Purification of a membrane‐bound sorbitol dehydrogenase from Gluconobacter suboxydans

A sorbitol dehydrogenase was purified from the membrane fraction of Gluconobacter suboxydans KCTC 2111 (= ATCC 621) by chromatography on CM‐, DEAE‐, Mono S and Superose 12 columns. The purified enzyme showed a single activity band upon nondenaturing polyacrylamide gel electrophoresis (PAGE) and three subunits of 75, 50 and 14 kDa upon SDS‐PAGE. When purified preparations of the enzyme were reconstituted with pyrroloquinoline quinone (PQQ), the specific enzyme activity was significantly increased (up to 9‐fold). The absorption spectrum of purified sorbitol dehydrogenase in the reduced state exhibited three absorption maxima (417, 522 and 552 nm) which is in accordance with the typical absorption spectrum of cytochrome c . The 50 kDa subunit appeared as a red band on unstained SDS‐gels suggesting its identity as a cytochrome. Fluorescence spectra of extracts from purified sorbitol dehydrogenase showed an excitation maximum at 370 nm and an emission maximum at 465 nm, which conformed to those of authentic PQQ. The purified enzyme showed a rather broad substrate specificity with significant activity toward D‐mannitol (68%) and D‐ribitol (70%) as well as D‐sorbitol (100%). The PQQ‐dependent sorbitol dehydrogenase described in this study is clearly different from the FAD‐dependent sorbitol dehydrogenase from G. suboxydans var. α IFO 3254 strain in its cofactor requirement and substrate specificity.