Synthesis of d-xylulose from d-arabitol by enzymatic conversion with immobilized mannitol dehydrogenase from Rhodobacter sphaeroides

d-Xylulose was produced by enzymatic conversion of d-arabitol using mannitol dehydrogenase from Rhodobacter sphaeroides. The purified enzyme was immobilized on CNBr-activated Sepharose ® 4B resulting in a yield of 40.5% with a binding capacity of 20.7 U g −1. The K mapp value of the immobilized enzyme for d-arabitol was determined to be 1.9 mM, the inhibitor constants for d-xylulose and NADH were 1 mM and 0.2 mM, respectively. Bioconversion of d-arabitol was carried out in a batch procedure, using a methylene blue/O 2/diaphorase system for regeneration of NAD +. The yield of the enzymatic conversion could be improved from 42% to 80% by complexing the produced d-xylulose with borate and thereby preventing a product inhibition of mannitol dehydrogenase. d-Xylulose was purified from the reaction mixture by ligand exchange chromatography on Ca 2+ loaded DOWEX 50W X8 with a yield of 77%.