Pentitol-Metabolizing Enzymes of the Uronic Acid Pathway
According to the sequence of reactions of the uronic acid pathway shown in the preceding chapter, two pentitol dehydrogenases (Fig. 1) linking L-and D-xylulose via xylitol are operative in this third pathway of carbohydrate metabolism in mammals, namely the NAD- and the NADP-specific xylitol dehydrogenase, systematically named xylitol: NAD oxidoreductase (D-xylulose-forming) (EC 220.127.116.11) and xylitol: NADP oxidoreductase (L-xylulose-forming) (EC 18.104.22.168), respectively. Mention has also been made already of the fact that pentosuric individuals, after administration of D-glucuronolactone-1-13C, excrete labeled L-arabitol in their urine . This observation may be taken as suggestive evidence for the existence in mammals of a third pentitol oxidizing enzyme interconverting L-arabitol and L-xylulose. There is no indication whatever for the occurrence in mammals of an enzyme possibly linking D-arabitol to D-xylulose. Moreover, D-arabitol has been shown to be a very poor precursor of glycogen in rats guinea pigs . Therefore, only the three systems mentioned above have to be discussed in the following sections.
KeywordsLiver Mitochondrion Pyridine Nucleotide Xylitol Dehydrogenase Polyol Dehydrogenase Dietary Ascorbic Acid
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