A novel enzymatic approach to the massproduction of L-galactose from L-sorbose

Wild-type strain of Pseudomonas cichorii ST-24 was unable to grow on D -psicose and inductively produced D -tagatose 3-epimerase (D -TE) with D -tagatose as an inducer. We have isolated a constitutive mutant, designated strain Ka75, which had acquired a new ability to grow on a mineral salts medium containing D -psicose as a sole carbon source. The D -psicose-metabolizing mutant synthesized a high level of D -TE. When grown on the culture medium supplemented with Mn(2+), the mutant strain produced around 250-fold higher activity than did the parent strain. Enzymatic properties of the constitutive enzyme were similar to those of the wild-type. Using the immobilized D -TE and recombinant L-rhamnose isomerase (L-RhI) from Escherichia coli strain JM109, a two-step enzymatic reaction was performed for massproduction of a rare aldo-hexose monosaccharide, L-galactose, from a common one, L-sorbose. In the first step, L-sorbose was epimerized to L-tagatose in a yield of 28%. The L-tagatose obtained was utilized as a starting material for L-galactose preparation by the immobilized L-RhI. At equilibrium, approximately 30% L-tagatose was isomerized to L-galactose. Finally, 7.5 g of L-galactose was obtained from 100 g of L-sorbose, viz an overall yield of 7.5%. The product obtained was purified and identified to be L-galactose by specific optical rotation and high performance liquid chromatography (HPLC) analysis, and was ultimately confirmed by (13)C nuclear magnetic resonance ((13)C NMR) and IR spectra.