Short-chain dehydrogenases/reductases (SDR) constitute a large protein family. Presently, at least 57 characterized, highly different enzymes belong to this family and typically exhibit residue identities only at the 15-30% level, indicating early duplicatory origins and extensive divergence. In addition, another family of 22 enzymes with extended protein chains exhibits part-chain SDR relationships and represents enzymes of no less than three EC classes. Furthermore, subforms and species variants are known of both families. In the combined SDR superfamily, only one residue is strictly conserved and ascribed a crucial enzymatic function (Tyr 151 in the numbering system of human NAD(+)-linked prostaglandin dehydrogenase). Such a function for this Tyr residue in SDR enzymes in general is supported also by chemical modifications, site-directed mutagenesis, and an active site position in those tertiary structures that have been characterized. A lysine residue four residues downstream is also largely conserved. A model for catalysis is available on the basis of these two residues. Binding of the coenzyme, NAD(H) or NADP(H), is in the N-terminal part of the molecules, where a common GlyXXXGlyXGly pattern occurs. Two SDR enzymes established by X-ray crystallography show a one-domain subunit with seven to eight beta-strands. Conformational patterns are highly similar, except for variations in the C-terminal parts. Additional structures occur in the family with extended chains. Some of the SDR molecules are known under more than one name, and one of the enzymes has been shown to be susceptible to native, chemical modification, producing reduced Schiff base adducts with pyruvate and other metabolic keto derivatives. Most SDR enzymes are dimers and tetramers. In those analyzed, the area of major subunit contacts involves two long alpha-helices (alpha E, alpha F) in similar and apparently strong subunit interactions. Future possibilities include verification of the proposed reaction mechanism and tracing of additional relationships, perhaps also with other protein families. Short-chain dehydrogenases illustrate the value of comparisons and diversified research in generating unexpected discoveries.

Sixteen characterized alcohol dehydrogenases and one sorbitol dehydrogenase have been aligned. The proteins represent two formally different enzyme activities (EC 1.1.1.1 and EC 1.1.1.14), three different types of molecule (dimeric alcohol dehydrogenase, tetrameric alcohol dehydrogenase, tetrameric sorbitol dehydrogenase), metalloproteins with different zinc contents (1 or 2 atoms per subunit), and polypeptide chains from different kingdoms and orders (mammals, higher plants, fungus, yeasts). Present comparisons utilizing all 17 forms reveal extensive variations in alcohol dehydrogenase, but with evolutionary changes that are of the same order in different branches and at different times. They emphasize the general importance of particular residues, suggesting related overall functional constraints in the molecules. The comparisons also define a few coincidences between intron positions in the genes and gap positions in the gene products. Only 22 residues are strictly conserved; half of these are Gly, and most of the remaining ones are Pro or acidic residues. No basic residues, no straight‐chain hydrophobic residues, no aromatic residues, and essentially no branched‐chain or polar neutral residues are invariable. Tentative consensus sequences were calculated, defining 13 additional residues likely to be typical of but not invariant among the alcohol dehydrogenases. These show a predominance of Val, charged residues, and Gly. Combined, the comparisons, which are particularly relevant to the data base for protein engineering, illustrate the requirements for functionally important binding interactions, and the extent of space restrictions in proteins with related overall conformations and functions.

The amino acid sequence of sheep liver sorbitol dehydrogenase has been fitted to the high-resolution model of the homologous horse liver alcohol dehydrogenase by computer graphics. This has allowed construction of a model of sorbitol dehydrogenase that provides explanations why sorbitol is not a substrate for alcohol dehydrogenase, why ethanol is not a substrate for sorbitol dehydrogenase, and what determines its specificity for polyols. An important feature of the model is that one of the ligands to the active site zinc atom is a glutamic acid residue instead of a cysteine residue, which is the corresponding ligand in the homologous alcohol dehydrogenases. This is one component of the structural change that can be related to the different substrate specificities, showing how altered enzymic activity might be brought about by structural changes of the kind that it is now possible to introduce by site-directed mutagenesis and recombinant DNA techniques.

