SDH

Sorbitol dehydrogenase (SDH) is a cytosolic enzyme encoded by the SORD gene in humans and belongs to medium-chain dehydrogenase/reductase protein family. SDH could convert sorbitol, the sugar alcohol form of glucose, into fructose in carbohydrate metabolism with NAD⁺ as a cofactor and together with aldose reductase. It contributes to the pathway for body to produce fructose from glucose without the participation of ATP, which is a crucial bypass to glycolysis for glucose metabolism and is believed to play an important role in the development of diabetic complications. As sorbitol has difficulty in diffusing out of the cells, and its oxidation procedure to fructose is slow, the accumulation of sorbitol owing to hyperglycemia not only enhances the intracellular osmotic pressure and thus promotes cell swelling, but also results in diabetic cataractogenesis. The catalyzed reaction is:

Figure 1. SDH catalyzed reaction.

where Zn²⁺ is also involved in the catalysis. SDH is also capable of oxidizing various polyols and other secondary alcohols into their corresponding ketones. SDH is expressed almost ubiquitously in all kinds of organisms from bacteria to humans and is most frequently used by liver and seminal vesicle. However, SDH possesses labile activity and has a short half-life. Herein, thus its utility is limited and its analysis must be conducted promptly.

Structure

Crystallization experiments and X-ray diffraction with a resolution of 2.20 Å have been conducted to determine the structure of human SDH, which consists of four identical chains of A, B, C, and D. Each one of them is 31% helical with 14 helices and 26% beta sheet with 23 strands, and has 356 residues and a catalytic site. According to MolProbity Ramachandran analysis, it is found that 97.1% of all residues are in favored regions and 100.0% of all residues are in allowed regions, with no outliers. The catalytic sites cover hydrophilic side chains of serine and histidine residues, which require the presence of NAD⁺ and a zinc ion for catalytic activity. In speak of the conserved zinc binding motif and the hydrophobic substrate-binding pocket, SDH has also been revealed to be structurally homologous to mammalian alcohol dehydrogenase. Recently, it is found that despite the overall 3D structural similarity between SDH and alcohol dehydrogenase, the zinc coordination in the active sites of the two enzymes is different. The available structural and biochemical data of SDH are being implicated in a structure-based approach to develop drugs for the prevention or treatment of the diabetic complications.

Subunit interactions in SDH

The non-covalent interactions between subunits of SDH could form a tetramer and consist of a hydrophobic effect, hydrogen bonds, and electrostatic interactions among the four identical subunits. For the homotetrameric protein SDH, the structure is considered to evolve from a monomeric to a dimeric and finally generate a tetrameric quaternary protein structure. There are various methods such as protein sequence alignments, structural comparisons, energy calculations, gelfiltration experiments, and enzyme kinetics experiments have revealed that a hydrogen-bonding network between subunits is important for the stability of the tetrameric structure. The general binding process in SDH without the gain energy can be determined though the rate of association and dissociation between subunits. SDH protein has a close evolutionary relationship with alcohol dehydrogenase. Both the SDH from silver leaf whitefly and alcohol dehydrogenase from yeast lack a structural zinc site and share a tetrameric quaternary structure. Thence, from a structural viewpoint the two classes of proteins exhibit a close evolutionary relationship.

Figure 2. Assembly of the four subunits (A,B,C and D) in SDH

Functions

One of the main applications of SDH is to help the metabolism of dietary sorbitol that is particularly important for the function of various organs in the body, although sorbitol is usually found in quite small amounts in diet and is not absorbed in the intestine as well as its related compounds glucose and fructose. In the polyol pathway from glucose to fructose (glycolysis), aldose reductase promotes the conversion of glucose into sorbitol accompanied by the oxidation of NADPH to NADP⁺ and SDH facilitates the oxidization of sorbitol into fructose along with the reduction of NAD⁺ to NADH. The sorbitol pathway is meaningful for it does not involve the input of energy in the form of ATP. Excessive glucose leads to the depletion of NADPH, an essential cofactor in the production of glutathione.

Figure 3. A polyol pathway involving SDH. (El-Kabbani O; et al. 2004)

In uncontrolled diabetes, large amounts of glucoses enter tissues where SDH is low or absent, such as in the retina, lens, kidney, and nerve cells and are then catalyzed into sorbitol by aldose reductase, which results in the accumulation of sorbitols and further causes water to be drawn into the cell due to the increased osmotic pressure, thus impairing tissue function. Symptoms like retinopathy, cataract formation, nephropathy, and peripheral neuropathy observed in diabetes are partly induced by this phenomenon. Therefore, there is a growing interest in SDH as a consequence of its implication in the development of diabetic complications and accordingly the exploration of drugs for the treatment of diabetes may be facilitated by its tertiary structure. SDH is sometimes included as biomarkers in clinical chemistry panels for hepatocellular toxicity, and generally regarded to be mainly liver specific.

Reference

1. El-Kabbani O, Darmanin C, Chung R P. Sorbitol dehydrogenase: structure, function and ligand design. Curr Med Chem, 2004, 11(4):465–476.