Substrate stereospecificity of the NAD-dependent mannitol dehydrogenase from celery

The NAD-dependent mannitol dehydrogenase (MTD) of celery catalyses the interconversion of D-mannitol and D-mannose. This 1-oxidoreductase is uniquely different from all NAD-dependent polyol dehydrogenases described to date, which are 2-oxidoreductases. The stereospecificity of mannitol dehydrogenase was tested in the oxidative direction in the presence of polyol and NAD cofactor and in the reductive direction in the presence of aldose and NADH. The enzyme would be expected to show the same stereospecificity in either direction. The stereospecificity in the reductive direction was tested by attempted reduction of all eight D- and L-pentoses and 15 of the 16 D- and L-hexoses. Stereospecificity in the oxidative direction was tested with the four pentitols and four of the hexitols. Mannitol dehydrogenase showed a marked preference for aldopentose and aldohexose substrates with the same absolute configuration at C-2 as that of D-mannose. Reduction of L-idose by mannitol dehydrogenase was the only exception to the stated stereochemical preference among 23 aldoses and eight alditols tested. The sugar D-threose that occurs rarely in nature is a competitive inhibitor (K-i = 18 mM) of mannitol oxidation. The physiologically important hexitols, galactitol and glucitol, are oxidized by MTD to aldoses that are not metabolized by higher plants. Copyright (C) 1996 Elsevier Science Ltd