarus clarkii is known to accumulate D-alanine remarkably in muscle after seawater acclimation, accompanied by an increase in alanine racemase activity. We have purified alanine racemase from crayfish muscle to homogeneity. The enzyme is a monomeric protein with a molecular mass of 58 kDa. It is highly specific to alanine and does not racemize L-serine, L-aspartate, L-glutamate, L-valine and L-arginine. The enzyme shows the highest activity at pH 9.0 in the conversion of L- to D-alanine and at pH 8.5 in the reverse conversion. Properties such as amino acid sequence, quaternary structure, pyridoxal 5'-phosphate (PLP)-dependency, pH-dependency and kinetic parameters seem to be distinct from those of the microbial alanine racemases. Various salts including NaCl at concentrations around seawater level were potently inhibitory for the activity in both of L- to -D and D- to -L direction.