Sorbitol dehydrogenase from sheep liver shows similarities to mammalian and yeast alcohol dehydrogenases.Comparisons based on peptides from segments of sorbitol dehydrogenase reveal that homologous regions with 38% identity include two ligands to the active site zinc atom in liver alcohol dehydrogenase, as well as further important residues. Similarities in other regions are less extensive, exactly as they are between different alcohol dehydrogenases. In all aspects, sorbitol dehydrogenase appears as a typical member of the alcohol dehydrogenase family. On the other hand, alcohol dehydrogenase from Drosophila, which has a shorter subunit, is not closely related to either of these enzymes, except for a region that probably corresponds to the first part ofthe coenzyme binding domain in many dehydrogenases. Instead, Drosophila alcohol dehydrogenase in its supposed catalytic region shows similarities toward Klebsiea ribitol dehydrogenase, which also has a small subunit. It may be concluded that both alcohol and polyol dehydrogenases show two types of protein subunit, reflecting an early subdivision of polypeptide types into "long" and "short" subunits rather than into different enzymatic specificities or quaternary structures. The relationships explain Imown properties of all these enzymes and provide insight into functional mechanisms and evolutionary interpretations.Sorbitol dehydrogenase (SDH) has some structural properties resembling those of mammalian and yeast alcohol dehydrogenases (ADHs) (1). All these enzymes have subunits ofsimilar size range, are sensitive to the same types of inhibitor, and contain reactive cysteine residues in similar structures, including one ofthe zinc ligands at the active site ofhorse liver ADH (LADH), 4226The publication costs ofthis article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact.

The first primary structure for a sorbitol dehydrogenase has been determined by analysis of the tetrameric enzyme from sheep liver. The ['4C]carboxymethylated protein was cleaved with CNBr and proteolytic enzymes. Peptides were purified by several methods, often utilizing exclusion chromatography for pre-fractionation and reverse-phase high-performance liquid chromatography for final purification. Different methods of sequence analysis complemented each other, mainly the manual dimethylaminoazobenzene isothiocyanate method and the use of liquidphase sequencer degradations.All eight major CNBr fragments were purified and form the basis of the work. Three minor CNBr fragments derived from an acid cleavage and from a partly resistant Met-Thr bond were also obtained, as well as evidence for a contaminating homologous polypeptide. Most of the tryptic peptides were purified, including all with methionine residues, thus overlapping the CNBr fragments. Combined, all data permit the deduction of a 354-residue amino acid sequence for the polypeptide chain of sorbitol dehydrogenase. The N terminus is acyl-blocked, the C terminus is formed by a proline residue, tryptophan is the least common residue (two, at positions 50 and 301) and there are 10 cysteine residues, including the residue previously shown to be especially reactive (at postion 43). Similarities to 'long' alcohol dehydrogenases have functional implications.Liver sorbitol dehydrogenase was recently found to be similar to zinc-containing alcohol dehydrogenases [I], constituting an apparent structural intermediate [2] between the distantly related alcohol dehydrogenases from yeast and horse liver [3,4]. The results suggested wide and parallel evolutionary relationships between groups of dehydrogenases [5] and emphasized relationships in a metabolic pathway from glucose [6]. The complete primary structure of a sorbitol dehydrogenase is necessary for further correlations.In the present work, the amino acid sequence of the entire protein chain of sheep liver sorbitol dehydrogenase was determined. The work was centered on purification and analysis of all CNBr fragments, which were overlapped by identification of the methionine-containing tryptic peptides. Throughout, [14C]carboxymethylation of cysteine residues was used to obtain suitable markers in several peptides of all digests. A 354-residue primary structure for the protein chain was deduced. Evidence was obtained for the occurrence of a minor contaminating component which appeared distantly homologous to sorbitol dehydrogenase in one region. Otherwise the preparations are homogeneous, suggesting all subunits in the tetramer to be identical. Results to support the amino acid sequence are given in this study, together with aspects of methodological interest. Further correlations in a protein family of zinc-containing alcohol/polyol dehydrogenases are given in an accompanying paper [7].

