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 126.96.36.199) and xylitol: NADP oxidoreductase (L-xylulose-forming) (EC 188.8.131.52), 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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- 6.Hollmann, S., G. Laumann: Hoppe-Seylers Z. physiol. Chem. 348, 1073 (1967).Google Scholar
- 10.Robinson, D., R. T. Williams: Biochem. J. 68, 23P (1958).Google Scholar
- 18.Touster, O., G. Montesi: In: S. P. Colowick and N. O. Kaplan, Methods in enzymology, vol. V, p. 317- New York: Academic Press 1962.Google Scholar
- 19.Hollmann, S., D. Lattermann: Hoppe-Seylers Z. physiol. Chem. 350, 106 51(1969).Google Scholar