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 ma nnitol dehydrogenase from Rhodobacter sphaeroides. The purified enzyme was immobilized on CNBr-activated Sepharose(R) 4B resulting in a yiel d of 40.5% with a binding capacity of 20.7 U g-1. The K(m app) value o f the immobilized enzyme for D-arabitol was determined to be 1.9 mM, t he inhibitor constants for D-xylulose and NADH were 1 mM and 0.2 mM, r espectively. Bioconversion of D-arabitol was carried out in a batch pr ocedure, using a methylene blue/O2/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 there by preventing a product inhibition of mannitol dehydrogenase. D-Xylulo se was purified from the reaction mixture by ligand exchange chromatog raphy on Ca2+ loaded DOWEX 50W X8 with a yield of 77%.