Capability and extent of xylitol metabolism in vitro was studied in a variety of non-hepatic tissues of rats, using enzymatic and isotopic methods. Only kidney was found to be able to utilize xylitol at a significant rate while in all other organs tested - lung, brain, heart muscle, skeletal muscle and adipose tissue - no uptake of xylitol was detectable using two independent methods. In accordance with these results the incorporation of radioactivity from [U-14C]-xylitol as substrate into the products CO2 and lactate was found to be very small in all organs with the exception of kidney and when compared to that from [U-14C]-glucose it was less than 5%. From these results it is concluded that besides liver only kidney can utilize xylitol to a significant extent while for the other tissues studied xylitol is not a suitable substrate.

Abstract— Glucose metabolism in the superior cervical ganglion for calves has been studied by incubating slices with [1‐14C]‐, [6‐14C]‐ and [U‐14C]‐labelled glucose at 37°C and pH 7.4. Glucose utilization and the metabolic partitioning of glucose carbon in products during different incubation periods ranging from 5 to 60 min were determined by isotopic methods. Separation and identification of labelled compounds have been achieved by anion and cation exchange chromatography as well as by TLC and enzymatic analyses. From the data obtained a carbon balance could be constructed showing lactate to be the major product of glucose metabolism followed by CO2 and amino acids. Measuring the release of 14CO2 from differently 4C‐labelled glucose, the existence of an active pentose phosphate pathway in the ganglion could be demonstrated although this pathway seems to contribute only to a small extent to glucose metabolism. The marked decrease of the C‐U: C‐6 and the C‐U:C‐1 ratios in 14CO2 observed in the course of incubation is discussed in terms of a time‐dependent change in the rate of synthesis of amino acids which are directly connected with intermediates of the citric acid cycle.

1) Erythrocytes are able to metabolize D-ribose, D-xylitol, D-xylulose, D-fructose and D-glucose; the rates of metabolism increase in that order from 2430 to 26200 ng atomC/m/ packed cells per 120 min of incubation.2) The utilization of the carbon of these substrates and its recovery in the products were found to be in balance, when the change in the 2,3-bisphosphoglycerate concentration was taken into account.3) The metabolic rates strongly affected the 2,3-bisphosphoglycerate level. Without addition of substrate the decomposition rate of this intermediate was found to be 1030 nmol/m/ packed cells per 120 min.4) The net decrease of the 2,3-bisphosphoglycerate concentration and the conversion of this compound into lactate provides a NAD® regeneration system which enables the red blood cells to utilize xylitol.5) The rate of carbon metabolism via the pentose phosphate cycle is determined by the NADPH requirement of the erythrocytes which was found to be 160 nmol/m/ packed cells per 120 min under the experimental conditions employed. Stoffwechsel des Kohlenstoffs und Wasserstoffs von menschlichen ErythrozytenZusammenfassung: 1) Erythrozyten sind in der Lage, die Substrate D-Ribose, D-Xylit, D-Xylulose, D-Fructose und D-Glucose mit steigenden Umsatzgeschwindigkeiten von 2430 bis zu 26200 ng-Atome C/m/ Zellsediment während zweistündiger Inkubation umzusetzen.2) Wurde die Änderung der 2,3-Bisphosphoglycerat-Konzentration einbezogen, so konnte in allen Fällen eine ausgeglichene Bilanz zwischen Aufnahme der Kohlenstoffatome von diesen Sub-

I i i s~ i t iite for P ti? sio log ica I c'he in is t r y . Ll nive 1-5 it y of E r la rig en -Nurem berg ( Received Aueusl 8 . 1977) I . Using different experimental conditions and applying various substrates, a wide range of metabolic rates could be observed in incubations with human erythrocytes.2. Lower rates of substrate utilization resulted in a decrease of the ATP and, more pronounced. of the 2.3-bisphosphoglycerate concentration while carbon utilization rates beyond 14 pmol C/ml packed cells in 120 min yielded constant levels of ATP and 2,3-bisphosphoglycerate indicating that steady state had been achieved.3 . I t can be concluded. that 21 carbon utilization rate of 14 piiol C/ml cells in 120 min is able to covcr Ihc energy requirement of the red cells under steady state conditions which could be calculated to amount to 3.6 pino1 ATP/mI cells in 120 min at 37 C.4. The contribution of the 2,3-bisphosphoglycerate bypass to carbon metabolism could be calculated to range between 15 -X",, at metabolic rates below 14 pmol Ciml cells in 120 niin (presteady state). When steady state has been achieved. the share of this bypass increases up to 61 ' l o with increasing metabolic rates.

1. Washed human erythrocytes metabolize 1,14 muMol xylitol in 120 min under the following experimental conditions: pH 7.4 and 37 degrees C. 2. The levels of 2,3-diphosphoglycerate and ATP decrease during the incubation with xylitol. This decrease is not a specific effect of xylitol, but depends on the amount of C-atoms taken up. This could be shown in control experiments with fructose and glucose as substrates given separated or in combination with xylitol. The highest uptake of C-atoms has been observed with glucose plus xylitol as substrate. 3. 99% of the metabolized xylitol was recovered in the products lactate, Co2 and D-xylulose. 4. There was no measurable oxalate production from xylitol.