Chirality, Hydrogen Transfer, myo-Inositol Dehydrogenase. The chirality of the hydrogen transfer to NAD catalyzed by myo-inositol dehydrogenase (myo-Inositol: NAD 2-oxydoreductase, EC 1.1.1.18) from Klebsiella pneumoniae (formerly classified taxonomically as Aerobacter aerogenes or Kleb siella aerogenes) was investigated. [4-3H] NAD was enzymatically reduced to [4-3H] NADH with non-labeled myoinositol and myo-inositol dehydrogenase. The stereochemistry of the prochiral center at C4 of the NADH produced was determined. It was found that the label was exclusively located at the (4S) position of the produced NADH. Since the hydrogen transferred from non-labeled myo-inositol to [4-3H] NAD must have entered the opposite of (R) position, myo-inositol dehydrogenase from K. pneumoniae should be classified as an (R) or A-type enzyme with respect to the stereochemistry of the hydrogen transfer to NAD.