Purification and partial characterization of Lactobacillus species SK007 lactate dehydrogenase (LDH) catalyzing phenylpyruvic acid (PPA) conversion into phenyllactic acid (PLA).

Phenyllactic acid (PLA) is a novel antimicrobial compound synthesized by lactic acid bacteria (LAB), and its production from phenylpyruvic acid (PPA) is an effective approach. In this work, a lactate dehydrogenase (LDH), which catalyzes the reduction of PPA to PLA, has been purified to homogeneity from a cell-free extract of Lactobacillus sp. SK007 by precipitation with ammonium sulfate, ion exchange, and gel filtration chromatography. The purified enzyme had a dimeric form with a molecular mass of 78 kDa (size exclusion chromatography) or 39 kDa (SDS-PAGE). The ratio of enzyme activity with PPA to that with pyruvate being almost invariable at every purification step indicated that, in Lactobacillus sp. SK007, LDH is responsible for the conversion of PPA into PLA. HPLC profiles of PPA transformation into PLA by growing cells, cell-free extract, and purified LDH of Lactobacillus sp. SK007 were also investigated. Results showed that the presence of NADH was found to be necessary for the enzymatic production of PLA from PPA. The purified LDH displayed optimal activity for PPA at pH 6.0 and 40 degrees C. The Km values of the enzyme for PPA and pyruvate were 1.69 and 0.32 mM, respectively. Moreover, because other screened LAB strains exhibiting relatively high LDH activity toward PPA produced also considerable amounts of PLA, LDH activity for PPA could be therefore used as a screening marker for PLA-producing LAB.