A pathway from glucose via sorbitol bypasses the control points of hexokinase and phosphofructokinase in glucose metabolism. It also may produce glycerol, linking the bypass to lipid synthesis. Utilization of this bypass is favored by a plentiful supply of glucose-hence, conditions under which glycolysis also is active. The bypass further involves oxidation of NADPH, so the pentose phosphate pathway and the bypass are mutually facilitative. Possible consequences in different organs under normal and pathological, especially diabetic, conditions are detailed. Enzymes with related structures (for example, sorbitol dehydrogenase and alcohol dehydrogenase, and possibly, aldehyde reductase and aldose reductase, respectively) are linked functionally by this scheme. Some enzymes of the bypass also feature in glycolysis (aldolase and alcohol dehydrogenase), and these enzymes, with the reductases involved, are proteins known to occur in different classes or multiple isozyme forms. Two of the enzymes (aldolase and alcohol dehydrogenase) both involve classes with and without a catalytic metal (zinc). The existence of parallel pathways and the occurrence of similar enzymic steps in one pathway may help to explain the abundance and multiplicity of enzymes such as reductases, aldolases, and alcohol dehydrogenases.Although alcohol dehydrogenase is widespread in nature, is common in liver, and has been structurally characterized from several sources (1), its exact role in mammalian organs generally has remained unclear. In addition to ethanol oxidation, functions in bile acid formation, fatty acid degradation, vitamin A metabolism, a hydroxysteroid reaction, and other areas of metabolism have been considered (2). Different and multiple functions of alcohol dehydrogenase would be consistent with the considerable species variations and extensive evolutionary changes found.Recently, another common liver enzyme, sorbitol dehydrogenase, was shown to be structurally, mechanistically, and ancestrally related to alcohol dehydrogenase (3). The substrates of sorbitol dehydrogenase, fructose and sorbitol, are considered important in special organs, such as male sexual organs (4), or special disease states, such as hyperglycemic cataract formation (5) and possibly diabetic neuropathy (6) and glomerulosclerosis (7), but a more general metabolic significance that is compatible with the structural similarity of these enzymes has not been clear.The structural link between alcohol and polyol dehydrogenases showed a type ofparallel or convergent evolution ofa second, different enzyme in each case with related specificity (3). Enhancement of alcohol metabolism by fructose gave one association between the substrates for sorbitol and alcohol dehydrogenases. This effect may stem from avoidance of the ratelimiting NADH dissociation off alcohol dehydrogenase by an in situ coenzyme reoxidation with glyceraldehyde formed from fructose (8). Increased operation ofthe glycerol phosphate shuttle could also mediate the effect (9).The structural relat...

Evidence is given that tetrameric sorbitol dehydrogenase from sheep liver contains one zinc atom per subunit, most probably located at the active site, and no other specifically bound zinc or iron atom. In alcohol dehydrogenases that are structurally related to sorbitol dehydrogenase, more than one zinc atom per subunit can complicate investigations of zinc atom function. Therefore, sorbitol dehydrogenase will be particularly valuable for defining the precise roles of zinc in alcohol and polyol dehydrogenases, and for establishing correlations of structure and function with other important zinc‐containing proteins.

Glucose-6-phosphate dehydrogenase from Saccharomyces cerevisiae (bakers' yeast) reacts with acetylsalicylic acid, and this is accompanied by inactivation and modification of essentially one lysine residue per subunit. The amino acid sequence of an 11-residue tryptic peptide containing the reactive lysine residue of the yeast enzyme is given and establishes the existence of different subgroups of glucose-6-phosphate dehydrogenases. Thus, the labeled yeast structure has few similarities to the known structure around the reactive lysine residue of the enzyme from Leuconostoc mesenteroides, although it has extensive similarities with a structure in the human enzyme. It is further shown that amino acid sequences around reactive lysine residues of dehydrogenases in general vary, even though similarities occur around reactive lysine residues in 6-phosphogluconate, glutamate, and glyceraldehyde-3-phosphate dehydrogenases.