Sorbitol dehydrogenase, reaction

Figure 5. Example of dehydrogenase reactions which can be coupled with the bienzymatic bacterial bioluminescent system. ADH = alcohol dehydrogenase (EC 1.1.1.1), SDH = sorbitol dehydrogenase (EC 1.1.1.14), LDH = lactate dehydrogenase (EC 1.1.1.27), MDH = malate dehydrogenase (EC 1.1.1.37).

It is also the case that enzymes showing sequence similarities do not necessarily catalyse the same reactions. Sheep liver sorbitol dehydrogenase (EC 1.1.1.14) does not utilize ethanol, though in primary structure it resembles both yeast and horse liver alcohol dehydrogenases [5,6]. 

The polyol pathway consists of two enzymes, AR and sorbitol dehydrogenase. AR has a rather broad substrate specificity for sugars and a fairly high Km for glucose.These two enzymes catalyze the following reactions ... 

In these coupled reactions, the overall transformation of glucose to fructose is accompanied by a transfer of hydrogen from NADPH to NAD. Studies on the activity of aldose reductase and sorbitol dehydrogenase as well as the presence of sorbitol in seminal vesicles of several species and rat coagulating gland are consistent with the operation of the sorbitol pathway as a major determinant of seminal fructose biosynthesis (248, 252-260). Nonphosphorylative conversion of glucose to sorbitol has also been demonstrated in placenta (258, 261, 262), smooth muscle of the aorta (263), the lens of the eye (264, 265), erythrocytes (266) and hepatoma-derived (HTC) cells in culture (267), and possibly normal liver cells (267, 268). 

Sorbitol is metabolized by the liver (Fig. 1) and oxidized to fructose by the enzyme sorbitol dehydrogenase in the course of which reaction NAD is reduced (Fig. 1), Fructose is then phos-phorylated to form fructose-l-phosphate by hepatic phosphofructo-kinase in a reaction requiring ATP. Through the action of hepatic 1-phosphofructoaldolase, the fructose-l-phosphate yields dihydrox-yacetonephosphate and glyceraldehyde. While dihydroxyacetonephos-... 

D-Glucose can be specifically reduced in the presence of other sugars by the enzyme, D-sorbitol dehydrogenase, using the reduced form of the coenzyme nicotinamide adenine dinucleotide (NADH). The reaction is reversible, and d-glucitol (D-sorbitol) can be specifically oxidized to D-glucose using the oxidized form of the coenzyme, NAD+. 

Sorbitol dehydrogenase is obtained commercially from sheep liver. It occurs naturally in humans in two specific locations, sperm and the eye. Its function in the eye is to keep the concentration of D-sorbitol low D-sorbitol is a factor in the formation of cataracts. It is, however, the reaction of sorbitol dehydrogenase in the reverse reaction with the high concentrations of D-glucose resulting from an uncontrolled diabetic condition that leads to diabetic cataracts. 

Different preparative procedures have been shown to yield protein fractions which are able to catalyze different types of reactions with respect to their requirement of either NAD or NADP as coenzymes [cf. Eqs. (19), (20), and (21)]. In sera of mice poisoned by carbon tetrachloride we found polyol dehydrogenases catalyzing the oxidation of the following polyols (a) with NAD sorbitol, ribitol, mannitol (b) with NADP sorbitol, ribitol. Erythritol and mt/o-inositol were not attacked at all. Figures 8 and 9 show the results of these determinations performed at pH 9.6. In the NAD system sorbitol and ribitol are oxidized at exactly the same rate, while in the NADP system ribitol does not reach the rate of sorbitol. The ratio NAD NADP for sorbitol is calculated to be 4.20 and for ribitol 5.50. Mannitol is oxidized at 23% of the rate of sorbitol. 

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