Thermolabile alanine racemase from a psychotroph, Pseudomonas fluorescens: purification and properties

A psychotrophic bacterium that produces a thermolabile alanine racemase was isolated from raw milk, and identified as Pseudomonas fluorescens TM5-2. The enzyme was purified to homogeneity from the cell extract, and characterized to be compared with enzymes from mesophiles (Bacillus subtilis and Salmonella typhimurium) and a thermophile (Bacillus stearothermophilus). The enzyme has a molecular weight of about 76,000 and consists of two subunits identical in molecular weight (38,000). The enzyme contains two mol of pyridoxal 5'-phosphate per mol as a coenzyme. The amino acid composition was different from those of other alanine racemases in content of valine. The amino acid sequence of the amino terminal region (from 1Met to 25Gly) had 21-33% homology with those of other alanine racemases. Kinetic parameters of the enzyme were similar to those of other alanine racemases. The enzyme is extremely labile over 30 degrees C, and shows the high catalytic activity even at 0 degrees C; it is thermolabile and psychotrophic.