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 descr
ibed to date, which are 2-oxidoreductases. The stereospecificity of ma
nnitol dehydrogenase was tested in the oxidative direction in the pres
ence 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 t
he 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 fou
r of the hexitols. Mannitol dehydrogenase showed a marked preference f
or aldopentose and aldohexose substrates with the same absolute config
uration at C-2 as that of D-mannose. Reduction of L-idose by mannitol
dehydrogenase was the only exception to the stated stereochemical pref
erence 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, galact
itol and glucitol, are oxidized by MTD to aldoses that are not metabol
ized by higher plants. Copyright (C) 1996 Elsevier Science Ltd