The Metabolic Fate of C14 Labeled Pentoses in Man

Abstract

The role of pentoses in intermediary metabolism has received much attention within the past few years. The phosphate esters of D-ribose, D-ribulose and D-xylulose have been identified as key intermediates in the pentose phosphate pathway of glucose metabolism in both plant and mammalian tissue (1). More recently, another alternate pathway of glucose dissimilation, the uronic acid pathway, has been described in which L-Xylulose, xylitol and D-xylulose are intermediates (2).Several pentoses have been identified in normal urine: the aldopentoses xylose, arabinose and ribose (3, 4) and the ketopentoses D-ribulose and L-xylulose (5). Large quantities of the latter sugar are excreted in the disease essential pentosuria (6, 7), which appears to be due to a defect in the further conversion of L-xylulOse to the sugar alcohol xylitol (8). In addition to L-xylulOse Touster and Harwell (9) have also reported the isolation of L-arabitol from the urine of pentosuric subjects.Because of increasing awareness of the biological importance of the pentoses, certain aspects of pentose metabolism in man have been investigated in this laboratory. Previous reports have described the physiological disposition of large quantities of infused pentoses (10), the effect of insulin on blood levels of infused pentoses (11) as well as studies of the metabolism of D-ribose (12). This paper reports some observations on the fate of C14 labeled D-xylOse, D-lyxose, D-arabinose and L-arabinose administered in trace quantities intravenously to normal human subjects. Some of the previously published data (12) on C14 ribose metabolism are included for purposes of comparison. METHODSFive experiments were performed in four normal male volunteers aged 18 to 21. Each subject had been on a 250 Gm. carbohydrate diet for at least three days and was fasted overnight prior to study.Five /Ac. of each sugar dissolved in 250 ml. of normal saline were infused over a 15 minute period. Expired air samples were collected for 4 minutes at 15 to 60 minute intervals for 6 hours. Urine samples were collected at hourly intervals for 6 hours and pooled thereafter till 24 hours had elapsed from the start of the experiment. A urine sample was obtained at the twentyfifth hour to determine if C" was still being excreted. Urine was preserved by freezing at -20' C. Blood wvas collected at 20 minute intervals.Methods for the collection and C' counting of expired CO,, for preparation and radioactive assay of blood samples, and C' counting and chromatography of labeled compounds in urine have previously been described (12).The C" pentoses D-xylose, D-lyxose, D-arabinose, Larabinose and D-ribose (Figure 1) with specific activity of 0.67, 1.65, 1.65, 1.11, 2.6 and 2.01 ,lc. per mg., respec-H C=O H-C-OH

The Metabolism of D-Ribose in Man

“…In the tracer experiment, the specific activity of the C1402 increased at a faster rate, reached a peak about 40 minutes earlier, and maintained a much higher activity throughout the interval of observation. Both of the curves appear to show a period of constant specific activity after the peak labeling has been reached, a finding not encountered in studying the C1402 excretion after the administration of other labeled pentoses (44).…”

Comparison of gavage, water bottle, and a high-moisture diet bolus as dosing methods for quantitative D-xylose administration to B6D2F1 (Mus musculus) mice

“…Earlier researchers (Segal & Foley, 1959) report that labelled D-xylose infusion in man resulted in an average 44% urinary recovery of the total dose. These researchers proposed that the pentose 169 sugars may be converted, in part, to glucose or may enter glycogenesis via the pentose phosphate pathway.…”

Functional Analysis of Degradative Enzymes for Hydroxyproline-linked ^|^beta;-L-Arabinofuranosides in Bifidobacterium longum

“…Bifidobacteria metabolize glucose and fructose through a particular metabolic pathway, socalled bifid shunt, and form lactate and acetate via D-xylulose-5-phosphate as major metabolites. L-Arabinose is a poorly absorbed sugar (33) and is scarcely metabolized in humans (34). Therefore, most of the L-arabinose is thought to utilize by intestinal bacteria.…”