Sorbitol Dehydrogenase Membrane Bound

Sorbitol ^ L-sorbose

Unlike the ubiquitous quinoprotein polyol dehydrogenases in acetic acid bacteria, one of which catalyzes oxidation of D-sorbitol to L-sorbose, a distinct D-sorbitol dehydrogenase (SLDH) was found in the membrane fraction of G. suboxydans var. a IFO 3254. SLDH was solubilized from the membrane fraction and purified as a covalently bound FAD-containing dehydrogenase [67]. The best solubilization of SLDH carried out in 10 mmol L-1 acetate, pH 5.0, containing 1% Triton X-100, 0.1 mol L-1 KCl, and 0.1 mol L-1 D-sorbitol gave the highest recovery of more than 100%. Purification of SLDH was readily done by DEAE-cellulose and CM-cellulose chromatography. The purified SLDH showed a rose-red color due to the presence of heme c and the absorption spectrum of SLDH after reduction with sodium dithionite gave absorption maxima at 551, 522, and 417 nm in the visible region. The heme c component was not reduced rapidly by the addition of D-sorbitol, primary substrate, until coenzyme Q1 was added to the enzyme solution, implying that ubiquinone mediates D-sorbitol oxidation, similar to the other membrane-bound dehydrogenases exemplified in this article.

SLDH dissociated into three subunits of 63 kDa (subunit I), 51 kDa (subunit II), and 17 kDa (subunit III). Subunit I was characterized as a flavoprotein and FAD was bound covalently to the enzyme. Subunit II contained heme c. SLDH catalyzes D-sorbitol oxidation to L-sorbose exclusively, which is distinct from PQQ-dependent polyol dehydrogenases catalyzing D-sorbitol oxidation to L-sorbose. The substrate specificity is restricted to D-sorbitol, and D-mannitol was oxidized only by 5% of that of D-sorbitol. Other polyols, such as D-arabitol, D-iditol, meso-erythritol, dulcitol, ribitol, and xylitol, are not oxidized by the enzyme.

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