Mechanism of D-alanine production by Corynebacterium fascians

The amounts of extracellular D-alanine accumulated by Corynebacterium fascians ATCC 21950 in a medium containing glycerol as the sole carbon source is increased to almost 12 mg/ml by adding pyruvate to the medium. Cell-free extracts of C. fascians were shown to possess both L-alanine dehydrogenase and alanine racemase activities. These results indicated that a mechanism exists that allows this microorganism to synthesize D-alanine from pyruvate. A study comparing the optical purity of the intracellular alanine and the extracellular alanine suggested that the cell membrane possesses the stereospecific permeability for D-alanine. Thus, it may be concluded that L-alanine is first formed from pyruvate by L-alanine dehydrogenase and then converted to D-alanine by racemase inside the cells. Subsequently, only D-alanine leaks out stereospecifically through the cell membrane, and large amounts of D-alanine accumulate in the extracellular medium.