Relative catalytic efficiencies and transcript levels of three d- and two l-lactate dehydrogenases for optically pure d-lactate production in Sporolactobacillus inulinus

As the optical purity of the lactate monomer is pivotal for polymerization, the production of optically pure d-lactate is of significant importance. Sporolactobacillus inulinus YBS1-5 is a superior optically pure d-lactate-producing bacterium. However, little is known about the relationship between lactate dehydrogenases in S. inulinus YBS1-5 and the optical purity of d-lactate. Three potential d-lactate dehydrogenase (D-LDH1-3)- and two putative l-lactate dehydrogenase (L-LDH1-2)-encoding genes were cloned from the YBS1-5 strain and expressed in Escherichia coli D-LDH1 exhibited the highest catalytic efficiency toward pyruvate, whereas two L-LDHs showed low catalytic efficiency. Different neutralizers significantly affected the optical purity of d-lactate produced by strain YBS1-5 as well as the transcription levels of ldhDs and ldhLs. The high catalytic efficiency of D-LDH1 and elevated ldhD1 mRNA levels suggest that this enzyme is essential for d-lactate synthesis in S. inulinus YBS1-5. The correlation between the optical purity of d-lactate and transcription levels of ldhL1 in the case of different neutralizers indicate that ldhL1 is a key factor affecting the optical purity of d-lactate in S. inulinus YBS1-5.