Utilization of Lactate Isomers by Propionibacterium freudenreichii subsp. shermanii : Regulatory Role for Intracellular Pyruvate

Abstract

Five strains of Propionibacterium freudenreichii subsp. shermanii utilized the l -(+) isomer of lactate at a faster rate than they did the d -(−) isomer when grown with a mixture of lactate isomers under a variety of conditions. ATCC 9614, grown anaerobically in defined medium containing 160 mM dl -lactate, utilized only 4 and 15% of the d -(−)-lactate by the time 50 and 90%, respectively, of the l -(+)-lactate was used. The intracellular pyruvate concentration was high (>100 mM) in the initial stages of lactate utilization, when either dl -lactate or the l -(+) isomer was the starting substrate. The concentration of this intermediate dropped during dl -lactate fermentation such that when only d -(−)-lactate remained, the concentration was <20 mM. When only the d -(−) isomer was initially present, a similar relatively low concentration of intracellular pyruvate was present, even at the start of lactate utilization. The NAD + -independent lactate dehydrogenase activities in extracts showed different kinetic properties with regard to pyruvate inhibition, depending upon the lactate isomer present. Pyruvate gave a competitive inhibitor pattern with l -(+)-lactate and a mixed-type inhibitor pattern with d -(−)-lactate. It is suggested that these properties of the lactate dehydrogenases and the intracellular pyruvate concentrations explain the preferential use of the l -(+) isomer.