Dehydrogenases Containing Pyridine Nucleotides

The ALDs are a subset of the superfamily of medium-chain dehydrogenases/reductases (MDR). They are widely distributed, cytosolic, zinc-containing enzymes that utilize the pyridine nucleotide [NAD(P)+] as the catalytic cofactor to reversibly catalyze the oxidation of alcohols to aldehydes in a variety of substrates. Both endobiotic and xenobiotic alcohols can serve as substrates. Examples include (72) ethanol, retinol, other aliphatic alcohols, lipid peroxidation products, and hydroxysteroids (73). 

The stereospecificity of hydrogen transfer for estradiol-17 and estradiol-17(3 dehydrogenases has been examined by George et a/.84>. These enzymes are both present in chicken liver, and have substrates which differ only in the chirality of their substituents at C—17. Both of these enzymes were shown to use the 4-pro-S or 4B proton of the NADPH. Since the steroid is a bulky substrate, the authors argue that the steric fit between pyridine nucleotide and steroid cannot be as important as the role played by the enzyme in directing the fit. This paper contains an interesting summary of other recent work on the stereospecificity of pyridine nucleotide dependent-steroid dehydrogenases. 

Liver cells contain two different but closely related enzymes glycerol phosphate dehydrogenase which is specific for NAD, and acylglycerol phosphate dehydrogenase, which is NADP specific. Both enzymes have B stereospecificity for the pyridine nucleotide 93. They apparently have different metabolic functions. 

Electron transfer between pyridine nucleotides and disulfide compounds is catalyzed by several fiavoproteins and three of these are well characterized. Lipoamide dehydrogenase functions in the oxidative decarboxylation of a-keto acids catalyzing the reoxidation of reduced lipoate by NAD+ (18, 19). Glutathione reductase catalyzes electron transfer between NADPH and glutathione ZO-22). Thioredoxin reductase catalyzes the reduction of thioredoxin by NADPH (5) thioredoxin is a protein of 12,000 molecular weight containing a single cystine residue which is the electron acceptor S3). 

NADPH was regenerated under anaerobic conditions by crude extracts of C. thermoaceticum containing formate dehydrogenase (FDH) as well as artificial mediator accepting pyridine nucleotide oxidoreductases (AMAPORs) and formate as electron donor. For electromicrobial redox reactions see Section 6. 

Adams et al. 177a) prepared the hydroxypyrroline carboxylic acid from hydroxyproline and studied its metabolic reactions. Acetone powder extracts of liver were found to contain an enzyme (A -pyrroline dehydrogenase) which oxidized this compound to 7-hydroxyglutamate, utilizing either pyridine nucleotide as a hydrogen acceptor. Support for this step,... 

Mitsuhashi and Davis 213) also discovered a dehydrogenase that oxidizes quinic acid to dehydroquinic acid in certain mutants of Aerdbactor. In this instance DPN is the required pyridine nucleotide. The bacterial strains that contain this enzyme respond to quinate as a growth factor. This dehydrogenase was purified in the same manner and to about the same extent as dehydroshikimic reductase. The pH optimum of quinic dehydrogenase was found to be at 9.8 Km values obtained were 4.9 X 10" M for quinic acid and 1.4 X 10 M for DPN at pH 9.4. 

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