Rat sorbitol dehydrogenase was expressed in Escherichia coli and purified to homogeneity, resulting in a protein with a specific activity of 4.7 U/mg, close to that of the enzyme isolated from mammalian liver. A Glu residue has been postulated to replace the Cys of alcohol dehydrogenase as a ligand to the active-site zinc atom of sorbitol dehydrogenase. This Glu (position 155 in the rat enzyme) was mutated both to Cys, in order to mimic the alcohol dehydrogenase relationships, and to Ala, as a control. A third mutation, Cys164 to Ala, was also performed since Cys has also been considered as a possible zinc ligand. With Ala at position 155, an inactive enzyme was obtained, showing that correct active-site relationships have been destroyed. With Cys at position 155, the enzyme is still partly active, but rapidly looses activity unless stabilized by the addition of ZnSO4. The catalytic efficiency in the oxidation of sorbitol is 120-fold less than that of the native form, and reduction of fructose is lost completely. In contrast, the activity of the Cys164Ala mutant is comparable with that of the native enzyme and, in fact, even increased in the oxidation of sorbitol. Combined, the results strongly suggest that Glu155 is a ligand to the active-site zinc atom. Zinc analysis of the different variants of sorbitol dehydrogenase establishes that all contain one atom of zinc/subunit, also when the catalytic function is lost. Apparently, zinc remains coordinated even after replacement with an amino acid residue (Ala) unable to ligand metal atoms.