THE CONVERSION IN VIVO OF XYLITOL TO GLYCOGEN VIA THE PENTOSE PHOSPHATE PATHWAY
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Abstract:
Studies in this laboratory designed to elucidate the metabolism of L-xylulose, the characteristic urinary sugar of essential pentosuria, have shown that n-glucuronolactone is a precursor of the ketopentose in guinea pigs and in humans (1, 2) and that there are enzymes in guinea pig liver which can interconvert L-xylulose and xylitol as well as xylitol and D-xylulose (3, 4). The discovery that n-xylulose 5-phosphate is an intermediate in the pentose phosphate shunt (6-phosphogluconate oxidation pathway) (5, 6) suggested that glucuronate and the xyluloses might be ultimately metabolized via this pathway (4, 7). A kinase has indeed been found in calf liver which can convert n-xylulose to n-xylulose 5-phosphate (8). These findings suggest that the following sequence of reactions occurs in mammals: n-glucuronate --) L-xylulose G= xylitol ti n-xylulose + D-xylulose 5-phosphate. Since the formulation of this sequence is based upon work done on different species and partly on enzymatic studies performed in vitro, it seemed desirable to obtain evidence in tivo regarding its physiological occurrence. The tracer experiments reported elsewhere (2) have demonstrated the conversion of n-glucuronolactone to L-xylulose in a pentosuric human through reactions involving loss of the carboxyl carbon of the former compound, with the aldehydic carbon becoming the fifth carbon of the pentose. The present paper reports a study of the metabolic fate of a compound which is intermediate in the reaction sequence shown above. Xylitol-l-Cl4 was chosen because it can be conveniently prepared from the readily available n-xylose-1-CY4. n-Ribose, known to be metabolized via the pentose phosphate shunt (9), and n-xylose were tested for comparative purposes. A recent report has shown that the latter is also metabolized via this pathway (10). It was first demonstrated that xylitol is oxidized in vivo to carbon dioxide. A study was then undertaken to determine whether the distribution ofKeywords:
Glucuronates
Pentose
Metabolic pathway
Yeast extract
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Pentose
Bioconversion
Lignocellulosic Biomass
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Yeast extract
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Xylose metabolism
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Glucuronates
Carbohydrate Metabolism
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Carbon source
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Yeast extract
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In this paper we first determined the adsorption isotherms of xylose and xylitol, and simulated their mathematics models. Then with Lumped model, we simulated the concentration of xylose and xylitol in columns by material equilibrium and isotherms. The simulated result was very satisfactory.
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